Before I rip out and my recently polished and installed MGA 1600 crank, as
last resort a stab to the collective wisdom on the list -
Dry fitting the con rods I have -
10 thou under crank pins .0004 below spec minimum and .0009 below max
New Federal Mogul 10 thou under big end bearing shells (same .0765 thickness
as the old ones at mid point to the parting line)
Big end clearance spec is .0010 to .0025
I am getting .003 calculated and measured
My .003 clearance on the big end is generally in the trade regarded as
excessive, and even the .0025 max spec seems a little on the high side, even
for race engines. Interesting that for the same crank in the 1500 the
clearance spec is only .0001 to .0016. (I suspect the minuscule .0001 might
be a typo in the MGA BMC official workshop manual).
Barney Gaylord measured his standard brg shells at .072, so maybe my old
shells were worn a 1/2 a thou and the new ones are a 1/2 a thou under spec.
Methinks not likely, but that would explain a one thou contribution to my gap.
Has anybody run into slightly undersize or oversize shells?
Triumphs have same diameter rod big ends and 1 thou smaller pins. Any chance
someone has chased clearances with these?
I know I am reaching for straws and will soon be pulling the crank for a 20
thou regrind, meanwhile here's some learning I can pass on -
Grind your crank last. Measure or have the shop measure the OD of pins (and
journals). Measure ID of caps with new shells torqued to spec. Decide on the
actual clearance u want and then grind. The 'assumption' route has not worked
out this time, since I was just lightly polishing with zero metal removal
(worked on the mains) and I also assumed that the loose original big end fit
was due to wear. With my crank pins a measly 1/2 thou under min spec, I guess
the tolerance game just stacked up unlucky.
Big end bearing caps are thicker at the mid point to the part line to account
for dynamic stretch forces at TDC.
For some American iron you can procure slightly undersize brgs, ie not just
.010 but also .011 and .012
Although some piss all over plastigage the brg company websites recommend it
(maybe they sell it). I suggest measure and calculate, dry fit with plastigage
and when possible verify with a dial indicator. In my case above, all three
methods coincided so that I can comfortably assume I am screwed.
Mike
MGA 1600 obviously in progress
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