Crankshaft advice please!

[trunt]

I started to assemble my newly machined engine and was not happy with the crank journals, I see some pitting, also the thrust surface is rusty.. The shop I used has done fine work for me a few times before but I think they dropped the ball here. The Crank was cut to -20 thou and I talked to him about nitriding (which he does not do) and he assured me that a "micro polish" will harden them fine. As it was a Mk2 crank I thought I would go with that.

I have another crankshaft (he had both and TBH I think his staff ground the wrong one) which is from an early Mk1 -10 thou now and I feel in better shape so I can have that one ground and micropolished or get that ground and nitrided at another place, it probably need to go -020 at least on the mains I may get away with the big ends. For some reason I cannot find a shop that does nitriding locally so would need to send it away.

What does the team think?? can I risk an early MK1 crank without hardening it at -20?

[jbuckl]

The carbo-nitriding or tuftriding is essential even on a -0.010 grind unless the car is in a museum. (I.E. virtually unused)

The only exception is if the un-ground journal surfaces can be re-used and proved to be 55-58 HRC. or harder, because a -0.010 grind will remove the hardening.

The journals must be checked for size and alignments and corrected after hardening.

Then they can be cleaned and or polished.

Its a shame the pits are still present, they shouldn't be.

Using the crank as shown and described is a risk sorry.

[flying farmer]

From experience I reckon the life of an unhardened crank is in the region of 30-40,000 miles.

The reason most didn't get done is because it is a time consuming and costly business but it is definitely a false economy not to get it done.

Neil

[stagmuffin]

Deffinatley get hardened as stated that grind will remove the original surface hardening ,and i am aware of cranks lasting a lot less than 30k if not done. those pock marks will abrade the new bearings and help them wear out in no time so should of been cleaned of on a grind and polish .
steve

[wilf]

Apologies, not really adding to this thread but musing on the Stag crank in general.

What exactly is it about the Stag crank that demands journals surface hardened by nitriding to give decent life expectancy? The vast majority of production engines do not use anything other than plain cast or forged cranks, and they seem to live for far longer without the need for additional hardening? Is the core metal hardness that poor?

My experience of hardening steel injection moulding tools via nitriding leads me to believe it is a fairly horrible, dimension-altering and warp-inducing process. No wonder crank specialists shy from it.

[Motorsport Micky]

Wilf,

Having prepared many race engines in TRs which are ALWAYS treated for hardness as part of the final process you are quite correct.
Often I've had a crank returned ground,heat treated and micropolished complete with warp or bend, the worst was a 10 thou warp over half the crank length !
However this doesn't give a problem, it was returned to the engineering firm who prepared it and collected 48 hours later straightened with no run out, and I mean NO run out, I thought my DTI (clock) was broken the needle never varied. Oh...and the mains and big end journals hadn't been reground either, they were the same size as specified originally and received the first time I collected it.

How they do it ?...brutally, the standard straightening method is being hit with a very, very large hammer in the correct place, apparently you have to be a Druid to discern where that is but they know. Also I've never had a crank failure with a TR whether it's been treated like this or not, (you do undercut some of the journals to get rid of stress risers where careless grinding leaves too sharp a corner).

As to why the Stag takes regrinds so badly, my own opinion is not a sufficient quality of steel in the original choice.

Micky

[jagorstag]

Hi all,

On a related note, does anyone have any experience of Rimmer's exchange crankshafts? They state that their crankshafts are "Reground, Hardened & Polished, and include shell bearings," which of course sounds good but what might the reality be in regards to the quality and so on?

Cheers,
Joakim

[jbuckl]

Not had a crank from R.B.
Should be ok in theory.
Do ask what undersize you will be sent though.
IIRC some were getting -0.060 or even smaller sent.

Hi all,

On a related note, does anyone have any experience of Rimmer's exchange crankshafts? They state that their crankshafts are "Reground, Hardened & Polished, and include shell bearings," which of course sounds good but what might the reality be in regards to the quality and so on?

Cheers,
Joakim

[DJT]

FWIW I had a crank reground and hardened by Oselli back in 1992 when the engine had +/- 130,000 miles on it. The same engine was still performing well when I sold it with +/- 190,000 miles on it. They don't list the service on their website, but I'm sure they could help if required.

http://www.oselli.co.uk/crankshafts/

[trunt]

From experience I reckon the life of an unhardened crank is in the region of 30-40,000 miles.

The reason most didn't get done is because it is a time consuming and costly business but it is definitely a false economy not to get it done.

Neil

And Theres the dilemma... 10 years the way I'll be driving it..


I went back to the shop and he went postal on me for questioning his 30 years of experience, quite a shock actually he seemed pretty laid back before.

Basically he says that the small imperfections are not a problem, just look at the oil hole! and I have seen that observation before but by the time I brought the thrust surface up he wasn't going anywhere - ITS FINE.. but I'm definitely don't agree with that, it feels like sandpaper. I just don't know why he used that crappy rusty one when there was a much better one right there.

Anyway I found an outfit in Ohio that will do it, I'm 3k into this engine now, another few hundred wont hurt!

Terry

[flying farmer]

Apologies, not really adding to this thread but musing on the Stag crank in general.

What exactly is it about the Stag crank that demands journals surface hardened by nitriding to give decent life expectancy? The vast majority of production engines do not use anything other than plain cast or forged cranks, and they seem to live for far longer without the need for additional hardening? Is the core metal hardness that poor?

My experience of hardening steel injection moulding tools via nitriding leads me to believe it is a fairly horrible, dimension-altering and warp-inducing process. No wonder crank specialists shy from it.

I believe the main problem is that the crank is steel rather than cast iron which is much more common in engines of its era.

I have read somewhere that cast iron work hardens on its surface, therefore it does not need further hardening (unless it is for serious stuff like race engines). Steel on the other hand does not work harden so needs a separate hardening process, but should be less prone to cracking as it is not as hard as cast iron.
If you whack a piece of cast iron with a big hammer it will shatter whereas steel will bend.

Of course there are a huge number of different grades of steel and cast, and I have virtually no knowledge of metallurgy!

The fundamental problem with the Stag crank is the main bearings are too small. If they had made them all the same width rather than narrower on 2 and 4 it would have helped quite a lot. By the time you account for the oil groove in nos 2 and 4 about 30% of the bearing surface is missing.
The Rover V8 has plain ungrooved bottom halves to its bearings, but this becomes a problem at high rpm as the big ends get starved of oil.
Neil