87s4. For those engine experts have some questions. I have asked Greg this question as well.....

What is the difference between the early 928 main bearings and those from s4? Are the cranks exactly the same?

I am curious as I have read that some people use earlier bearings "more surface area"

Also I looked at Porsche pet and it lists main bearings different for early cars and later cars. Why? Are the bore sizes different and if so how much? If bore is different can one bore to use earlier bearings?

Also I notice that both bearings for early and late can get undersized but only early bearings have oversized....

 

The main issue I guess between early and later cars are

Are the crankshaft specs exactly the same,

Are the main bore sizes exactly the same

 

The thrust bearing surface did change....

 

The tech spec books have details.
(Am assuming you wouldn't be into a project like that without the morehouse CD's).

 

Use the correct main bearings and install a Super Clamp for peace of mind. Did you have a thrust bearing failure?

 

The thrust difference that Jim mentioned is the later being larger on the thrust face which requires the block to be machined for the additional 'flange'. I would guess that's the larger area you've heard about.

The front bearing also changed to a one piece which makes sense as there's no need for it to be split and block alignment is helped.

Why do you ask? Are you building an engine?

 

Not sure this helps: tech spec booklet snippets

80 thru 84 (first image)

S4 (second image) footnote refers to two diff color markings)

 

The specs look to me to be the same unless I am missing something.

Looks like they give you one oversize on the mains....and several for going under on the crank.

That begs the question why bearings are different parts numbers for early vs later s4 cars....
The bore diameters are identical and crankshaft d1 diameters are the same

 

Do you understand what thrust bearing surface means ??? it is the side of the bearing and is what locates the crankshaft front to back in the block. Keeps the clutch pressure plate from pulling the crank out of the block when you hold down the pedal......
Also note that even though various under oversizes are listed on the spec sheet few exist !!!

 

The key issue is are the crankshaft spec identical between early years and s4 in 928 engines. Based on information I have seen and I could be wrong and stand corrected if I am misinformed but the cranks are identical.

Another statement that I think is true is that the bore diameters are indentical for the mains between the two.

What is different is the single bearing on the front number 1 bearing versus 2 split halves and the thrust bearings between the 2 have different faces. So the short blocks are different in that respect...i am guessing but it is machine job to accept a bigger diameter face.I am not sure if short blocks between the years also have differences in dowels pins etc and that may be the case.

Another corollary question is are girdles swappable between each s4 years.

 

The next step in logic is that the number 2 main bearings and number 4 and number 5 end cap bearings are identical between early years and s4.

Again I welcome any disagreements to this logic from those who have built these engines.

 

The girdles are NOT changeable from one block to another reguardless of the years...they are final machined bolted to the block. You can not overbore them to make them match up as oversize O D main bearings are not made.

 

Oversize mains seem available for early year cars and again I could be mistaken as the specs show .25 over ...

Even if they are not available...one can always take a standard size and nickel plate the back .25 over ....it's done all the time.

 

Mike and Jim are correct, the front bearing became one piece I think that was around 83 MY and I think the thrust bearing face doubled is width around the same time, I could be wrong but certainly the 84 MY had the single piece front bearing and the wider thrust bearing.

I am not sure if they went to 360 degree grooved main bearings in the S4? Certainly that is not what you want due to increased friction. Modern cars like the SLS Mercedes use 140 degree bearing which is what the current NASCAR race engines use. 180 degree is what they were in the 928 and is not a bad option.

 

On my 87s4 my front bearing main num 1 is a single piece....my thrust and all other bearings are 2 piece...2 half moons.....