A typical problem with MGB (5-main) engines is their crankshaft
centerline is not parallel with the deck: this leads to different piston
heights and compression ratios from front to rear. The last MGB 1798 I
rebuilt was .138" (over 1/8") off of true from front to rear...horrible
tolerances!
It's a good idea to first check the distance from the main bearing
pockets front and rear to the deck, and then decide on an amount to remove:
it may be at an angle, and not just a simple milling operation. It might be
the crank is running "uphill" or "downhill" in the block. A good machinest
can check for you. Either way, I would deck the block as little as possible
to square it, but verify the align bore.
A crankshaft align bore can be tricky: if too much material is removed,
the timing chain may run loose, and the lag will change the valve timing.
Sometimes, a compromise can be reached by milling half of the material off of
the deck, and then correcting the align bore half-way. This helped keep MGB
dependible and competetive in SCCA Production racing for years: there were no
special hypo blocks available, just hand picked parts and good machine work.
Jim Leach PTC Seattle
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