[DeTomaso] Torque plates-long

[JDeRyke at aol.com]

When one torques a head onto a block, the forces involved will warp the block 
slightly. Enough so the bore diameters distort, affecting the ring seal as 
the pistons slide up and down it. Smokey Yunick got the idea of honing a bored 
cylinder with a head torqued on so the bores would then be straight. The 
warpage depends on the engine- Smokey was working on 265 and 283 Chevys and found 
that torquing one head affected all 8 bores; torquing both heads made it worse, 
and the same with torqued main bearing caps. In fact on those blocks, 
tightening the oil pump hold-down stud measurably distorted the bores. Smokey's first 
'torque plate' was a stock iron cylinder head that he'd punched bore-sized 
holes clear through the combustion chambers so an engine hone could be used while 
the block was warped into its running condition. Later, it was found that to 
really do the job of making a block's bores round under running conditions, 
you needed to hot-hone it (at around 160-180 degrees) with torque plates and 
main caps on- and adding studs later changed things again. Hot-honing is 
unpopular with engine builders since its uncomfortably hot and smokey, but it does 
work. In 1996, a Nor-Cal member took 160-some bore measurements both before & 
after attaching a torque plate on his 351-C (POCA Newsletter Aug 96). Most of the 
bores distorted the same way but two distorted the opposite direction. After 
honing with the plates, he removed them and took yet another set of 
measurements- and the finished bores were all over the place, presumably to straighten 
out after the heads were added. The max bore distortion was about half the 
specified piston clearance, so the plates do work if you need that last 1% 
possible power or reliability. FWIW- J DeRyke


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