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View Full Version : General Tech Crankshaft Main Bearing and Oversize Puzzle



KVH
07-22-2015, 03:49 AM
The spec for the crank journals on my TR4A is 2.479, and my bearings are .010 oversize. Doesn't that mean my crank should have been ground to 2.469?

If the bearing width goes up by .010, I'd think the crank must go down by the same.

However, my crank journals are measuring between 2.471 and 2.475, barely below spec.

Am I doing something wrong, or just not understanding?

TomMull
07-22-2015, 06:50 AM
I'm with you, crank should be 2.469. Was this just re-ground? If so, 2.471 to 2.475 would also seem a bit of a wide range. Perhaps your machine shop can shed some light? Might also be useful to put the crank in and use a plastigauge, or mic the shells in place.
Tom

Rut
07-22-2015, 06:52 AM
I would take any parts you have that are in question to the machine shop and get them checked. I've got all the tools necessary for me to measure just about everything on my engines, but my technique is different from the machine shops and engine builders in the area and I come up with different numbers. If they polish your crank they are responsible for the measurements! I do follow up with Plastigage on main and rod bearings to verify the work done since that is about fool proof.
Rut

tomshobby
07-22-2015, 08:43 AM
Did you check your bearings? When I did my TR6 engine these were in the set I first got. They were from a reputable vendor and mfg. But things can happen.
What kind of tool did you use to measure your crank? For example, a caliper, digital or dial, is not a very reliable tool and should not be trusted for anything closer than +/-.005". And like Rut said, your technique can vary from the shop's and even between uses.
I completely agree with the use of Plastigage for a final check. My engine builder used it and they gave a written 1 year 12,000 mile guarantee. That engine is far past that now by several times and still running great.

https://www.britishcarforum.com/bcf/attachment.php?attachmentid=38424&stc=1

CJD
07-22-2015, 10:45 AM
NAPA sells plastigage. It's a simple little strip of wax that you lay on the crank journal. You then install the rod cap, torque, and remove the cap. The amount the plastigage "squishes" tells you what your actual clearance is. You don't care so much what your journal measurement is, so long as your clearance is correct, and plastigage is how you tell...

KVH
07-22-2015, 11:31 AM
I used a good micrometer to measure but I guess I'll recheck. The angle can be tough while avoiding scratches to the crank so it's possible I got off a bit. I'll use plastigauge.