Honda CM400T 1981 twin piston ring failure

[sono]

Hi everyone !
I've made the mistake to buy a Honda CM400T 1980 without performing a compression test because It ran very smoothly and there was no visible smoke or oil on spark plugs. The engine had 25000km and bike condition is consistent with that. The price was low enough to cover for the risk of a major failure.

10 days and 600km later It started to smoke (bluish white) for 10 seconds then it stopped. Now, the amount of smoke is much greater (until the engine is hot) and I received my compression tester... BAD NEWS:

Engine specs are 13 bars (185psi +- 15 psi)
Left cylinder has 7 bars plain, 12 bars with 5ml of oil piston
Right cylinder has 5 bars, 7 bars after I drop some oil on the piston.

There is no question my piston rings are worn out and my valves are OK. I've read that a motorcycle engine with less than 7 bars must be rebuilt.

I have not opened the engine so far but this bad surprise does make me beleive either poor storage condition caused rust pitting into the cylinder or oil changes were not done often enough.

Two out of four motorcycle mechanics I consulted told me it was better to swap the engine by one that has proper compression. They told me that even if I fix the twin cylinder and engine head to specs, I could end up breaking the base of the engine that is already worn up to an unknown point.

Two told me considering the engine had only 25000km it was better to get cylinders and head rebult to specs because a used engine had good chances to use a lot of oil and fail prematurely. A rebuilt engine should last much longer they say.

Swapping the engine is less expensive, but I don't want to redo the same every year. I'm driving 12,000km a year on my motorcycle, so I don't want a short term solution. Some make me fear about the timing chain, some are confident it is OK. I'm confused.

On such an old motorcyle, getting new oil seals on the valves and renewing all gaskets will certainly be a good idea, but if there was rust pitting on cylinder, there could be some on the crankshaft and transmission bearings too, isn't it ?

Does anybody has some advise and explanations that could help me make the best decision ?

Thanks !

[tricky1970]

Tricky here, in my experiance repair/rebuild generally is better as after market or replacement parts are mostly of a better quaitity than the origanal components. However you also have to way up the cost involved and your situation. I've had 4 bikes and 2 of them were engine jobs. 1 was a rebuild and the other was a replacement engine. Both were good after each repair, however this may have been due to mantainance carried out after the bikes were put back together.  Either way should be good as long as you maintain the bike well after the repair. I am not tell you what to do in your cercomstance. If it was me I would rebuild, but the decission is yours.

[Thundertek450]

Hey, I agree with the techs too, as I am one too, and own a 1980 hawk that came as a 400cc when i bought it but many dollars and extensive modifications later it is a 450 now. with alot of LOT of mods to bike and frame, but I have two spare engines. 1 from a 1980 CB400T, The other from a 1979 CM400T. one has the head removed due to a broken exhaust stud. The other is COMPLETE! They both run! have Top notch Compression, And Carb banks For both, They are "PICKELD"(preservrd), I also have alot of odds and ends, a Complete exhaust system with the crossover cat. with a few scratches. If you are intrested in any of these items please email me at csaeroent@aol.com

[sono]

By the way, I'm based in Qc, Canada, so shipping and duties for a complete engine is usually too high unless you are yourself based in Canada.

Thanks for the offer, but I already have a spare for cylinder block, pistons, a complete head and carb and two gaskets sets. I even have a spare clutch and starter engine which can be used on both CM400 or CM450 engines.  

I am certainly going to fix the CM400T engine in the next few months to keep as a spare engine (I am riding a lot). I got recommended a machine shop that would recondition the head and cylinders and check if I need new STD piston or oversize. Once that is done, I may even end up remounting the whole engine by myself with the help of a friend who is a pro mechanic.

Estimate on CM450 engine replacement including install is coming soon. Once my vendor is finished testing what he has I will get a final price, likely the same or lower than getting rings pistons and head and rebore cost.

Still on standby