Connecting Rod Bearing Nip or Crush

[Crofter]

Am in the process of getting a new connecting rod ready to put in the 10-1. I have a set of plain upper bearing from John F to get rid of the upper shell holes and grooving. Will be installing a hollow dipper on bottom cap and have decided to upsize it from 5/16 to 3/8 - 24. Just have to trim it to length. I have seen mention that too much splash may not be good for an engine breaking in. This one will have a bigger profile but I suppose it could be narrowed down and still leave lots of meat.

I find the method of torqueing up the rod bolts with the bearing shells installed, then slacking one nut off and see if the part line gapes 4 to 6 thou. is not  a very good indication of whether or not you actually have grip on the shells. Here is why. I got a reading of .004 or a bit less but if I torque down the empty rod without the shells and slack one side off, I get nearly the same ready. The way the metal is distributed around the rod bolt causes it to tilt up even without contact with the shells. I am thinking of lapping a bit off the cap and rod faces. I will have to shim a bit for bearing clearance anyways and plan to let the shims come between the ends of the inserts. The Indian shims stop short of the bearing shells and leave the shell loose to be located solely by the tip of the dipper.

I have heard of a few problems of inserts shifting and wearing that hole egg shaped. I wonder of the finding of the 4 to 6 thou. gap gives a false sense of security.

[mobile_bob]

i would not want the shims extending into the shell, and here is why

the upper shell has ramps that are used to help establish the hydraulic wedge, if you have a shim extending into this area
you "may" disrupt this ability.

personally i think 2 or 3 thou crush is adequate to keep the brg held firmly, and the oiler can maintain position

i also am very sure you are going to be very pleased with the longevity of this conversion

bob g

[Crofter]

i would not want the shims extending into the shell, and here is why

the upper shell has ramps that are used to help establish the hydraulic wedge, if you have a shim extending into this area
you "may" disrupt this ability.

personally i think 2 or 3 thou crush is adequate to keep the brg held firmly, and the oiler can maintain position

i also am very sure you are going to be very pleased with the longevity of this conversion

bob g

Bob, you are spot on in regard to the oil ramps. The shims will have to be relieved enough to keep clear of these. I think actual crush does not need to be near two or three thou. I would be happy with a tad under a thou. The opening at the untorqued side is probably what is calculated (extrapolated) to result from about this amount of discrepancy beasring OD with rod bore ID. I just thought it "velly intellesting" that the so called correct amount of gaping could be produced without even having inserts in there. You could have zero or a negative crush and be lulled into thinking you had a positive grip. Yep, the tip of the dipper will locate the shells but it is not what is supposed to be keeping them from spinning.

I will have to shim a slight amount and it appears likely that the clearance near the part line may have to be scraped. I think most of the Indian renditions had this done. New shells ID appear to be just about crank diameter so shimming will establish top/bottom but leave the sides tight.. I think some were bolted up tight and hand cranked expecting to flow the babbit to where it was needed, Lol! Some pictures I have seen of their scraping looks like it was done with a farrier's rasp. If you cannot place any needed shims under the bearing ends then it appears to me that the nip will be lost if it was only marginal when bolted tight without shims.

Their idea of precision inserts is a fair bit removed from what a lot of modern mechanics have been spoiled by. The original old engines have some nostalgia but you can't say their modern clones are a snap to assemble into a 10,000 hour engine.

[mobile_bob]

sorry Frank, when i mentioned 2 or 3 thousands, i was referring to "nip"
which probably would make for maybe a thou tight?

seems like some of the guys in years past filed down the nip and then had issues
with fretting, but i don't know if i ever heard of any issues as long as there is some nip
the question is how much is too much?

if it were me i would be tempted to remove the rod, and have the big end reconditioned at an automotive machine shop
they would fit the cap properly, torque it properly and then hone the thing perfectly round, after which i would fit the brg shells
to provide for about .003 nip.

the shims used back in the day, were likely used with brg shells with a fairly thick babbit, i am not at all convinced the new production
shells from india have anything but a very thin babbit much like what is used in current production engine's that use no shims.

probably not telling you anything you don't already know, huh?



bob g

[Apogee]

I second what Bob said.

I was just going to write that I don't understand why folks just don't have the rods re-sized at a machine shop to the correct dimension and be done with it.

Cheap $$$ in the long run imho.

Steve