Torque Wrench Trouble Stretch head bolts HELP!

[sanzomat]

Making some more progress with my VVC K series engine but maybe a set back.

I set up the VVC timing yesterday evening including installing the new gaskets to the VVC mechs and fitting the new oil seals and had a bit of time left so the next job is to put the head back onto the block.

I positioned the new gasket, carefully lined it up and placed the head onto the dowels. I checked the length of the new stretch bolt against the ones I'd taken out and they seem to be identical (might come in handy later!)

All ten new bolts went in gently and hand tightened to just start to pinch then out came the trusty old torque wrench. They are supposed to be tightened to a torque of 20Nm and then turned a further 360 degrees (no final torque figure given)

I set the wrench to ten to start with and then followed the sequence giving each bolt a quarter turn at a time.

After (I think but might have lost count) 6nr quarter turns on each bolt I felt that the bolts were starting to give a reasonable resistance to turning and then it struck me that the wrench should have been clicking by then as it was only set to 10Nm. I tried the wrench on one of the engine stand bolts and tightened it hard and still no click. I tried putting the 1/2" square in the vice and still couldn't get it to click whatever torque I set it to. It seems the wrench is not functioning!!

So, I don't know how much torque I've put on the bolts and whether I've maybe overtightened them.

I'm hoping that the distance I've turned them should mean that I haven't as if I have only done 6x 1/4 turn then even if it was up to the 20Nm when I started I've only gone 1/2 a turn beyond the extra 360 degres after 20Nm. I'm guessing that at least 1/2 turn from 1st pinch would probably be needed to get to the 20Nm.

The bolts must be around 400mm long and have a very fine thread so each turn can't be more than 1mm - if so then even a whole turn too much would only stretch them by 0.25%.

I'm after some reassurance and advice - should I back the bolts off to loose again and start again with a good torque wrench? If I do this I'm concerned that the MLS gasket might not like being relaxed again?

Should I just loosen them a little, say until it takes 20Nm to turn them back again, and then do a full turn?

Should I take just one bolt out fully leaving the others as they are and measure the bolt for stretch (against one of the old bolts) my worry with this is that it would leave uneven compression and maybe induce a warp.

Or take all the bolts out, buy new bolts and another new gasket

Anyone got any experience in how tight they become during the final 360 degrees of tightening? I'd guess that from experience my human torque gauge was giving it around 30Nm at the point I felt it was way more than 10?

[Lucky Ed]

Dave,

Dave Bence is our resident expert on the K-series, so I'll give him a nudge to read this. I'm unsure about the stretch bolts as they're so long, but I'm currently rebuilding my Zetec, and have been told it is ok to reuse the ARP head studs, so that's what I'm going to do. The MLS gasket on the other hand should be fine as they can be reused even after running the engine, so the outcome might not be too bad.

I'll see if I can get Dave to give you his thoughts ASAP

Ed

[CC Cyclone]

Seems to me that if you get a working wrench set to 20NM you should be able to tell if they are torqued to 20NM yet? If not I would have thought you could keep going with the procedure. If it clicks at 20NM immediately, you don't really know where you have got to, in which case bolts out, get new bolts and start again? Either way you need a working torque wrench and so that would be my first step.

I may have misunderstood, in which case ignore all that...

[benchmark51]

20Nm is sod all is it? My torque wrench starts at 40Nm and goes to 200Nm.
Which is 30 lbf.ft to 150 lbf.ft. Most 1/2" torque wrenches are near that. Perhaps a 3/8 drive one would be better, although not all them will go as low as 20Nm. Draper and Sealey do them though

I think I would get a good torque wrench and , in sequence, slacken each bolt and retighten to the specified torque. Then do the 360 degree as instructed.
You haven't subjected it to heat or fluids so I would say you would be alright.

[dave_bence]

When I collected my tuned cylinder head from Dave Andrews (DVA Power), he recommended always using new genuine Rover bolts, torqueing them all evenly to 20NM then 40NM to settle everything down, then slackening them all off and doing the 20NM, half a turn, half a turn method. However, I wasn't using a MLS gasket. The final half a turn always feels worryingly tight – almost as if the bolts are about to snap.

I'd not take any risks with this part of the build. I'm a big fan of the K-Series engine, but it was originally designed as only a 1.1 or 1.4 litre engine, so it is a highly stressed unit. If there is any weakness, it'll show up. At the very least, replace the MLS gasket.

Bear in mind that if the head gasket does fail in the future, not only will you need new bolts and gaskets, but you'll probably have to spend a lot of time and money getting all the 'mayonnaise' out of the engine and the entire cooling system, which is a horrible job

If I were you, to make absolutely sure, I'd renew the gaskets and bolts and start again. I have a calibrated 3/8" torque wrench that is ideal for these engines, if you need to borrow it.

[sanzomat]

Thanks for that Dave. The biggest positive for me from what you've said is that the final half turn feels worryingly tight. I definitely didn't get to a point where it was feeling that tight so I'm feeling a lot more confident I didn't get the bolts beyond the yield point. As I was expecting just to be tightening them to a settled position and was expecting a "click" at a low torque I realised there was something wrong and stopped to check the wrench. I read something at lunchtime that was saying about how experienced people recognise the yield point of a (traditional) bolt with a sinking feeling knowing they've over done it and that you get that feeling when you reach the yield point of a stretch bolt - I've sheared bolts and stripped threads in my youth and I can remember that feeling and I know I didn't get it last night so there is hope! I'm starting to think I've most likely taken the bolts beyond the starting point of the angle torqueing but not to the end of it. If that's the case then slackening them back a bit, finding the 20Nm and re-doing the 180, 180 should work.

