Couple of questions: JE Pistons/Glyco bearings

[matt930s]

Hi Pelicans-

I have my 78 SC 3.0 torn down and cleaned. Reason for the tear down was broken head studs. My main bearings, pistons and cylinders, crank, rods all looked good and spec out. (I already have SSI's and an aluminum SACHS pressure plate).

That being said, if I wanted to go with a 9.5:1 JE pistons, would I have to do anything to my Mahle Niki cylinders to drop the JE's in? They are in nice shape with the cross hatching still visible. Or would I need them machined?

Next question, can someone tell me the thickness of new Glyco bearings? I have read a few threads where folks have received questionable quality main bearings. If that's the case, I may re-use mine.

Thanks

MattR

[brighton911]

Matt, in my opinion, if your mains are not showing any signs of wear or damage, no reason not to re-use them. And if the JE piston to cylinder clearance is within spec, then a hone of the cylinders will do the job. So, you will need to know the sizing of the JE pistons (or buy the set) to determine if they will work out. Keep in mind the Nikasil requires a special hone. There are a few threads on the matter within this forum.

[Alan L]

The issue will be matching the new JEs to the old cylinders? They normally come as matched P/C pairs.
Regards
Alan

[Trackrash]

If your cylinders are in spec you can de-glaze them using the scotch bright method. Or you can send them out to be honed by someone who is an expert in Nicasil cylinders.

If your cylinders are in spec you can use JE pistons in your de-glazed cylinders without any special work. The "off the shelf" JEs are a correct fit for Mahle cylinders. Of course you would want to verify the fit.

Or you could send your cylinders to someone who will match them with JEs for you.

Just depends if you trust your own measurements and judgement or someone else s.

Not sure what the deal is with Glyco bearings. The ones I got last year were fine. (so far)

[targa72e]

Cylinders are grouped by size (like rod weights) and marked on side of cylinders. You should really measure the bore and make sure it is in spec for ovality. The cylinders can look perfect and be out of round. The standard JE piston would have the correct clearance with the lower diameter cylinder group. Larger groups would have too much piston to cylinder clearance. You can have pistons made to the size to have correct clearance with larger group cylinders if needed. Generally cleaning up the bore by hand with red scotchbrite is all that is needed to get cylinders ready to reuse.

thanks,

john

[mikedsilva]

Some have had issues with the "generic" Glyco bearings as opposed to the ones that are sold from Porsche.
The ones that Porsche sell, are still Glyco, but they are all individually measured to make sure they are within spec, before they go in Porsche packaging.
I went through this personally last year; bought Glyco generic, and measured but there was more clearance than with my stock bearings. Then I splurged and bought a full set of the Porsche/Glyco and you can see the 3 sets of "measurement marks" on each bearing shell.. they were a lot "tighter" than the generic ones.
In hindsight, I should have saved the money and just used my orignal bearings again. If yours are within spec, and have no wear, then re-use them and save the money and time.

As long as you marked which bearing came from with journal and which case half...

[Trackrash]

I'm not sure at what point you would want to replace bearings due to age, even if they are still in spec. Does it matter?

Many years ago I reused a set of rod bearings that measured out perfect. A year later I had a rod knock and then a spun bearing. Upon disassembly all the rod bearings had suffered from the babbet material flaking off the shells.

Not a Porsche, and none of the experts I knew at the time had any idea what went wrong.

[Alan L]

It is a bit of a tough one. The golden rule I work with is 'if it ain't broke, don't fix it". When you have something that is running well, it is with reluctance that I tear in to it and do stuff other than what is needed. That said, if you have gone as far as splitting a case, you would have to have a good think about not doing the whole job.
When I rebuilt my SC engine from crank up I did new valves/guides, timing chains, crank bearings , rod bearings etc. I had broken head studs. But for some reason chose to leave the rod end bushes - they still measured. A year or so later I tore it down again, to do the rod ends.
But if the bearings are still well in spec why not polish the crank and fit new ones - as long as you can replace with same as was in there? Since you are in there. Obviously a shop would want to err on side of caution. As DIYers we get to choose, and live with the results.
Now I must go and find what blew in my track engine y/day :-( Hoping just the turbo.
Regards
Alan

[Speedy Squirrel]

Bearings are subject to fatigue failure. The steel shell is coated with the load bearing intermediate layer. The intermediate layer is coated with the outer layer (something softer with low friction).

This all lasts a long, long time, but 200k miles might be a good point to consider worrying about cyclic fatigue failure.

Basically, small cracks in the intermediate layer develop and grow. When they intersect small pieces start to flake off.

Originally Posted by Trackrash

I'm not sure at what point you would want to replace bearings due to age, even if they are still in spec. Does it matter?

Many years ago I reused a set of rod bearings that measured out perfect. A year later I had a rod knock and then a spun bearing. Upon disassembly all the rod bearings had suffered from the babbet material flaking off the shells.

Not a Porsche, and none of the experts I knew at the time had any idea what went wrong.

[Trackrash]

Originally Posted by Speedy Squirrel

Bearings are subject to fatigue failure. The steel shell is coated with the load bearing intermediate layer. The intermediate layer is coated with the outer layer (something softer with low friction).

This all lasts a long, long time, but 200k miles might be a good point to consider worrying about cyclic fatigue failure.

Basically, small cracks in the intermediate layer develop and grow. When they intersect small pieces start to flake off.

That was the best guess at that time. It looked like the bearings were pounded to the point of fatigue.

I am not opposed to reusing bearings, but I would want to know their history before doing so. In the big scheme of things bearings are not that expensive.