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Yamaha ME420STI Diesel Engine

Yamaha TRP Sterndrive

Yamaha ME420STI Diesel TRP sterndrive

The engine is basically a Toyota 1HDT but with Yamaha pistons for different compression ratio, modified ratio injectors, pump and turbo charger for extra fuel and boost. The availability of far more efficient water for cooling the engine as opposed to air for its automotive counterpart means that more fuel and air can be delivered to the engine for a lot more output HP, however if any restriction or deficiency occures in the cooling system and the engine runs hotter than it should then ultimate failure is eminant.

Yamaha ME420STI Diesel TRP sterndrive 3500 hours

My Engine 3500 hrs

Yamaha ME420STI Diesel TRP sterndrive Cracked pistons

No 2 and 5 pistons

Yamaha ME420STI Diesel TRP sterndrive holed piston

Side of no 2 piston, cracked and hole completely blown through

Yamaha ME420STI Diesel TRP sterndrive watermixer

Yamaha ME420STI Diesel TRP sterndrive Blocked water mixer

Rust scale removed from elbow water jacket after 1 year.

Yamaha ME420STI Diesel TRP sterndrive Restricted cooling pipe

Intermediate houseing water inlet hose

Yamaha ME420STI Diesel TRP sterndrive Fuel bypass hose

The other end of this hose just "T"s into the main fuel line before the lift pump.

Yamaha ME420STI Diesel TRP sterndrive bearing failure

Lowercase uppermost bearing, most common failure, note that this bearing is cooled by exhaust water which fills the little cavities surrounding the bearing as shown. Reduced exhaust water flow due to blocked or restricted exhaust mixers reduces or prevents the cooling of this bearing.

Yamaha ME420STI Diesel TRP sterndrive failing bearings

Close up of that bearing damage

Yamaha ME420STI Diesel TRP sterndrive failed bearings

Uppercase, main pinion bearing set, second most common failure, a Yamaha only available bearing set and hugely expensive.


Yamaha ME420STI Diesel TRP sterndrive broken F gear


 Yamaha ME420STI Diesel TRP sterndrive corroded steering arm

Intermediate housing, Steering Arm, wear inside arm.


  Yamaha ME420STI Diesel TRP sterndrive water pump seal

Sea water pump seal failure, usually caused by corrosion and breakage of seal spring on seawater side of pump due to electrolysis, the seal is often not worn and the spring can be replaced by removing the impellor and spring fragments and replacing without major dissection of the pump.

Replacement Airfilter

Yamaha ME420STI Diesel TRP sterndrive Air Filter

Air Filters block rapidly due to fanbelt dust, OEM replacement price in NZ is $300+ I rebuild my own filters using FRAM PSA35 pods. You need to retain the intake fitting of your old filter by melting it off the old cartridge with a heat gun. The new pod needs to have the rubber nozzle cut off (as per remnant sitting on top of turbo in Photo) The pod is then a perfect fit into the old intake fitting and I use plenty of epoxy glue to hold it in place. The engine blowby hose just rests nicely into a hollow on the pod.

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Gimbal Bearing

Yamaha ME420STI Diesel TRP sterndrive Gimbal Bearing

Once again the OEM item is well over $300nz. This bearing can be rebuilt useing SKF bearing YET 209. The SKF bearing housing is narrower than the standard bearing but for some reason lasts a whole lot longer. When re assembeling bearing ensure that you have the grease channels and port on the correct side. The YET bearing comes with another metal ring (Top right). which can be discarded.

Engine Wiring Diagram

Yamaha ME420STI Diesel TRP sterndrive Engine wiring diag

1st Engine failure, 2300 hours use.

My story and the beginning of my journey to understand and maintain my own unit.