Yes please to the loan of the torque wrench! I'll PM my phone number.

[Facial Hair Optional]

What CC Cyclone said makes sense Dave, the 1st thing I would do is get an accurate working wrench and find out how tight you have done them already. Easy to do, then you are more able to make an informed decision based on that and all the other good advice given here on which way to go next mate. It maybe be they are only 21Nm for all you know, or they could be 80!!!

Dave Bence sums it up mate, if they have been way overtightened, renew the gasket and bolts or you are taking a chance on future trouble. Borrow his wrench, find out how tight they currently are and then decide what the hell to do dude!

[CC Cyclone]

What CC Cyclone said makes sense

If you could let my Mrs know that would be really useful.

[Camber Dave]

Like all mechanics I have changed about a dozen K series gaskets.

On one job the threads in the block stripped.

That got you panic-ing didn't it.

In the sump there is a alloy block that is bolted to the main bearing carrier and the head bolts tighten into it - so closing the entire sandwich.

I have a couple of old engines here if you would like to try a second hand one.

B T W I once met a bloke that did engineering at Manchester and he knew the bloke that invented the stretch bolt.
He told me that the tighten procedure (see Dave B's post) is critical to its function.
During the first heat cycle the bolts contract and tighten the gasket down tighter.
This is why they shouldn't be used again. Or they are subject to length check.

Dave

[Facial Hair Optional]

I also seem to recall what Camber Dave is talking about, during my engineering days I'm pretty sure I was told stretch bolts are a one time use item, once stretched they should not be used again as they will not then properly perform the function they are designed for!!

I have a brand spanking new torque wrench here Dave (birthday gift!), unfortunately it is from 30Nm to 210Nm so I doubt it would be what you are after?

[sanzomat]

Okay, all sorted and happy again

Borrowed Dave Bence's torque wrench - you have to admire a home mechanic who has an up to date calibration certificate for his torque wrench!

Anyway, checked what I'd torqued the bolts to previously and they were clicking at 35Nm so my human torque calibration is 5 out as I'd guessed 30. So, based on the Dave Andrews method (and as far as I'm concerned anything that is good enough for Mr DVA is good enough for me) I needed to tighten them up to 40, back them off, up to 20 again and then do the 180 then 180. So, that is what I did. I checked what the torque was for the last few degrees and it was 58Nm so confident that I hadn't got anywhere near to the yield point of the bolts previously. I have to say that I expected it to be a bit tighter at the end of the 360 degrees though and I didn't get any feeling of the bolts starting to yield. I'd say that they are still in the elastic phase so now I'm wondering if they need to go further. I'm guessing the original method of 20Nm, 180, 180 was for the original type gasket, not the MLS that ford introduced after they bought Landrover but all the advice I can find still says to use the 20Nm, 180, 180 so thats how I'll leave it. (I chose to go MLS because the one that was in there was MLS and there was no sign of it having leaked)

I've also fixed my torque wrench too. It was actually my late father in law's. I dismantled it and found the problem was that the part that the spring pushes to hold the bearing against an angle on the drive end was rusted solid to the inside of the tube. A bit of release oil and a long narrow drift pushed it out. Everything cleaned up and greased and back together I checked it's calibration against Dave's by holding the shaft of mine in the vice. A 15mm 3/8 drive socket on Dave's fitted perfectly over the 1/2 square on mine. It was then a matter of trying various settings and checking whether they clicked in harmony. It turns out mine is within 2Nm of Dave's at every 10Nm from 10 to 60 so that's good enough for me. There doesn't seem to be any way of adjusting the calibration on mine (could maybe add shims between the screw and the spring) but mine was low at the low end and high at the high and spot on in the middle so I'll leave it alone.

Thanks for the loan Dave - I'll return it as soon as possible - if you are at the Supercar Saturday next week that would be ideal.

[Facial Hair Optional]

Good job Dave. Out of interest mate I just got myself one of these...

http://www.ebay.co.uk/itm/131613975905?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT


That one turned out to be top top quality and bargain price in my view. You can get sets of them to cover all needs too 

[Lucky Ed]

Good work Dave, fingers crossed it should be ok

[Daley Down Under]

Excellent news - sounds like a good result 

[sanzomat]

I found this graph that really helps understand how these stretch bolts (aka torque to yield or TTY) work. Traditional bolts should always stay in the plastic range but you'll see from this graph that the stress (in the case of a head bolt the clamping force) varies very steeply with length. For an alloy head this is bad news as there is a fair expansion/contraction through heat cycles so the clamping force would be all over the place and for a given strong high tensile bolt would produce an excessive clamping force when the head was hot that would over squash the fire rings etc (or if it was set to be right at high temp would be too weak when cold). The graph shows how much flatter the stress v stretch relationship is once you leave the elastic phase and get into the plastic phase. So, if you use a weaker bolt and take it into the plastic phase the clamping force will be much more constant as the head expands/contracts with heat. Constant clamping is a good thing!

The only big issue is how close you have taken the bolt to its ultimate stress (failure) point. Hence the need to do this accurately!

This graph explains very well to me that a bolt that never left the elastic phase can be re-used but once you've taken it past the yield point you'd be daft to try!