A Bit Of A Muse
Turbo Mini Forums

During the engine rebuild, I found one of the big end bolts a significantly different colour. Musing on why, I came to the conclusion, that it may have been suffereing cyclic stress? None of the bolts were loose, and all have been torqued with a high quality calibrated torque wrench. Anyway, not happy using it, I ordered a new set of bolts. ARP 200-6210, same part number, and identical in shape, look, colour and size. However, the instruction sheet that came with the newl bolts, states to torque the bolts to 30lbft rather than the 26lbft the instruction sheet stated that came with the original bolts. The stretch values are also different, but interestingly the old set with lower torque value has a larger stretch, than the new bolts with a higher torque value.

Anyway, new bolts will be torqued to 30lbft as per instruchtion sheet, I am just musing

Note to self; Need to buy a proper stretch gauge

Just out of interest, what do you mean by a "stretch guage" ???

The only ways that I'm aware of of measuring "stretch" (ie, elongation (but actually strain)) is

(a) with guage rods, which can only be installed in large high integrity bolting, or
(b) ultrasonics, which can be used on smaller stuff but is not cheap.

I'm just intrigued as to what you have in mind.....

Schrödinger's cat - so which one am I ???

Rod, here you go

http://www.amazon.com/Proform-66788-Bolt-S...e/dp/B000CCG1E0

Obvious I suppose :)

I've only ever worked on big stuff that you can only get at one end of (because it's allways studs and nuts)

Hence guage rods or ultrasonics.

Also, rarely torque but hydraulic extension (then wind the nut down) to achieve the required stress levels, hence strain, then check it with guage rods or ultrasonics.

I must try and forget the big stuff

Schrödinger's cat - so which one am I ???

Spot the difference

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Edited by Sprocket on 23rd Jul, 2010.

oh yeah, third one in, strange.

You may also have spotted that there are only seven, lol, off to find where the eighth has gone

Strange but is it something "baked" on the surface ???

Is it worth trying a bit of wire wool to see if it is just a surface deposit ???

It would be hard to believe one single bolt could have reached a high enough temperature to change its heat treatment (hence colour).

Is there any discolouration on the associated conrod ???

Stress, cyclic or otherwise, (as per your original thoughts) won't change the colour unless there is something else there, ie, heat or chemical contamination.

But if the loading is correct (ie, "stretch") the bolts will be below yield anyway so there should be no stress related effects unless a service load took it beyond yield - which is very unlikely for just one.....

If they have dimples in the end as you say (I've never used ARP bolts) just borrow a micrometer with pointed ends and see if that one bolt behaves differentely for the same torque tested on a conrod out of the engine.

It might have a dodgy thread form causing torque vs achieved load to be wrong - torque is a really crap way of loading bolts accurately hence why (in my industry) we use studs/nuts and tension them hydraulically.

Schrödinger's cat - so which one am I ???

Its deffo a colour change of the material rather than anything on the surface. Problem is I am unsure if it was like this when I put them in the engine originally. For what it was worth, I just bought a new set of bolts that way eliminating any doubt.

I would think that cyclic stress would generate heat as you are working the bolt? thats what I thought anyway. I am no expert

The little end had the bit of the piston that went missing stuck to it, but easily scraped off. If it was due to the blowby of the failing piston, I don't understand why the head of the bolt has not changed colour either. nothing else shows such heat distortion.

Odd, and chances are I will never know why.

Edited by Sprocket on 23rd Jul, 2010.

Where these bolts fitted when you had your O ring incident?

was they on the rod with the holed piston?

Yes i moved to the darkside

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