Crankshafts 101

[f3nr15]

According to service receipts, the last time my 2.7 was rebuilt in 1992 (~130K kms ago), the crank didn't require machining, but new -0.25mm bearings were installed at the time, so I assume that the crank had already been reground to first undersize some time before that.

With the current rebuild under way, I'm looking at sourcing another crank, rather than having the current one machined to second undersize;

I have been after a std/std crank, but a buddy of mine is trying to convice me to source another first undersize, with the reasoning that an undersize crank reduces reciprocating mass and thus provides a slight performance increase.

My understanding is that grinding a crankshaft is generally for repair, rather than performance? Any comments?

[Eagledriver]

Your buddy is correct but the difference will be unmeasurable. I think the chances of a correct regrind are smaller than the chances of finding a good std/std crank. Are you sure that the bearings are for a smaller crank and not an alignbored case?

-Andy Simpkinson

[f3nr15]

Oops, good point about the possibility of an alignbored case, I had better check that out...

[Dave Nuccetelli]

I may have a crankshaft from a 2.7 that may work email me if you are interested

[f3nr15]

Originally posted by Eagledriver
...I think the chances of a correct regrind are smaller than the chances of finding a good std/std crank...

Does a perfectly reground crank offer ANY advantages over a std/std one?

If not, why do people bother machining their cranks at all? Wouldn't it be easier to source a used std crank in better condition?

[911pcars]

Originally posted by f3nr15
Does a perfectly reground crank offer ANY advantages over a std/std one?

If not, why do people bother machining their cranks at all? Wouldn't it be easier to source a used std crank in better condition?

The designation, "std/std" by itself implies the journals are within spec. and suitable for installation. If so, physically, a reground crank should be equivalent save for .25mm (0.010") smaller diameter bearing journals.

In terms of surface finish, a properly reground and prepped crank should be better compared to a typically used, "std/std" crank. Some hold to the theory that an underground crank has less material and thus weaker than an uncut crank.

Of more importance is the very thin layer of hardened, heat-treated surface layer (Tenifer) which is pretty much gone by the time .25mm is removed and must be reproduced with an equivalent, but lower temp. process (Tuftride) that's rapidly disappearing in this country. Just checked, couldn't find any in the US. Closest is Canada.

And for some engines, bearings for ground crank journals are priced at a premium - I presume because of the relatively small quantities in demand or because of some marketing scheme to pry money out of our tight-fisted hands or from relatively pliable wallets, whichever the case may be.

Sherwood

[ChrisBennet]

When you get it apart, measure the crank. It costs $550 to have a 2.7 case resized at Ollies so you can use std bearings. Believe it or not that is quite a bit cheaper then undersized bearings for a case that has been align bored.
-Chris