 Whilst on a routine 50km run Medium loading, calm sea, 3200 rpm, I noticed a sudden jump in the engine operating temp from the usual just above 84c to just below 100c.  I initially thought a water blockage and over the next two months whilst continuing to work the boat systematicaly went through both salt and fresh water systems, replacing impellors, thermostats, cleaning heat exchangers, intercoolers, etc.  Below 3200 rpm the engine would run at about 90c all the time but would heat up if ran any faster. The only pump that I had not checked was the engine water pump as removal of timing gear required for this but I had got to the point where I thought this could now be the only reason for the problem.

Then the oil drain from the intercooler began to intermittently dump a lot more oil than usual and I began to think one of the turbo oil seals as beginning to leak also.

This was peak season with bookings every day for the next two months so I was hoping to nurse the unit through to the winter layoff for a thorough check.

This was not to be the case.

On a short run the engine suddenly died down to 1000 rpm with clouds of smoke from the exhaust. Opening the engine bay revealed oil pouring out of the blowby hose into the air filter.  I removed the oil filler cap and was confronted with about 10 p.s.i. back pressure and black oil being sprayed everywhere.

We managed to idle home under our own steam where removal of the engine head revealed pistons 2 and 5 cracked right through.


Possible reasons for Piston failure.


(1) Piston design, I have since received at least 20 reports of cracking in the same position on these OEM Yamaha Pistons. (NOTE: It appears as though the piston has been redesigned for the engines 2006 release.)


(2)  Engine running temp higher due to blocked exhaust watermixer. Internally the cast iron water jacket rusts and flakes off blocking the water flow. 

 I have found that these need replacing every 2-3 years if boat left in the water.

 A quick way to check is to squeeze the rubber hose feeding the salt water into the mixer at about 3000 rpm, if the hose is hard and under pressure the mixer is probably blocked. We now manufacture S/S water mixers which I can supply.

See Picture.


(3) Fuel quality and injector servicing.

My injectors in some instances were jetting. I now service injectors at 1000 hours.

(4) Injection timing, the exact timing of the fuel injection is crucial and needs to be done precisly according to the Yamaha specs. The alignment marks stamped onto the housing should not be trusted.


(5) Other cooling water restictions. The Intermediate housing was found to be corroded where raw water hose passes through. This causes narrowing of hose (See photo). The raw water hose through the intermediate housing at both ends, is a huge source of problems for many people who contact me. Many including myself have sought a separate 'Through hull' solution to the problem. The water which now passes through the leg by way of elaborate hoseing I now pipe into the plastic cover on the upper drive to assist in cooling it.

2nd Rebuild 3450 hours

Near the end of my 2005 summer season I began to notice an increase in exhaust smoke all the time and an increase in running temperature when the engine was again pushed over 3200 RPM, I decided that this was a good opportunity to remove the engine and inspect.

When the head was removed it was found that the inlet valves were carboned up and in one instance the buildup allowed the valve to make contact with the top of the piston. As some of the valves had become distorted it was neccesary to replace them all. Several cracked pistons were again replaced.

Probable reason for carbon buildup, Oil

Since the first rebuild a visit to my Caltex oil merchant suggested that Caltex Delo 400 would be a suitable oil for my engine. Unfortunatly this was wrong. This oil is an extremely good diesel engine oil but not for Japanese diesels which require different additives, sulphur levels etc. The Oil which I should have been running is Caltex CXJ, J stands for Japanese. Now Jan 2008, I have been advised that a better oil for the engine is Caltex Super Tractor.

Main pully balance wheel loose.

Not long after my first rebuild the main pully came loose on several occassions and it took a fair amount of loctite and torque to secure it properly. During this rebuild it was noted that the crankshaft, main pully and timing gear had all been damaged and required replacement as a result of the loose pully. There is  design fault here as only one little woodruff key holds all this together. I now use plenty of locktite and bearing retainer compound on all mating surfaces.

 Do not be tempted to use the main engine pully to drive additional components, pumps, refridgeration what have you, as this pully is not up to it.

