Cossie head depth (part 2) HELP
#1
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Righto i have a wee problem.
Cossie head needs skimmed after a melt down.
Current head depth is 137.53 mm.
According to cossie workshop manual minimum depth is 138.68 mm and thats with a max of 0.13 mm machined off.
Now 0.13 mm is 5 thou. My head measures almost 40 thou UNDER the MIUIMUM limit!
So wtf is going on? could my head really have been machined soooo far over the limit?
Can anyone confirm that 138.68mm is minimum head depth?
Heeeeeeeellllllllllllllllp
chop
Cossie head needs skimmed after a melt down.
Current head depth is 137.53 mm.
According to cossie workshop manual minimum depth is 138.68 mm and thats with a max of 0.13 mm machined off.
Now 0.13 mm is 5 thou. My head measures almost 40 thou UNDER the MIUIMUM limit!
So wtf is going on? could my head really have been machined soooo far over the limit?
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Can anyone confirm that 138.68mm is minimum head depth?
Heeeeeeeellllllllllllllllp
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chop
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my head is only 137.5mm and runs greys low comp t34 and mapped at msd.
afaik ford have min thickness to avoid valve/ piston contact but this would not apply if valves pocketed. some tuners say that the seat inserts come too close to head face and others say this is not important. some say that the face of the head is hardened and that you lose the hardness by excess skimming (this is the case with k series heads) but this can only be true if the material is exotic stuff like LM25 which is capable of hardenning but i dont know if YB heads are made of this. spoke to karl about this and he says skimming the head has no effect at all.
afaik ford have min thickness to avoid valve/ piston contact but this would not apply if valves pocketed. some tuners say that the seat inserts come too close to head face and others say this is not important. some say that the face of the head is hardened and that you lose the hardness by excess skimming (this is the case with k series heads) but this can only be true if the material is exotic stuff like LM25 which is capable of hardenning but i dont know if YB heads are made of this. spoke to karl about this and he says skimming the head has no effect at all.
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Gareth
I guess![Sad](https://passionford.com/forum/images/smilies/icon_sad.gif)
Dan
Hahahahahaha, i'd rather get my cheque book out
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PGT
Interesting info mate. I take it you mean pistons pocketed? Well mine aren't![Sad](https://passionford.com/forum/images/smilies/icon_sad.gif)
On my head you can see that the piston has touched near to cylinder 3. Theres a small dent where it looks like the plug tip was hit by piston and made a wee mark.
I really dont want any fucking about so i guess its a new head
I guess
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Dan
Hahahahahaha, i'd rather get my cheque book out
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PGT
Interesting info mate. I take it you mean pistons pocketed? Well mine aren't
![Sad](https://passionford.com/forum/images/smilies/icon_sad.gif)
On my head you can see that the piston has touched near to cylinder 3. Theres a small dent where it looks like the plug tip was hit by piston and made a wee mark.
I really dont want any fucking about so i guess its a new head
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#11
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I know that, but wasn't sure about the pistons you have currently.
I thought that they have a slightly less stroke.......i.e. lower compression?
I thought that they have a slightly less stroke.......i.e. lower compression?
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Originally Posted by Lambchop
Gareth
I guess![Sad](https://passionford.com/forum/images/smilies/icon_sad.gif)
PGT
Interesting info mate. I take it you mean pistons pocketed? Well mine aren't![Sad](https://passionford.com/forum/images/smilies/icon_sad.gif)
I guess
![Sad](https://passionford.com/forum/images/smilies/icon_sad.gif)
PGT
Interesting info mate. I take it you mean pistons pocketed? Well mine aren't
![Sad](https://passionford.com/forum/images/smilies/icon_sad.gif)
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Chop
other problem is the cambelt... obviously its closer...
My mate who has my old white car has the same prob now as he's skimmed over.
I explained.
"If the valves hit the pistons its only gonna happen once"
I think he spoke to Karl/Ollie who suggested some verniers would dial out the problem?????
