[redearth329]

Head gasket is leaking and the leak is accelerating to the point where I can't ignore it any more. Maybe the gasket between the cylinder and the crankcase, too. I want to replace those, and I figure if I start pulling gaskets off I gotta put all new ones in when I put it back together. I do have a shop manual and have read it over. I've done my own work on this bike and my car, but this would be the most advanced thing.

I was thinking of getting something like this:
Engine Rebuild Kit - Honda CB/CM400 - 1978-1981

I'm wondering if I'll need anything else? If I don't have to replace the gasket under the cylinder, would I have to pull the pistons? If I pull the pistons should I replace the rings? Compression is good now. Anything else I should do while I've got the engine out? It's got 40k miles (1980), my bike for 8 years, and no problems other than the oil leak right now. Actually: there's a leak from the carburetor cross-over tube, so I'm going to fix that too while I've got it apart.

Any other advice before I start? I'm planning to take lots of pictures as I go, keep parts organized in labelled baggies, and have the shop manual open on the bench.

Thanks!

 

[HamHock]

Have you done a compression test? That would tell us more about the state of the piston and rings. Either way it sounds like you just need to rebuild the top end. That'll mean removing the cam, cylinder head and cylinder, you will need to inspect the bores, you might only need to give it a light hone and a re-ring, but if there is a ton of wear or any gouging you'll need to buy oversized pistons, or buy a good used cylinder block. Barring any abnormal wear you shouldn't need to rebuild the head but check the valve seats for wear and the valve stems for mushrooming. Be careful when taking out the cylinder head bolts as bike left on their side stand are prone to collecting water down the bolt hole which can cause corrosion and broken bolts. Vesrah or any semi reputable brand will have a good rebuild kit for gaskets and buying a whole engine kit will save you some money. I would recommend replacing the carb boots at the same time as they can be had for around $10 on ebay. The walk through Jim posted, which you linked, is a great recourse as is the FSM, which you say you have. Lots of pictures, baggies and labels help the reassembly go smoother.

 

[eunsan]

I redid the top end on my cM400T today, and agree that the detailed instructions in the shop manual are good, except.

There is very little slack in the cam chain so it is very important to fully raise the tensioner (about 1" of travel). Although I did that, and got the chain onto the cam sprocket, I could not move the sprocket onto the shoulder on the camshaft until I temporarily installed the rocker assemblies and the 8 long bolts and tightened them just a bit. Problem solved.

Cheers / Gary

 

[DD23]

Cool, so a 6 valve ( 3 per cylinder ) engine. You did clean the crank case cylinder head bolt holes ?

Bolts back in correct holes ?

No sealer anywhere near those bolt holes....

Did you check the balance chain adjustment and DOT ?

Dd23🤓