I need help on a potentially serious problem regarding main bearing
clearances with my TR6 engine rebuild. Prior to bringing the block to a
rebuilder, I made a platigage measurement with my old bearings and got
.0025" clearance between the mains and crank. I just got it back from
the rebuilder, who polished the crank and said it miked within spec "on
the high side". He recommended a new set of standard size bearings,
something I think anyone would probably recommend. So I get a set of
standard bearings that are labeled "County", but made by King Bearings.
Put them in and I get .004" clearance. My rebuilder suggested I check
the block for out of round bearing holes, so I made measurements with a
three jaw inside mic, and get .0004" out of round for one hole, .0002"
for the others. I also made a measurement of one hole with the bearings
in place and I could account for .0035" clearance between crank and
mains. I also corroborated the rebuilders measurement for the crank, I
get right around 2.3115 for all four journals.
So now I found a rep in Pa. for King Bearings and he says the bearings
should be .0720" thick (at the vertex of course). Mine were a little
under .072" (ranging from .0715 to .0720). Of course my rebuilder threw
out the original bearings so I cant go back and check them against the
new ones. Also as a check, I took my inside mic readings and subtracted
the bearing thickness and get very close to the inside mic measurement I
made without bearings. My rebuilder found a set of Clevite bearings
(made by Vandervell, now part of Dana) and those miked between .072 and
.0725. Put them in and I get around .003" clearance between mains and
crank. He also said that Clevite's data showed a nominal bearing
thickness of .0720" and also a maximum main to crank clearance of
.0036". Nowhere in any of my books is there a spec on the clearance,
although Vizard suggests .0015" to .002".
So what gives? I'm in spec according to Clevite, but I've asked three
professionals who think .003" is on the loose side, and that .0015" to
.002" is more like it. If so, then where is my problem if the crank is
ok?
1) could the bearing holes in the block be distorted or enlarged? I
dont have any spec data on this.
2) has anyone had any experience with King or Clevite bearings recently,
or has anyone used .072" bearings with better results than mine?
3) is it possible that both bearing manufacturers are getting it wrong,
or are Clevite bearings also made by King meaning that only one
manufacturer got it wrong?
4) does anyone have any old bearings that had decent main to crank
clearances that they would be willing to send to me for comparison, or
have any measured bearing thickness data?
Sorry for this lengthy post. I dont want to line bore the block and put
in oversize bearings unless I absolutely have to, and given that my old
bearings produced good or at least better results it kinda points to a
bearing problem. If so, I doubt if the bearing manufacturer will be in
a position to do anything for me other than perhaps hand selecting some
bearings, which King said they would consider but I'm not sure if they
can go a full .001" over their spec. Can anyone out there help shed
some light on this?
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