Topic: How to check compression of spare engine, out of frame?

[markreimer]

Hi there,

So I've got a '74 750K with an engine case cracked by an over-tightened oil pressure switch. It's JB welded together after two failed attempts at welding it properly, and it's held for the last few hundred KM just fine. But it makes me nervous, so when I happened across a spare '74 engine for sale I jumped on it.

The spare looks to be in roughish shape. It's missing the starter, the cam-chain tensioner cover and bolt isn't there. No ignition. I really don't know anything about it. But I bought it because I figured I could take the top half of my current engine (only a few hundred KM on a complete top-end rebuild with 2nd oversize pistons, new valve guides, etc) and plunk it onto the lower-half of my spare engine.

Two things come to mind - maybe this spare engine is in good shape and I should just do a complete swap. Save some cash and time. I'd like to do a compression test but I can't get it hot first and don't really know how to go about doing this with an engine out-of-frame. COuld I just put a starter motor in it, hook it up to a battery directly and do that? No carbs attached, no exhaust, etc.

Second thing - this engine has obviously been stripped of a few parts as mentioned. Makes me wonder if it's not in such good shape. It turns over and the guy I bought it from received it with a 750 he bought. He had the cam cover off, said it turned over, dumped a bit of oil up there and in the cylinders and packed it away. But then again...who really knows. I'm a total newb with the bottom-half of these engines, and engines in general. Are there some good ways of checking the condition of the bottom half? You know, before I spend a weekend swapping a cracked bottom for a dud bottom  ??

[HondaMan]

Hi there,

So I've got a '74 750K with an engine case cracked by an over-tightened oil pressure switch. It's JB welded together after two failed attempts at welding it properly, and it's held for the last few hundred KM just fine. But it makes me nervous, so when I happened across a spare '74 engine for sale I jumped on it.

The spare looks to be in roughish shape. It's missing the starter, the cam-chain tensioner cover and bolt isn't there. No ignition. I really don't know anything about it. But I bought it because I figured I could take the top half of my current engine (only a few hundred KM on a complete top-end rebuild with 2nd oversize pistons, new valve guides, etc) and plunk it onto the lower-half of my spare engine.

Two things come to mind - maybe this spare engine is in good shape and I should just do a complete swap. Save some cash and time. I'd like to do a compression test but I can't get it hot first and don't really know how to go about doing this with an engine out-of-frame. COuld I just put a starter motor in it, hook it up to a battery directly and do that? No carbs attached, no exhaust, etc.

Second thing - this engine has obviously been stripped of a few parts as mentioned. Makes me wonder if it's not in such good shape. It turns over and the guy I bought it from received it with a 750 he bought. He had the cam cover off, said it turned over, dumped a bit of oil up there and in the cylinders and packed it away. But then again...who really knows. I'm a total newb with the bottom-half of these engines, and engines in general. Are there some good ways of checking the condition of the bottom half? You know, before I spend a weekend swapping a cracked bottom for a dud bottom  ??

Yep, you can install a starter, remove the plugs, and check the compression on each cylinder: done that many times.
The cracked one: you can also bore that hole and insert a custom-made "plug" reducer in the hole to re-install the oil switch. The "plug" should be bigger than the crack, though, which might be the limit?

[markreimer]

Yeah the crack goes all the way through that little raised section of the case, and down to the main galley. I cut it open, had it welded twice. Both times leaked and the welder said he just didn't have proper access without the engine out of frame and the cylinder block removed. Also the upper threads are totally gone.

So I tried high heat JB weld, and threaded a tapered brass male-to-male tube into the remaining lower threads with permeated thread sealant. About an inch long. Then on the end sticking out, put a female to female fitting on so I could thread the pressure switch on there. At the moment I just have a plug installed. Looks absurd but has held tight so far.

Time to find my spare starter.




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