Reusing main bearings?
 

I'm rebuilding my 1980 SC engine, and I'm looking at the main bearings. They literally look perfect.. Should i bother changing them? I'm thinking i should just get new ones. Help me!

You should be able to reuse them if they look fine.

That said, they're one of the main wearing parts of the engine and replacing them is pretty cheap as far as 911 parts go. So, I'd probably replace them anyways.

Same thing I'm doing. They look in pretty good condition, but it's worth replacing them so I don't have to tear it down again if something goes wrong.

I would reuse the bearings if they look good. You know for sure that set is the correct size.

If it's a engine in a car that you race/track, that is torn down occasionally to "freshen up" and the bearings look ok - then I re-use them.

If it's a street engine that you (obviously) have completely apart and plan on it lasting another 150,000(?) miles; why anyone would NOT replace the main and rod bearings, no matter how good they looked - I find that logic misplaced. YMMV.

*If you can LOOK at a bearing or piston skirt and determine its clearances are CORRECT - you are a better man than I.

It all has to do with oil pressure and the resistance to flow. If your oil pressure is good, I ve opened aluminum engines with 300K and the bearings still look great but realize they could be better.
I reload bottom ends on SC and Carrera all the time..All depends, Whats it look like..
Bruce

I have always been a "why would you not" kinda person when it comes to bearings. maybe, MAYBE, if it was a racer and had only a few hours on it. For a street car hell no.

If dimensionally within spec and good oil pressure, I would probably leave them alone.

If a bearing set is running good, that's a lot of reason to leave it alone. A lot of folks assume that a new part is always within spec and should be better, but anytime you introduce something new, especially being so critical, you have to do do all the checks to make sure it's good but even then, there can be manufacturing defects that you can not see or measure with common tools.

I'm not trying to be cheap here, the main reason im asking is because of what AlfonsoR said. You never know if theres a slight defect in the new part, and i can't recall the oil pressure i had before the teardown. I'm going to just replace them and hope for the best.

Depends how frugal you want to be. You could just plastigauge the existing setup after cleaning everything and if within clearance specs, reuse. That is if the bearings all look uniformly good.

I hear the bottom ends of 911 engines are robust and seldom have wear unless put under lots of stress (racing) or were not maintained correctly (dirty oil).

A counterweighted vs non counterweighted (T) crank may make a difference on wear, also.

I took apart my spare '68 engine (with counterweighted crank) and the crank looks fine and measures to std specs. The bearings looked good also.

Todays OEM new main and rod bearings are known to be junk; your existing factory parts, if in measurement spec, are far better than what's available new.

I have multiple sets of used, low mileage bearings.

If anyone wants a set, PM me.

- presumably you have a quality micrometer set in hand

measure the crank journals, clamp the brgs together and measure ID - or temporarily bolt the case together -

that is all described in any good generic engine book


./

if they look good you can reuse them on this engine, never in a V-8 would I reuse these

In the race engines we hand sand with 600 wet and have the bearings dry film coated.
If your used bearings are in good enough condition (judgement call) I would think the same process would be a reasonable option.

why? it sounds like you have it backwards. they're far easier to change on a v8. you don't even have to pull the engine out of the car!

V-8 sets are what, 40 bucks or so? cheap and good insurance

After the measurements I've recently taken for my rebuild project, I'd agree entirely with you on this. The original "STD" shells I removed from the mains did look (to my fairly newbie eyes) in remarkable condition for an 84k mile car that showed no pressure issues and certainly no failures.

I trial fitted the new Glyco's that I ordered and Plastigauged them...3 times, as I couldn't quite believe what I was seeing....every journal was instantly out of Porsche spec! With the mic I was using, it appeared to be down to material thickness, pure and simple.

Thankfully I kept the original ones, bagged and numbered. Cleaned them up, replaced and re-checked with Plastigauge (in case my method was causing the issue) and they were bang-on and in-spec.

Long story short, the Glyco's have now been swapped out for some more "original" Porsche ones (i.e. with the triangular identification symbol stamped on them) and the clearances are back to between 0.038 and 0.050.

If I were doing it again, I'd probably do what Henry says (and another very well regarded builder on PP said) and get the original shells dry-film coated after a little prep.....I just wanted to use the new ones after buying them!! :rolleyes:

(as an aside, my initial measurements and calculations for the Glyco rod shells, however, suggests they're in spec, but am about to double-check with more Plastigauge)

Spence

Great replies everyone. Where exactly would you get the bearings coated? Would a machine shop do that sort of thing?

Well I've been chatting this (and other stuff) through with Chris Carroll over at TurboKraft in AZ....I'd pick up the phone to him and either buy a new set (coated) or ask whether he can get yours coated from wherever he uses.