Kevin in Atlanta

I am in the midst of reassembling a 1990 engine taken apart by the PO to hone one cylinder (never completed) and think that while I am reassembling it i should replace the thrust bearing and rod end bearing caps (already have the new rod nuts on the shelf).

Could somebody explain how to determine if I should use bearings other than standard? The engine has around 100k on it. I have plastigauge on hand.

Thanks.

jcorenman

How do the bearings look? At 100K I would expect the rod bearings to look like new, unless some damage has occurred. Can you post pics?

Same for the thrust bearing-- any visible wear? Is this an automatic?

Kevin in Atlanta

It is an automatic and absolutely no signs of wear. I will post pictures.

Imo000

If there is no sign of wear then don't replace them.

GlenL

Changing bearing sizes is only done following machining the crankshaft. The crank doesn't wear that much or at all, really. The bearings take all the wear. The crank gets machined after damage or very long use.

Measure the crank to be sure you're getting the right bearings.

And these engines are never "honed" like iron blocks. There's a machining process for boring and overboring but using the classic three-stone hone on a drill is never done. If you take out scratches (that probably aren't problems, anyways) you'll need new piston(s) to mach the tolerance. Best to just re-ring it and put it all back together.

mark kibort

I plasticined all my bearing fits and made sure that the tollerances were good. on a motor that old, it sure is worth replacing rod bearings in my opinion. they may look good but might have wear patterns that will make the wear out faster than if you have a new set to put in. Ill leave that advice for the experts here, but thats what i have always done. i always see what the clearances are though too with the plasticgauge.

as glen says, there is no "honing" to be done. the bores are forever, unless they are really damaged and scractched up. the process should be done by a shop that has done these alusil motors before. I only did that when the bores were really bad, or i bored the block out. (in nor cal there is a great shop here that does it in Mt view, ca )

jcorenman

100K miles? That's almost new, for a street motor anyway.

Here you go, find #2 and #6 and then guess the miles:

mark kibort

I dont have a trained eye, but that looks like the bearings are worn. usually, the first surface is more of a matt finish, and when they get shiny, they are getting into the other bearing materials, no???

Rob Edwards

0.0546 to 0.0548?

Edit, whoops, I meant 0.0586 to .0588. Pictures to serve my penance.

'91 GT, 160K miles, #6 rod bearing halves vs. a new Porsche 'yellow' shell. Identical between all three, 0.0586"

Upper:



Lower:




New:

fiatrn

If you have the new ones already, and took the old ones out, there's no good reason to not put the new ones in.

mark kibort

even though they are mic'ed out as good as spec, the coating needs to be intact, right? If the coating is compromised, (i.e. shiny or you see some copper) can it stilll be in spec?

Rob Edwards

If you see copper, it's not going to measure the same.


The only thing the Cobalt car got in the bottom end was new rod nuts on 2 and 6. and a new OPG.

Pic of the surface of 2 and 6, 163K miles.

ptuomov

I have nothing intelligent to add to this, other than that it's one sad engine that doesn't have cheap enough and reliable enough high performance rod bearings available that it would always make sense to change the bearings when the engine is open at 100k miles...

Background reading on bearings:
http://www.mahle-aftermarket.com/med...s-brochure.pdf

mark kibort

the ones i pulled from the holbert car looked a little better than those and so did my 84 part euro 5 liter..... (remember it had 4 days at willlow sprigns, 1:33 to 1:34 lap times for most all the race laps)
I dont like that wear spot (shinny) on the upper two bearing halves.. I dont know for sure, but that is a wear sign and was told that look requires a bearing change. the fact that the motor has that many miles, wouldnt you think that those parts, have served you well and need to retired?

i know copper showing would be a measured problem, but when the dull mat finish is gone , that might not measure out to be a problem in wear tolerance, but might be a progressive wear spot to worry about.

platigauge the bearing on the crank (torque the nuts) and show me the tollerance there... thats more real life, right?

GregBBRD

Seriously true!

I'm not sure everyone here understands this as much as people that have spent their entire life around engines do.

Re-using rod bearings is about the same as re-using an oil filter, out there in the real world.