>A friend of mine who is into national level competition in motorcycles
>claims that one important ingredient in an engine like mine is to
>minimize the piston crown to head clearance in the squish portion. He
>says that motorcycle people assemble engines so close that they 'click',
>and then shim apart. On the other hand, there are some pistons available
>for Triumphs that achieve higher compression ratios with a pop-up
>section in the open area of the combustion chamber.
>
>What's the current state of knowledge on this subject?
Jack,
.040" is considered the MINIMUM piston to head clearance for most engines.
I don't have any personal experience with the TR-4, but don't know of any
reason why that wouldn't hold. The physics and geometry are the same. By
the way, .040" is only for an engine that won't be over-reved. .100" is
considered the safe, conservative number. These numbers are for piston to
head with the head gasket and installed and compresed. So if you have a
.040" head gasket (compressed), the piston could actually reach the top of
the block.
In deciding what you want to do, keep in mind that there is clearance
between the piston and the cylinder wall, and that will allow the piston to
rock as it goes throught TDC. Cranks do flex, and there is some stretching
of the rods. In engineering terms, it is strain, expressed in inches of
stretch per inch of length.
If you go with pop up pistons, don't forget to look at the issue of piston
to valve clearnace. Be advised that the minimum clearance IS NOT when the
piston is at TDC!
There are many excellent books on engine building. Typically they are for
Chevys, but the principles are the same. I particularly like those by David
Vizzard. There are several. You might want to get them from Classic
Motorbooks.
Good luck!
Earl Zwickey
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