Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board B44 Cylinder head

Is it just me or have anyone else had this problem?

I am trying to fit a different cylinder head (larger inlet vale, ported) to my B44. I cannot get the damn head to seal. Head is flat,( had it checked by machine shop,) valves good (head filled with Kero overnight, no leakage). The head looks like it has been skimmed, perhaps once too often. New, well annealed thick copper gasket.
Compression is leaking out of the oil holes by the valves.
I am assuming that the recess in the head is now too shallow so the upstand on the liner is bottoming out before the gasket is sufficiently "crushed".

I can use a micrometer as well as the next man but the question is how much "crushing" is needed ? i.e. height of upstand on barrel minus gasket thickness compared to depth of recess in head.

The infuriating thing is I had the same damn problem 18 months ago with the original head, and I don't remember how I solved it! but solve it I did, you could stand or even jump up and down on the kickstart against compression.

Are you sure that you're losing compression? The issue you mention, that the fire ring is contacting the head before the gasket is making good contact, is a common problem when someone has skimmed the head. The other thing that happens when the fire ring is contacting the head is that you can get oil leaks because the oil that drains down thru the bottom of the valve pockets and then down thru the holes in the cylinder will seep out between the head and the cylinder and make a mess, even if compression is not being lost.

Other than getting a thicker head gasket made, the head can be re-machined to restore the depth of the recess. As long as it only requires a few thousandths of an inch, no other changes should be required.

Tom

The question is how much is "a few thousandths of an inch" presently the gap left for the gasket to fill is about 4 or 5 thou less than the thickness of the gasket. I guess that this is not enough, but what is enough?

If the head gasket is solid copper, I doubt if it compresses more than 1 thousandth of an inch, (I don't know the exact amount), so anything more than the space you have, maybe 2 thousandths of an inch more than the projection of the fire ring above the gasket should be sufficient. In other words, make sure the gasket contacts the head and the cylinder surface before the fire ring contacts the top of the recess in the head.
Tom

This is your own solution from last year

http://www.britbike.com/forums/ubbthreads.php/topics/761190/b44-cylinder-and-head#Post761190

Oh for heaven sake! I remember now! it is remarkably difficult to fix a problem that doesn't exist. I was testing without the rockers on again so first kick "hey that's better!" then all subsequent kicks were crap. Air puffing out of oil drains by the valves at BDC! crank case pressure! not TDC, cylinder pressure!

Thank you Kommando, I must learn how to use the bloody search feature. I tried looking with every combination of key words I could think of and got lots about every make of bike known (and unknown) to man, but could I find my own posts from only last year ……………?

If you click on your name you can then select to see all your own posts.