Is it just me?

[zedonist]

I don't have to post on this forum you know!

 

[hard top]

Yes you do.......

 

[zedonist]

Is that a challenge?

 

[hard top]

Nope, just telling you........
You already have to deal with bonzo and I have my hands full with Bad Boy.

 

[hard top]

And I did not mean his hampton......

 

[Mint]

Did you know a boy called Potter?

Don't bring me into this - I only met Mr Zedonist once - great bloke

 

[oldcarman]

You are correct Frankie, the gaskets also corrode over time from the combustion gases. I doubt very much if you could measure the stretch in a bolt with conventional home tools, and it's a mute point, because on stretch bolts (torque to yield) the bolts have been tightened just up to or past there yield point, so that they exhibit permanent set. A bolt is designed to work like an elastic band, I.e when you stretch it and release it it tries to go back to its former length, in doing so it induces a clamping force, the more you tighten it the more clamping load it exhibits, until it goes past its yield point at which point it continues to stretch. On many cylinder heads, bolts are tightened to there specified yield point and then turned x amount of degrees to be sure. as this invariably induces a permanent set condition, and the bolts are said to have stretched, hence the name. If you were to re-apply the torque to these bolts they would stretch further, in doing so the grain boundaries fail, the material becomes weaker, the result is the clamping load falls off, and the head becomes loose. This is why you should fit new bolts.

Just to add to your comment Zed, ARP sells specialty tools that allow for accurate measurement of head and all bolts which are previously torqued and need to be within a certain length. It is beyond the average persons ability to accurately measure old bolts compared to new ones and I agree, use new head bolts, particularly with aluminum heads and blocks. New bolts are not that expensive in the overall scheme of things when compared to the cost of rebuilds due to reuse of stretched bolts. The tensile strength also plays a major part in all of this. JIM

 

[oldcarman]

I was going to go deeper, glad I didn't, I trained as a metallurgist with Merlin, in my youth and as a young man.....

I though you were still there!! Did you sleep with Morgana in Camelot?

 

[zedonist]

Just to add to your comment Zed, ARP sells specialty tools that allow for accurate measurement of head and all bolts which are previously torqued and need to be within a certain length. It is beyond the average persons ability to accurately measure old bolts compared to new ones and I agree, use new head bolts, particularly with aluminum heads and blocks. New bolts are not that expensive in the overall scheme of things when compared to the cost of rebuilds due to reuse of stretched bolts. The tensile strength also plays a major part in all of this. JIM

You are nearly right Jim, Tensile Strength does play a part in this but only in deciding which grade of bolt to use against the desired clamping force required. The tensile strength of a bolt is actually the point at which it breaks in two (to keep it simple), the proof load (yield) is the point at which the bolt starts to go beyond the elastic limit, when bolts are tested for use, the key pass criteria is to have the minimum proof load applied to the bolt and for the length of the bolt to be unchanged to the un-stressed length, if it is longer it is said to have gone into permanent set and failed.

When tightening bolts on normal joints you usually apply only 60 - 80% of the proof load, that way you are guaranteed not to send the bolt into yield and plastic deformation.

That said sometimes it is important to achieve maximum clamping force, which is where tightening to yield comes into play, in order to achieve this these bolts have to be made from really good alloy steel, closely controlled on heat treatment for both core and surface properties, and then the shanks are sometimes ground to ensure the body diameter is almost identical (but not much benefit, as failure occurs in the smallest diameter - threads) on each bolt and then coated with manganese phosphate to ensure a consistent coefficient of friction, and then these bolts are then torqued up to just below the yield limit, and then a degree of turn added, called angle, this is normally done on a special machine with transducers to accurately monitor the torque.

I did say i was going to do a write up on bolting and materials, must get round to it.

 

[zedonist]

I though you were still there!! Did you sleep with Morgana in Camelot?

No but i am married to a witch

 

[Cooper]

@zedonist and @FRANKIE great write up on bolts, very interesting stuff. I notice in the Bentley manual that they often insist that new bolts be used when some components are removed and replaced but I did no know why - now am educated!

 

[oldcarman]

Thanks Zed, another point I meant to add was that once the bolt has reached it's permanent stretched state as to length there is the chance that they may bottom out in some bolt holes and never again be able to be torqued properly. That is where it's important to know the depth as well as utilize the stretch gauge. JIM

 

[zedonist]

More importantly Cooper, make sure you get a torque wrench and apply the correct torque as per BMW. I would also add only use replacement bolts from BMW as the coatings on them have a controlled coefficient of friction, which is important as 70% of torque on a bolt goes into over coming friction, and only 30% into clamping load, but by playing with the coefficient of friction you can make the bolts more slippy, and add more clamp load, therefore using a different coated bolt could mean you have an under tightened joint. Hope that makes sense.

