Over-size crank bearings

Spotted in 911&PW mag that Westwood Cylinder Liners have sourced over-sized crank and big end bearings for the 996 / 997 engine.

I have always wondered why re-grinding Porsche cranks has never been an option? Might be a solution for those with worn cranks and the shortage of replacements!

I wonder what Hartech's view is on this?

Ian

I thought that the surface heat treatment is a major problem when doing the regrinds. They can be physically reground, but reproducing the surface hardness will be hard (no oun intended).
That said, it seems common on other engines, so why are these so different?

MC

In my mini days, cranks could be reground and tuftrided for extra hardness.

Ian

The technical issues with these cranks are part of the report I am very soon going to make available to interested parties by invitation and request. So our opinion about these will soon be available and I don't want to reproduce all the detail here. But briefly these original cranks are only nitride hardened to a very shallow depth which is ground through when moving to oversized bearings.


We have had access to 'oversized" bearings for many years and previously re-nitride hardened them successfully but it is not as straightforward as you might think.

The surface grows as you nitride it creating a white layer on top that is not suitable for running a bearing on and needs polishing back to size again afterwards. Polishing does not create a perfectly round journal so you have to grind slightly undersized to allow for it and polish just the surface afterwards.

You might think it should be possible to grind off the white layer but most crank grinding machines cannot pick up on a journal diameter accurately enough to just grind off a few tenths of a thousandth of an inch afterwards.

Special Billet crankshaft materials (like EN40B) can allow for nitride hardening deep enough to grind to size after hardening and even allow for a re-grind but many mass produced crankshafts are not (because they never expected the engine to be rebuilt anyway or on cost grounds).

We have carried out this process many years ago and it does work OK but costs a similar amount to finding a good used crankshaft instead (or maybe more) , especially if you are going to regrind all the journals.


If it is carried out there is then the question of who is responsible if the bearing fails again, the heat treatment business, the crank grinder or the engine builder. In fact this is the problem with a lot of the cheaper repair options some offer since they often last long enough to get the supplier off the hook but then fail prematurely and requiring a second rebuild with no supplier come back.

This is a problem because the cost of stripping out the engine and a full rebuild is much higher than the cost of any usually sub contracted repair – which usually limits their responsibility to the cost of the part of the job they did (that is if they accept responsibility anyway). This is also the advantage of using a specialist that carries out all their work themselves 'IN HOUSE" like us, as it makes our position clear from the outset.

Thinking about 'repair products" in general, I'm a little concerned that a layman may be swayed to run an engine until failure thinking 'it's OK, I can just get XYZ repaired easily" – the problem with that approach with crank bearings is that cranks are very expensive and it isn't only the crank which is affected. The carrier and connecting rods are usually damaged and then the debris usually wrecks the oil pumps, pistons & potentially cylinders etc and in a worst case the rod will break resulting in the internals being smashed beyond repair.

We still think a preventative re-build is the best route to go down when the mileage is creeping into 'classic crank bearing failure territory".

Some owners would then like to consider using the opportunity to increase the capacity so they feel like they're getting a little more out of it.

We've currently got 2 cars in for re-build for other reasons and the bearings are down to the copper and very close to failure. One of them had a very recent oil analysis which didn't pick it up but the rate of deterioration at this point is usually very fast.

Baz


(picture not the above engine in question but typical or what we find internally.