cyl head bolts

[RDfella]

Wondering whether anyone makes yield bolts for the cyl head of an A10.
Reason I ask is because when I stripped down the GF to fit new rods, some of the cyl head bolts were barely more than finger tight - and that was less than 500 miles after a total rebuild. Which means after a head re-fit the bolts should be checked after 50 miles and again at 200. A real pain given the aggravation involved. It's not as if it's a side valve! Hopefully yield bolts would obviate the need for that work.

Clarification: I was referring to TTY - Torque To Yield
On many modern cars, one tightens a bolt to a certain torque, then applies further rotation, say 90 degrees. Such fastenings retain a certain elasticity that ordinary bolts don't.

[scotty]

What’s a yield bolt?
Enquiring minds and all

S

[berger]

it's a bolt that thinks it's elastic  that stretches when tightened to its specifications  as i understand it, like what they use on many cars, not to be used again for the same purpose

[BagONails]

Possibly RD but they would need to be specially made for the conditions. It sounds like relaxation in the joint or threads rather than than an issue with the bolts. Conventional bolts should be within their yield stress and therefore applying clamping force to the head like springs when set. As we all know it is important that threads are clean and very lightly oiled, joint faces are clean and flat, gaskets properly annealed etc.

This is all about Youngs modulus of elasticity if anyone wants to look it up! Bolts designed to be 'torqued to yield' are very accurately necked down under the head so that beyond a certain torque value/clamping load the bolt shank, in the necked down area, actually starts to stretch in a controlled way, in other engineery words it 'elongates'. The necked area is stressed beyond its elastic limit (ie. beyond this point it is permanently deforming) and to an extent you can continue applying rotation to the bolt head but the clamping force and the torque does not increase (because the bolt is getting longer).  This is why you often see progressive torque increments plus an angle figure on modern car cylinder head tightening specs.  In this way manufacturers can even out the clamping force applied by each bolt and don't need to control torque so accurately. Something that became much more important as we transitioned away from good old cast iron blocks and heads to dodgy aluminium! It also avoids the issues of all the other variables that affect torque verses clamping force ie. friction under the head of the bolt, friction in the threads etc.

Another aspect of these bolts is that once you have set them you don't go back and tighten them further after 500 miles, 'cos there's no point!

Once these fasteners have been used and 'stretched' they should not be re-used as their characteristics have permanently changed.  They are also very expensive due to having ground threads and shanks and being made from exotic steels and specially heat treated.

Would be an interesting project, designing and making these special bolts for A7/A10 customers, I hate to think what they'd cost though. Not so bad when your buying about 3 million a year....

Cheers Ian
(ex automotive engine manufacturing engineer)

[Rex]

That said it's still a big PITA to retighten A series head bolts after the first 300-500 miles, but something which can't be ignored or left to chance.
I just hope that the Drags-supplied head bolts are of the correct metallurgy...

[JulianS]

SRM supplied bolts fine no problems.

Had some stainless outer ones some years back - big mistake they would not hold torque, they stretched. Not yield bolts just the wrong material.

Have also been supplied some with metric 13mm heads

[Rex]

Always puzzled me why anyone would buy stainless head bolts, nuts and studs...

[JulianS]

Because they dont rust like the zinc plated or chemically blacked offerings and because they were supplied by a reputable supplier so reasonable to assume a suitable material was used. Molnars still list them.

[BSA500]

I have never gone back and re-torqued my head bolts on the a7 and so far no issues in 30 years(I have had the head off many times.

[Rex]

Because they dont rust like the zinc plated or chemically blacked offerings and because they were supplied by a reputable supplier so reasonable to assume a suitable material was used. Molnars still list them.

Yeah, I get the cosmetic appeal, but as you've found out the metallurgy can be suspect, and stainless can be a pig to remove when it's been in close contact with other metals.
So far, the Drags zinc-plated items are holding up well.

[berger]

BSA500 i am the same, i soften the gasket pull it down leave it for a day then tweak them again. not with the brazil nut cracking hand though that could be a disaster

[rocker21]

i use the carbon fiber head gasket, does not need to be re torqued , did 500 mile and took the rocker box off, tested them, no movement so not bothered since, also use copper rockerbox gaskets, brillant, no oil leaks.

[RDfella]

I realised the hope that yield bolts might be available was a very long shot, but I'm not prepared at my time in life to start making my own - an exercise which would doubtless need access to heat-treatment and possibly cylindrical grinding facilities (which I no longer have now my mate George has departed). And the last time I used machines to calibrate tensile strength was in the early 60's at RAE Farnborough, when everything was recorded on UV recorders.
Have never been a fan of solid copper gaskets, so will investigate the carbon fibre ones mentioned by rocker21. Certainly anything that avoids the necessity of re-checking those head bolts at least once after fitting must be worth it.

[KiwiGF]

After a full rebuild and a blowing head gasket before 1000 miles that was caused by a 40% over reading torque wrench......I annealed the solid copper gasket and refitted it, like others on here I did not properly re-torque the head bolts after 500 miles  etc. I just checked the bolts I could see and when they were hardly changed didn’t bother with the hidden one’s. I reckoned if I could go near 1000 mikes with the bolts done up so loosely that it can’t be that mission critical to have them all at the exact same torque.

I think I read somewhere of a technique of using a heat gun to heat the head up a few times and avoid a 500 mile re-check?

Edit: I actually re fitted the head gasket twice.....as shortly after re fitting the blowing gasket I had to completely strip the engine and re-torque the big end bolts, once I fully realised the implications of owning an over reading torque wrench 

[BagONails]

Always a good idea to calibrate torque wrenches. I have about 3 of them and check them against each other. Of course they could all be wrong ...Alternatively a spring balance and a measuring stick will do the job too!