Replacements of those damaged parts does not come cheap

 Inspection also found the Fuel pump was not working, however the injector pump was able to draw fuel through the bypass system without any noticable difference. If you do not have a bypass system fitted, you should do so.(See Photo)

TRP Sterndrive


I am the owner of 5 TRP  and 1 single prop sterndrives.

Unfortunatly unless underpropped it appears as though some parts in these drives are not able to handle the torque of the ME420STI engine they are attached to.

Legs sometimes only operate for about 300 -500 hours before they fail.

The impending failure of these legs is usually indicated by stiff gear changing and smelly black oil which is caused by metal particals usually from damaged  bearings and broken oil seal rings becoming trapped in the gear selector valve, if left unchecked some of these particals will  find there way into the oil pressure relief valve eventually jamming it open resulting in no oil pressure and lost drive. The black oil is caused by clutchplate wear and overheating as a result of the above.


The other type of failure is usually more  sudden. And involves the breaking off of pinion gear teeth, usually the top,  this results in sudden lockup and in one instance also resulted in the destruction in the 2 stage rate coupling on the engine by sheering the rubber bobbins.



Post Mortems and analysis of some of my leg failures.



Leg 1, 1500 hrs

Failure of lower case driveshaft uppermost bearing,  Yamaha Part no; 93332-OOOUA

Failure of uppercase pinion bearing set

Failure of inner/outer propshaft bearing, Yamaha Part no; 93332-OOOUB

Fractured tooth lower case driveshaft pinion gear, Yamaha Part no; 6U4-45551-00

The metal fragments from the above parts caused extensive damage to the gear selector valve and jammed the oil pressure relief valve.




Leg 2, 300 hrs

Sudden failure and tooth loss of Gear 2, upper pinion gear, Yamaha Part no; 6U4-4588E-00

Attributed to a 'Momentary impact' with some sort of object.




Leg 3, 500 hrs


Failure of lower case driveshaft uppermost bearing,  Yamaha Part no; 93332-OOOUA

Failure of 2 oil seal rings, Broken teeth

The metal fragments from the above parts caused some damage to the gear selector valve and jammed the oil pressure relief valve.


Leg 4, 500 hrs

Failure of lower case upper driveshaft bearing,

Breakage of 2 steel oil seal rings on clutch carrier shaft.


Probable cause

(1) Design, The lower case upper driveshaft bearing is insufficient for the job. I now replace this bearing with SKF equivalent and replace at 150 - 250 hours, I have also reduced preload on this bearing (Newsflash) 2006 I hear Yamaha have upgraded this bearing.

(2) Better standard of oil must be used only one specific oil is now recommended for this sterndrive and should be used; Castrol SAF XO 75w.

(3)Steel oil seal rings on clutch carrier wear and small teeth lugs break off. I actually suspect that this may be the cause of subsequent bearing failure.

(4) Overheating of bearing due to restricted exhaust water flow (See diag)


Intermediate housing, Transom plate


Loose floppy steering arm. This is a cast iron/steel steering arm on an s/s shaft. The main upper bush and the seal in the transom plate become worn allowing sea water to creep up and leak in . Since the shaft and steering arm are different metals electrolysis/corrosion sets in between  the two and steel arm begins to rust/corrode and move quickly. No amount of tightening the clamp bolt on the arm will rectify the problem and it will have to be replaced, Regular greaseing of the shaft is the only way to minumise this problem. Since my boat is permanantly moored I have run extension grease tubes from both this and my gimbal bearing into the hull.


2 Stage rate coupling

 I replaced the bobbins twice when the coupling became chattery at idle in drive and then later replaced the old damper plate setup with the new solid rubber one.

No problems since.


 Drive Cooling

It is apparent that at times the temperature of the drive become high as indicated by discoloration of the drive housing cover.

The fitting of some type of cooling is required and there are several options.

