Why final 220deg turn on a turbo cylinder head bolt

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masood_ilyas
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Why final 220deg turn on a turbo cylinder head bolt

Post by masood_ilyas »

Over this weekend I will be in the final stages of Head gasket replacement on a Mk2 1.9TD Xantia.
Reading the BOL it suggests three stages of head bolt torquing down and tightening, 20Nm, 60Nm and 220deg final rotation. A 20Nm followed by 60 Nm is ok but not being a mechanical engineer I can not fully understand the logic for 220 Deg final stage. Why not a further two stages of as a guess of 100Nm and 200Nm or 110deg followed by another 110 deg.
Now if the engine was on a bench there would be no problem, however with tight space I can only manage a maximum of around 200deg. Interestingly when I did manage a close to 220 degrees, I noticed after releasing the tension there was still room for the bolt to turn but of course I chickened out as I did not wish to break a bolt at this late stage of the job.
By the way for a non turbo the spec is not so tight, for turbos it is 220deg.
Any explanation would be greatly appreciated.

Thank you

Masood
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Re: Why final 220deg turn on a turbo cylinder head bolt

Post by Xantidote »

I would think this is to do with the fact that you are using stretch bolts. 220 degrees rotation will stretch them a pre-determined amount (as intended by the designer). In the old days, there was just a staged sequence for tightening, as you did initially, but I don't think stretch bolts were then in use.

I stand to be corrected. Wiser persons will be along soon :)

When I did the head on my 1.9TD, I was quite apprehensive about such a large rotation, but the book said Do It, so I did it. To date, the head's dehaved itself.
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Re: Why final 220deg turn on a turbo cylinder head bolt

Post by CitroJim »

Xantidote wrote: I stand to be corrected.
No need, you are absolutely and utterly correct sir :-D :-D :-D
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Re: Why final 220deg turn on a turbo cylinder head bolt

Post by Gibbo2286 »

The reason for this is that they are trying to simulate the 'feel' that the old fashioned mechanic had for bolt tensioning so that anyone is able to rebuild an engine without breaking the studs, straightforward torque setting won't do it, too many variations in materials often leave the studs too loose and head gasket failure, so they experimented in the metallurgical laborotories with ways to match the mechanic's natural skill and this is what they came up with.

Well that's what the guy from GKN told me. :)
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Re: Why final 220deg turn on a turbo cylinder head bolt

Post by citronut »

i always split the degrees into to stages 220deg to 110deg throught the complete tightening sequence twice,

as i dont like to wind one bolt at a time straight down to 220deg in one hit,

in fact i think in later car torque setting they follow this method also,

regards malcolm
masood_ilyas
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Re: Why final 220deg turn on a turbo cylinder head bolt

Post by masood_ilyas »

Thanks Gibbo that sort of makes sense.
May be I should have paid more attention in my Tribology class when the prof was teaching us about metals instead I thought I was going to be an Electronic Engineer and this stuff was irrelevant .

Malcolm's comment above is music to my ears I too felt 220 was an impossible angle to go through with the engine in the car. Thanks Malcolm I can now get on with the job.

Best regards
Masood
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Re: Why final 220deg turn on a turbo cylinder head bolt

Post by Peter.N. »

I followed the instructions exactly in pulling down my XM diesel head but when I checked the bolts by feel they were all different, so I pulled them down until they all felt about right and the same - it did 70k without problems until my wife wrote it off.

Peter
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