 

[Barry Gadd]

Great appraisal of headbolt forces Zed.
I still puzzle as to why theses are tightened to 30Nm, then 90*, then 90*
Why not tighten to a specified Nm final torque but do this in say 2 or 3 steps building up to whatever the final figure is?

 

[zedonist]

It's becsuse you have a gasket in between the joint faces, so torquing 3 times takes into account joint relaxation from compression of the gasket material and ensure you have correct clamping load.

The use if degrees of turn is because they use automated tools on the engine build line, you could in effect torque to figures, but easier to turn 90 degrees.

 

[robertjones]

Commiserations Robert.
Joe and myself have been working on both our Z3's (M44) as both showed loss of water / blown hoses /blown radiator at various times.
I noticed the same problem as you have, of a mysterious loss of water (never very much) each time I looked in the expansion tank.
Joe and myself contacted a great mechanic who tested the rad pressure by coupling his tester to the filler neck.
As the engine heated up the pressure increased considerably enough for him to switch the engine off quickly.
His diagnosis was warped head.
Both of us have had cylinder heads skimmed as a result.
The annoying thing is both the gasket and head look fine after removal. It is only when the engineering shop check the head with a proper straight edge that any problem can be seen.
The engineering shop we use is based in Ashtead (near Leatherhead) and charge £30 which seems very reasonable.
One problem we found is that after skimming you can't easily re-set the cam timing with the cam clamp and flywheel locking pin which is the accepted way of doing it.
The timing is a gnats whisker out if you do this and both Joe and myself call in the mechanic to finally set the timing (otherwise the engine starts and runs ok at high revs but ticks over very roughly )
I do hope you don't need a head skim but it sounds very likety to me.

mine has never blown any hoses, the problem seems to be with the plastics connected to the cooling system, up to now i have had to replace the rad, also the top hose because the plastic connector split, then a hose at the rear of the engine again because the plastic connector was breaking up and now its in to have the thermostat housing replaced, because its leaking from there,seems to me that,once one leak is repaired , another appears. One thing I noted of interest is that your mechanic tested the coolant pressure with the engine running, my garage fits the gauge with the engine off, then allows it tostand for a few minutes before checking the pressure? also i feel that , with them pressurising the system then it keeps finding the weak spots,is that possible?

 

[FRANKIE]

It's becsuse you have a gasket in between the joint faces, so torquing 3 times takes into account joint relaxation from compression of the gasket material and ensure you have correct clamping load.

The use if degrees of turn is because they use automated tools on the engine build line, you could in effect torque to figures, but easier to turn 90 degrees.

Previous quote: " It's because you have a gasket in between the joint faces, so torquing 3 times takes into account joint relaxation from compression of the................"
After all the information one needs to be aware of JUST ON BOLTS, you really need a little 'joint relaxation'.

 

[David Cullen]

very interesting reading guys ;-)

 

[Barry Gadd]

Hi Robert,
The mechanic is looking for any excess pressure that would exceed the accepted hot level such as to cause the sort of blow that both Joe and myself have seen. He found the level on my engine exceeded the max acceptable 20 psi (or thereabouts) and advised this could only have come from cylinder pressure and stopped the test. His diagnosis was warped head or bad gasket.
Frustratingly both head and gasket look fine on removal but the engineering shop could see warp on the head when checked with a pukka straightedge. Hence the skim. He is coming to set the timing in next day or so hopefully I can then try again for an MOT.
I haven't had any plastic parts fail as such. The rad had a postcard size hole blown out but this was due to very high pressure. (the MOT tester was not impressed !! Luckily he didn't get wet)

 

[robertjones]

Hi Robert,
The mechanic is looking for any excess pressure that would exceed the accepted hot level such as to cause the sort of blow that both Joe and myself have seen. He found the level on my engine exceeded the max acceptable 20 psi (or thereabouts) and advised this could only have come from cylinder pressure and stopped the test. His diagnosis was warped head or bad gasket.
Frustratingly both head and gasket look fine on removal but the engineering shop could see warp on the head when checked with a pukka straightedge. Hence the skim. He is coming to set the timing in next day or so hopefully I can then try again for an MOT.
I haven't had any plastic parts fail as such. The rad had a postcard size hole blown out but this was due to very high pressure. (the MOT tester was not impressed !! Luckily he didn't get wet)

Ok my rad. Had not blown as such but it seems to have failed between the rad itself and the header tank, presumably there is a seal around there somewhere