(1) The addition of a shower to squirt water onto the cover while underway (Minimal cooling effect on drive)

(2) Piping of water directly into the plastic (grey) drive cover, This option I have used for several years.

(3) There are several internal options I have seen which involve, blocking and drilling but these are best only used if you choose to no longer use the drive pickup for the engine water cooling. Note: that water must be able to freely move through this water intake as it is the main water cooling area for the drive.


Rapid wear of Engine Fan Belts


  The belt pullies on these engines are standard automotive and just steel, with infrequent use they develop surface rust which eats away at your belts like sandpaper. The only remedy if yours is bad is to repolish all pullies with emery paper and regularly treat with something like CRC 'Beltgrip" especially if engine not going to be used for several weeks at a time.


Faulty trim sender unit

  These arent the best design, have replaced twice in 10 years.


Corrosion, leaking heat exchanger

Repair this at least once a year now, I see that the later models have sacrificial anode plugs. I have replaced the drain plug with the small engine type screw in anodes. It actually seams that the Zink now shed by the anodes actually blocks any little holes that form.


Faulty reading rev counter

Havnt got to the bottom of this one yet. Start engine, counter climbs to 5000 rpm then settles back, rev counter sometimes reads 400 rpm over.


Excesive oil discharge from intercooler

 Due to excessive blowby, worn engine. Oil due for changeing, however I have established that the oil trap on the blowby hose which returns excess oil to the sump can work in reverse (i.e) when a piston is cracked which can force oil up the drain pipe and into the oil filter. I have actually shortened the drop tube into the sump so that it is above the oil level in the sump. If you read about my first engine failure, this is why oil was pouring out of the air filter. I have now added a collector to the end of this pipe and no longer feed it onto the air filter.


Corroded Turbo oil  pipes

 This little blighter rusts from behind and one day lets all your oil out while you are cruising somewhere, be sure to paint him properly the first chance you get, I am considering totally replacing mine with rubber almost up to the turbo.


Filling leg with oil from top

If you do this and fill up to correct level on leg and close it up then run it you will find that the oil level has dropped about 30mm. That is because if you fill the leg from the top, the oil pump and several of the oil galleries become air locked. The only way to do this properly is to inject the oil up through the drain hole in the bottom of the leg. If you do fill from the top, run the leg and then top up before relaunching.


Yamaha ME420STI Diesel TRP sterndrive Ringnut

My Removal Tool

Yamaha ME420STI Diesel TRP sterndrive Ringnut remover

If sacrificial anodes are not replaced frequently as required, one of the first places to corrode is the ringnut and its thread into the lower case. Even when new, I remove the ringnut from the leg and coat threads with CRC Softseal. If your ringnut is badly corroded it is best to just drill it out and replace with a new one. Softseal on all surfaces.

Universal Yoke modification

Yamaha ME420STI Diesel TRP sterndrive Drive yoke modification

The universal yokes are only cast and suffer rust pitting if you have ever had water in your boot. I have turned all of mine down and fitted stainless steel sleeves.

Oil Strainer Magnet

Yamaha ME420STI Diesel TRP sterndrive Oil strainer magnet

To absolutely minimise risk of drive failure out at sea due to stuck oil pressure relief valve, I have upgraded filter strainer magnet to one taken from the strongest magnetic pickup tool possible.

Follow this link to my web albums page on several How to's

Intermediate housing problems

replace gimbal bearing internals

Aftermarket exhaust mixer

Aftermarket exhaust mixer

CRC Softseal

Yamaha SKF bearing replacements


Yamaha sterndrive oil seal rings

Emails received and replies.

From: Jamshed M.....
Sent: Tuesday, 20 November 2007 7:57 a.m.
To: Diveworks@xtra.co.nz
Subject: Hi from India



 I am Jamshed M.... from Mumbai India.

 Your Report on the Yamaha engine and drives was very useful. We are a company maintaining Speed Boats in Mumbai India and have a Yamaha built boat ( Model is SEA ODYSSEY) with twin Yamaha ME_ 420STI with the stern drives from Dubai.


