The Stovebolt Site Forums The Shop Area Engine Shop Rod bearing and main bearing clearences

I need to know what the min. and max. clearences are for rod bearings and main bearings on my 1954 Chevrolet 235 c.i. engine.

I've done a search on this site and found contradicting info.

What are the correct tolerences for main and rod bearings?

I intend to use Plastigauge. The engine is still in the panel truck.

Thanks, Jerry

Jerry,

rod bearing - Plastigage method - 1954 Shop Manual

main bearings - 1948-51/52 Shop Manual - Plastigage not used in this technique (and, that earlier technique is not recommended in 1954 Shop Manual, which is also not a Plastigage technique).

Maybe someone will comment on a technique for 1954 main bearings?

OK, according to the Federal-Mogul master engine bearing specification manual:

Main bearings 1-3: oil clearance .0005"-.0033"
Main bearing #4: oil clearance .0013-.0043
Rod bearings: .0007"-.0032"

To get an accurate idea of main bearing clearance with the engine in the vehicle, use a small hydraulic jack placed on an appropriate amount of support like blocks of wood to put a slight amount of upward pressure on the crankshaft near the main bearing you're checking. This pushes the crank toward the upper bearing shell. (If the engine is upside down on an assembly stand, don't worry about counterbalancing the weight of the crankshaft.) Wipe the crankshaft surface and the bearing in the cap clean of oil and torque the cap to specification, then remove the cap and measure the width of the flattened Plastigauge. A small squirt of penetrating oil helps to remove the flattened PG, but if a small residue remains, it's designed to dissolve as soon as it's exposed to motor oil. Continue with the support procedure and clearance checking until all the bearings are checked. On the rods, just wipe the crank surface and the bearings clean and check the clearance- - - -the weight of the piston/rod assembly is negligible. I like to see clearances somewhere in the .0015" range, + or - a little. More clearance = less oil pressure. The green PG should be in the right clearance range for unworn bearings. Red is another option if you're measuring bearings with a little wear on them.
Jerry

Hotrod Lincoln Jerry:

This is exactly the information that I need.

The manual says to use the "spin freely" method. Try that with the
motor still in the vehicle and me upside down. My idea of "spin freely"
may be entrely different than someone else's idea of "spin freely."

Thanks for the precise info. Jerry.

Jerry Bubolz - "Skinnedknuckles" (I sure got 'em.)

The old school method (pre-Plastigauge) was to insert a small piece of shim stock between the bearing and the crankshaft, torque the cap, and see if the crankshaft could be turned. For instance, if I wanted a .002" rod bearing clearance, I'd insert a piece of .002" shim stock about 1/8" wide under the bearing, and add or remove shims until I could turn the crankshaft with a firm drag, not quite locked up. Precision-insert bearings that don't use shims can be checked the same way, but you'll need a good selection of shim stock thicknesses!
Jerry