Tohatsu 2.5 hp overheat damage and repair.

[dtj]

2 month old, 4 stroke, impeller failure and subsequent overheat damage with loss of compression.

Factory manual is on order, but while waiting for delivery I began teardown and have not found any damaged parts to the point of case separation and crankshaft removal.

The only failure indicators are no start, no compression, and leakage thru the exhaust port with visible indicator of a 1/4" shiny band area on the exhaust valve guide toward the piston direction (see photos). Exhaust and Intake valve stem heights seem to be identical.

Is a valve guide movement a common occurrence in overheat damage and loss of compression?

Can valve guides be replaced?

Thanks..................Don

 

[pvanv]

Re: Tohatsu 2.5 hp overheat damage and repair.

The head must have gotten so hot that it expanded enough to release the press fit on the exhaust guide. End result is you seem to have dropped a guide. AFAIK, those are not available from Tohatsu as separate parts. Even if they were, there is a good possibility that the head casting is damaged/warped, so that a new guide won't fit correctly. The head and cylinder is cast as a single unit on this model. If that is the only damage (what about piston and rings, thermostat, and all the small plastic bits, as well as seals?), you would need 3GTB011001 CYLINDER CRANKCASE $397.72.

There's a distinct possibility of a bent valve and damaged piston from piston-valve contact as well. If so, you would want 3GTB870902 ENGINE SHORT BLOCK, 3.5B/2.5B/2B $965.92 (instead of the cyl/crank/head casting), which is about the price of a whole new motor. Probably better off to consider this one a parts motor, or find another motor with blown LU and mate the good sections of the two.

 

[dtj]

Re: Tohatsu 2.5 hp overheat damage and repair.

The head must have gotten so hot that it expanded enough to release the press fit on the exhaust guide. End result is you seem to have dropped a guide. AFAIK, those are not available from Tohatsu as separate parts. Even if they were, there is a good possibility that the head casting is damaged/warped, so that a new guide won't fit correctly. The head and cylinder is cast as a single unit on this model. If that is the only damage (what about piston and rings, thermostat, and all the small plastic bits, as well as seals?), you would need 3GTB011001 CYLINDER CRANKCASE $397.72.

There's a distinct possibility of a bent valve and damaged piston from piston-valve contact as well. If so, you would want 3GTB870902 ENGINE SHORT BLOCK, 3.5B/2.5B/2B $965.92 (instead of the cyl/crank/head casting), which is about the price of a whole new motor. Probably better off to consider this one a parts motor, or find another motor with blown LU and mate the good sections of the two.

Hi Paul, thanks for the feedback.

So far, no melted plastic parts, so I am hoping that casting is not warped or cracked due to overheat. Thermostat seems o.k. but I will replace since I already have the old one out.

If new valve guides are not available as replacement parts then I may have to knurl existing guide o.d. and press fit back in place.

Next step will be to split the case and examine cylinder, piston, rings, seals, and get a final determination on the valves and valve guides. I will post my findings.....................Don

 

[dtj]

Re: Tohatsu 2.5 hp overheat damage and repair.

Just finished teardown and inspection of outboard motor damaged by impeller failure and overheating.

The ultimate cause of lack of compression was the exhaust valve guide movement inward toward piston resulting in the failure of the exhaust valve to fully close, an appx. .050" gap openning existed allowing for no compression.

Everything else seems in working order and all that is needed is to press fit the exhaust valve guide back into the cylinder case at the proper height using the highest temperature locktite available, re-assemble the motor with new impeller kit, thermostat, and gaskets.

Wonder if there is a total rebuild gasket kit?......................Don


Cylinder openning and valve seats/guide area showing exhaust valve guide protruding more than intake guide;

Same thing except from perspective on top of rocker arm area;

Good piston, no contact damage with valves;

Good exhaust valve, checked on lathe and no warp, bend, or damage, intake valve passed liquid leak test;

Offending exhaust valve guide that will be re-press fit into place at proper height with high temp. locktite and possible slight knurl machining although probably not necessary as it took alot of force to remove guide. So as long as engine is properly cooled guide should stay where it belongs;

Plate tool that is used to remove and re-install problem guide;

 

[pvanv]

Re: Tohatsu 2.5 hp overheat damage and repair.

As long as the valve guide has enough of a press fit, you should be OK. Just keep in mind that the aluminum head has a lot of thermal expansion, so you need a lot of press fit. Similar to any aluminum-head 4-stroke, such as Porsche, original VW, or Chevy Corvair, although those air-cooled heads expand even more than these water-cooled OB heads. The gaskets you need are contained in 3AB871210M POWER HEAD GASKET SET $62.02. The thermostat is 3H6010300M THERMOSTAT $27.53. Also a good idea to get the full wp kit 3AB873220M $21.08

 

[dtj]

Re: Tohatsu 2.5 hp overheat damage and repair.

Finally found some time to get back to this motor...............see the following photo's;

Fine taper knurled the upper 1/4" of the exhaust guide to .004" over factory diameter, froze the guide to -110 f. and re-installed to the same height as the factory installed intake guide (since there are no published dimensions for this replacement work), re-installed valves, springs, rocker assemblys, and liquid leak tested both intake and exhaust valve integrety..............both passed with no leaks, I am back in business.

Paul, thanks for the parts listings................I will be doing a final inventory tomorrow of all possible items I may have missed or need to complete the rebuild and will get back with my findings.....................Don

 

[pvanv]

Re: Tohatsu 2.5 hp overheat damage and repair.

Good work. You are probably good to go.

 

[dtj]

Re: Tohatsu 2.5 hp overheat damage and repair.

Good work. You are probably good to go.

Finally got the motor back together using home made gaskets. Ran it for a few hours to test all systems and everything seems alright, no leaks, no mix of oil and water around gaskets, about the only hiccup is a slight hesitation in the throttle up to high speed. Motor actually seems to start easier, almost always with first pull, wonder if some of the settings that I did during re-assembly may have improved start ups...................Don