The drive pinion gear was found to be damaged within the first 380 hrs of engine /drive run. We repaired the drives and are working fine now  but will keep a close eye on the maintenance schedule and open her up after 350 hrs run again as advised by the manufacturer.. If you have any additional advice for the maintenance on the engine and drives it will be welcome.

 Add me to your list of people who have asked for advice from you for these engine and drives..


Been on your site and like your boat .






 Tech. Manager



Hi Jamshed

 Many thanks for you information and compliment,

Your email just arrived as I am forwarding below information to Ireland.

Your photos are excellent and damage is common, gears are very hard and break easily if foreign metal enters. Yamaha do not acknowledge and attribute gear breaking to ‘momentary Impact’ although no external signs of impact on leg or props. Fortunately impact with object if substantiated you can claim Insurance for accident in New Zealand.

I would like to use some Photos and letter on website, Thank you.

 Phil van Dusschoten


New Zealand


From: m d...... Sent: Saturday, 17 November 2007 11:59 a.m.
To: diveworks@xtra.co.nz
Subject: Yamaha hydradrive


hi Phil
I am currently running an American built osprey 26 pilothouse sportfishing boat
here in Ireland and have done about 1000hrs since aug 2004,
as a yamaha hydradrive owner i was quite horrified to see your problems
on your site, to date i have blown 1 leg "700hrs",  and my replacement leg is now
showing signs of wear with swarf on the filter magnet, my leg is a single prop unit.my engine model is me 421 sti ,
After all the issues you have had would you not have been economicaly better off to repower?
do you believe that over propping is an issue ? i know in my own case that my laden max rpm is 3600
and i was considering dropping pitch to increase rpms .what was your max laden rpms?
also phil is it possible to fit non hydradrive props to the leg ? as the originals cost 3000 US $ !
also phil are the toyota lancruiser crank ,valves, pistons etc compatible ? probably cheaper.
phil congrats on your technical know how im really impressed
i look forward to hearing your response
tight lines
mike d......


Hi Mike

Thanks for your enquiry

I also have a close assn. to many boats running other drive units and problems are comparable.

Fortunately for me the TRP leg is very easy to work on and maintain with the right tools and know how, I remove legs, strip and replace certain parts every 200 hours.

As I get to understand these drives more I see the same patterns.

Generally failure of the lower case uppermost bearing and the damage its fragments then cause to the rest of the leg.

I am now beginning to suspect that the actual culprit for these failures is the breaking off of the small lugs which form part of the oil seal rings on the end of the clutch carrier shaft as at lease one of these is often broken and missing when drives fail.


Basically now all the drives cost me is a $60 bearing, an $8, steel oil ring about $25 oil seals (I always replace outer shaft seals and input shaft seal, about $80 oil Castrol SAF-XO, and about 4 hours labour every 200 hours.

Its like aircraft preventative maintenance, I replace things before they fail and the leg stays clean and undamaged, 2000 hours without breakdown now.

 Over propping seems to create more problems for the engine than the leg, 3600 W.O.T. is at the very lower end of OK.

I don’t know about non hydra drive props, If you look at them closely you will note they are different sizes, I am sure that it is very scientific so have not even considered it, You could have them repitched which is what I do.

 Landcruiser parts with the exception of Pistons and fuel system are compatible, Yamaha pistons run at different compression ratio than Toyota and fuel/air delivered at different rates.


The efficiency of water cooling in a marine engine allows more fuel and air to be delivered to engine consequently more HP output than its automotive counterpart, how ever if engine allowed to overheat due to sea water restriction then problems become more dramatic.

 I hope this helps


Phil van Dusschoten


New Zealand.