I dont know for sure though.
Jake
other problem is the cambelt... obviously its closer...
My mate who has my old white car has the same prob now as he's skimmed over.
I explained.
"If the valves hit the pistons its only gonna happen once"
I think he spoke to Karl/Ollie who suggested some verniers would dial out the problem?????
I dont know for sure though.
Jake
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Originally Posted by PGT
some say that the face of the head is hardened and that you lose the hardness by excess skimming (this is the case with k series heads) but this can only be true if the material is exotic stuff like LM25 which is capable of hardenning but i dont know if YB heads are made of this. spoke to karl about this and he says skimming the head has no effect at all.
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mmmm all good info.....
Chop - get the cheque book out..... or pay cash and Gemma will never know
hehehehehe
you may have to invest in a set of verniers to dial the cams in right if too much has been taken off your head, as long as there is no contact with the pis-tonskis you should be braw min!!
Hope you didnt eat too much cake the other night
Chop - get the cheque book out..... or pay cash and Gemma will never know
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you may have to invest in a set of verniers to dial the cams in right if too much has been taken off your head, as long as there is no contact with the pis-tonskis you should be braw min!!
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Hope you didnt eat too much cake the other night
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#17
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Originally Posted by Billabong
Originally Posted by PGT
some say that the face of the head is hardened and that you lose the hardness by excess skimming (this is the case with k series heads) but this can only be true if the material is exotic stuff like LM25 which is capable of hardenning but i dont know if YB heads are made of this. spoke to karl about this and he says skimming the head has no effect at all.
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We may be at cross purposes here, possibly my fault for quoting you. I wasn't suggesting that skimming the head face will soften it, simply that seriously overheating the cylinder head will soften it - how else would the head warp/bow?
I was also under the impression that if the head is overheated too much, then the head could be basically scrap, as the material properties could change (regardless of bowing/running out of true). I can't recall where I got this info' from (which may render my comments useless
, but it is something I remembered - if not the specific details), and so yes I could be wrong. If you or someone else knows different, then I'm happy to be better advised
I don't know the makeup of the cylinder head material, the Cosworth web site used to have a lot of info' on the casting processes etc. but it's all pretty much gone after successive buyouts and redesigns of the website and the company's change in focus. Again, my memory of this is that it was Cosworth's superior casting techniques that made them such a good head for a production engine, not necessarily the materials used.
I was also under the impression that if the head is overheated too much, then the head could be basically scrap, as the material properties could change (regardless of bowing/running out of true). I can't recall where I got this info' from (which may render my comments useless
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I don't know the makeup of the cylinder head material, the Cosworth web site used to have a lot of info' on the casting processes etc. but it's all pretty much gone after successive buyouts and redesigns of the website and the company's change in focus. Again, my memory of this is that it was Cosworth's superior casting techniques that made them such a good head for a production engine, not necessarily the materials used.
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Interesting comments about the overheating - it makes sense.
What I recall from the last time that the matter of the head thickness was raised I can comment as follows;
The minimum thickness does not relate to the valve piston clearance (assuming stf cams etc). This I can absolutely assure you as I have in the past run below the proposed minimum thickness with no such issue in this regard.
The wisdom is that it actually relates to the amount of material around the valve seats. Specifically on the inlet side (i believe) it gets dangerously thin, which theoretically can allow the valve seat to become dislodged.
In addition, I believe that it alters the squish area of the cylinder head, which can adversely affect the combustion.
JJ
What I recall from the last time that the matter of the head thickness was raised I can comment as follows;
The minimum thickness does not relate to the valve piston clearance (assuming stf cams etc). This I can absolutely assure you as I have in the past run below the proposed minimum thickness with no such issue in this regard.
The wisdom is that it actually relates to the amount of material around the valve seats. Specifically on the inlet side (i believe) it gets dangerously thin, which theoretically can allow the valve seat to become dislodged.
In addition, I believe that it alters the squish area of the cylinder head, which can adversely affect the combustion.
JJ
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