Discussion between Yamaha drive owner and Yamaha-motor.co.uk



16/07/2007 03:44 PM

Check out Yamaha ME420STI Sterndrive - Diveworks




Hi Wouter,

In reply to your e-mail which was forwarded to me...............................

Yamaha ME420STI Sterndrive - Diveworks
Hello Mark, I send you report about Sterndrive I found on the internet
I would like to get yamalube for sterndrives but only in 20 or 30 l cans
can I use castrol SAF XO 75 Yamaha have not officially approved any oil other than Yamalube for use in Hydradrive (however I am aware that other oils are being used) The specification of the required gear oil is SAE-90 gear oil, to GL-5 specification, NOT containing Boron (the not containing Boron bit is very important). If the Castrol SAF XO 75 meets or exceeds this, it can be used.
or do you know how to get smaller amount.Yamalube is only available in 20litre pack
is yamalube synthetic Yamalube Sterndrive gear oil GL-5 is a mineral oil not a synthetic based oil.
now in Leg gl4  mineral from netherlands but this might end up in faillure and loosing of leg. GL-4 is not the recommended grade of gear oil.
had new leg 400hrs ; this one now 250 hrs
would you advise changing lower case driveshaft uppermost bearing? This bearing should not require changing at 250 hrs if regular oil changes using the correct oil have been done.
is yamalube same as castrol saf xo 75 ? The Yamalube GL-5 has approvals...........ZF: TE-ML-01,  ZF: TE-ML-05 and ZF: TE-ML-07,08.  I have no specifications for the Castrol oil.
please inform me

I hope this answers all your queries. If you need any more information, please do not hesitate to contact me.


Jerry Woods
Technical Advisor
Marine Section
Service Division
01932 358144


  Dear Sir,

As requested, please find following, information regarding Yamaha Oil for Sterndrives:

Yamaha "Yamalube" Gear Oil weblink:

Yamaha "Yamalube" Engine Oil weblink:

Best Regards,



Hello Phil,
Seeing you are a heavy Yamaha sterndrive user and also Dutch, I want to share some Yamaha details with you.
I have been using a ME420 engine with single 19" prop drive for 7 years now without big problems (only 500 hrs light duty)
The only thing was a clodded mixing elbow. I replaced that in 2005 with a stainless one, with good results and more engine power.
This year I could trade in my engine with little loss and got me a ME432 withe a TRP drive with 22"props.
I thought, more power would mean a little more speed en torque.
This is not the case, I stille have the same speed en torque, so I am missing 75 HP.
To spare the drive I equipped it with a little driveshower.
Below are my measurements. Can you tell something about it?


Motor ME420 STI 240 PK enkele prop            
toerental liter/mijl km/uur liter/km liter/uur te leveren vermogen   knopen bereik km  
1000 0,45 10,7 0,24 2,60 13,00   5,78 1646,22 rompsnelheid
1500 1 14,2 0,54 7,67 38,34   7,67 740,80  
2000 1,85 17 1,00 16,98 84,91   9,18 400,43 halfglijden
2500 1,3 30 0,70 21,06 105,29   16,20 569,85 plane
3000 1,67 40,3 0,90 36,34 181,70   21,76 443,59  
3300 1,67 49,4 0,90 44,55 222,73   26,67 443,59 kruissnelheid
3400 1,7 53 0,92 48,65 243,25   28,62 435,76 Topsnelheid
Metingen d.d 20-08-2005 in Zeehavenkanaal zonder stroming en wind 1 persoon aan boord volle tank
Schroef 19 inch kan kleiner om maximaal toerental 3800 te halen      
Drive reductie 1:1,5                
Lukt nu niet 3400 max (240 pk)              
ME432 315 PK duoprop               
toerental liter/mijl km/uur liter/km liter/uur te leveren vermogen        
1000 0,47 11,005 0,2538 2 10        
1250 0,955 12,115 0,5157 5 25        
1500 1,705 13,54 0,9206 10,5 52,5        
1750 2,26 14,95 1,2203 16,5 82,5        
2000 2,27 18,2 1,2257 21 105        
2250 2,015 23,2 1,088 24,5 122,5        
2500 1,71 30,5 0,9233 27,5 137,5        
2750 1,655 36,95 0,8936 32 160        
3000 1,73 41,25 0,9341 38,5 192,5        
3250 1,84 47,6 0,9935 47 235        
3650 2,2 55 1,1879 65,335 326,6739        
Metingen d.d. 06-05-2007 bij Schiermonnikoog geen wind 1 persoon aan boord volle tank  
Drive reductie 1:1,68                
Zusterschip Marina met Yanmar 315 PK en Bravo 3 vaart dik 70km/uur met 5 personen en 3950 toeren

Best Regards
Hendrik K.....
The Netherlands






Hi There


Great lot of info on the Yamaha motor and leg on your web site. I have had a Yamaha Diesel in 730 Rayglass for approx 12 months

Just clocked up 1000 hrs so I hope I don’t have any of the problems you had, at least I know a few things to look out for.

It is the Me420sti with the duo prop stern leg, do you know the best service manual to get for this model and where to get it?

Happy Boating!


          Cheers Terry Chitty


> From: K, Mike-G

> Sent: 28 July 2008 14:23

> To: 'Diveworks@xtra.co.nz'

> Subject: ME420 Outdrive

> Hi, my name is Mike and I have a Yamaha ME420 engine and outdrive on

> an Osprey - about 3 ton. It has about 200 hours on it from new -seven

> years old. When using it this year the speed of the boat has dropped

> considerably for some reason. The engine is fine - new filters each

> year etc. It seems I am not getting all of the power to the prop -

> hard to get the boat over the planing hump. I am getting 18-20 mph at

> 3000 revs instead of 33 mph.

> I was thinking - could my clutch be slipping ? When the boat is in

> gear - engine off - should it be possible to rotate the prop by hand ?

> Something I never checked before. I pulled up the outdrive and rotated

> the prop to make sure it felt ok - it did - also I tried it in gear

> with the engine off of course - it still roates easily.

Subject: RE: ME420 Outdrive

Hi again, talked to a guy in Redabay boats and his opinion is I should clean my hull - a sever growth at the waterline. And no the prop should not be in gear - engine has to be running for it to be in gear - does that make sense ?

Hi Mike

Re your query

(1) The gearbox is hydraulic and requires oil pressure to engage gears, engine turns oil pump continuously when running, no engine running, no oil pressure means no gears can be engaged. Prop will freewheel.

When you use your gear lever you are changing a hydraulic diverter valve only, in one direction the oil is diverted to a clutch which operates forward gear set, to the other, the reverse gear set. This valve can become damaged or fouled by metal fragments from gear or bearing failure within the leg, consequently stiff gear lever is usually the first indicator of problems inside the leg.

(2) The main clutch carrier shaft has steel oil seal rings separating the forward gear oil flow from the reverse gear oil flow. These rings break (Usually a small lug which holds them closed forming a tight seal breaks off, this is diamond hard about .5mm square, imagine the damage this does while circulating around your leg, another story). When the seal is broken it means that when in one gear a small amount of oil pressure can seep through to the opposing clutches of the opposite gear creating drag and heat, consequently black, thick, smelly burnt oil noted during your oil change.

(You still get the same engine RPM at throttle setting because engine governor compensates for extra load, reduced WOT rpms could be an indicator or increased fuel use.

(3) As full engagement of gear clutches relies on good oil pressure to them to prevent slipping, the following factors could effect this also.

(a) A partially blocked oil strainer

(b) Incorrect or insufficient oil

(c) Partially obstructed or sticky oil pressure relief valve.

Mike if you suffer any of the symptoms I have described you probably have a leg problem.

Hope this helps,

I will post your question and my answer on my website, first name only


Phil van Dusschoten


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