Lotus Twincam motor
#1
I own a Birkin with a 1760cc big valve tall block original Lotus Twincam motor. 85 mm pistons.
My cylinder head has been repeatedly skimmed by previous owners and the last skimming actually removed part of the outer edge of 2 valve seat inserts.
My head measures 4.462 in high. Factory new spec is 4.598 which means .136 in has been removed. I have read of heads still working at 4.520 in.
My head has a small crack through to the water jacket on the small flat area inside the combustion chamber immediately adjacent the bowl area in cylinder #2.
This can be welded up but the head will then need further skimming.
I also have several very minor cracks running from spark plug hole to the outer edge of the valve seat insert. They appear very shallow and compression was still good.
I am hoping that there is an expert out there with extensive knowledge of the twin cam head who can assist with the following.
1. My current head gasket is .051 in thick and a .065 in replacement is available. This should compensate for the additional skimming. ( Hopefully minimal )
2. Is there a risk of the cylinder head combustion flat area failing during use as a result of having the thickness reduced by .136 in (3.45mm) which is excessive?
3. As the bowl area is not directly affected by skimming I am not too concerned about this area.
4. Should the valve seat become a problem protruding beyond the head face is it feasible to install shallower seats and obviously compensate with overall valve/shim length?
5. Clearance of valves to the pockets in the pistons is a concern but the lower seat insert would solve that I guess.
6. My cam shafts have full vernier sprockets so timing will be carefully done allowing for the head height affecting chain length.

All comments and assistance would be greatly appreciated. Smile Smile Smile
Retaining this classic motor in my very special original Birkin is very important to me.
I am aware that replacement heads are available from Burton and other manufacturers but these are way outside my budget.
Finding a Twink head sitting on a shelf or residing in a scrap yard would be unbelievable.

Stuart Donkin
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#2
(07-23-2017, 10:07 AM)Stuart Donkin Wrote: I own a Birkin with a 1760cc big valve tall block original Lotus Twincam motor. 85 mm pistons.
My cylinder head has been repeatedly skimmed by previous owners and the last skimming actually removed part of the outer edge of 2 valve seat inserts.
My head measures 4.462 in high. Factory new spec is 4.598 which means .136 in has been removed. I have read of heads still working at 4.520 in.
My head has a small crack through to the water jacket on the small flat area inside the combustion chamber immediately adjacent the bowl area in cylinder #2.
This can be welded up but the head will then need further skimming.
I also have several very minor cracks running from spark plug hole to the outer edge of the valve seat insert. They appear very shallow and compression was still good.
I am hoping that there is an expert out there with extensive knowledge of the twin cam head who can assist with the following.
1. My current head gasket is .051 in thick and a .065 in replacement is available. This should compensate for the additional skimming. ( Hopefully minimal )
2. Is there a risk of the cylinder head combustion flat area failing during use as a result of having the thickness reduced by .136 in (3.45mm) which is excessive?
3. As the bowl area is not directly affected by skimming I am not too concerned about this area.
4. Should the valve seat become a problem protruding beyond the head face is it feasible to install shallower seats and obviously compensate with overall valve/shim length?
5. Clearance of valves to the pockets in the pistons is a concern but the lower seat insert would solve that I guess.
6. My cam shafts have full vernier sprockets so timing will be carefully done allowing for the head height affecting chain length.

All comments and assistance would be greatly appreciated. Smile Smile Smile
Retaining this classic motor in my very special original Birkin is very important to me.
I am aware that replacement heads are available from Burton and other manufacturers but these are way outside my budget.
Finding a Twink head sitting on a shelf or residing in a scrap yard would be unbelievable.

Stuart Donkin
Quote:Stuart you can have the head metal sprayed with aluminium back to standard thickness - speak to Richard Twell - 021 558 3713.  Mention that I gave you his details.  He is a superb engine builder / machinist / engineer - he built the Climax motor in my Lotus 14. My Ph is: 082 924 9616 if you need to discuss.  Hope this helps.
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#3
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(10-20-2017, 08:45 PM)Richard Clarke Wrote:
(07-23-2017, 10:07 AM)Stuart Donkin Wrote: I own a Birkin with a 1760cc big valve tall block original Lotus Twincam motor. 85 mm pistons.
My cylinder head has been repeatedly skimmed by previous owners and the last skimming actually removed part of the outer edge of 2 valve seat inserts.
My head measures 4.462 in high. Factory new spec is 4.598 which means .136 in has been removed. I have read of heads still working at 4.520 in.
My head has a small crack through to the water jacket on the small flat area inside the combustion chamber immediately adjacent the bowl area in cylinder #2.
This can be welded up but the head will then need further skimming.
I also have several very minor cracks running from spark plug hole to the outer edge of the valve seat insert. They appear very shallow and compression was still good.
I am hoping that there is an expert out there with extensive knowledge of the twin cam head who can assist with the following.
1. My current head gasket is .051 in thick and a .065 in replacement is available. This should compensate for the additional skimming. ( Hopefully minimal )
2. Is there a risk of the cylinder head combustion flat area failing during use as a result of having the thickness reduced by .136 in (3.45mm) which is excessive?
3. As the bowl area is not directly affected by skimming I am not too concerned about this area.
4. Should the valve seat become a problem protruding beyond the head face is it feasible to install shallower seats and obviously compensate with overall valve/shim length?
5. Clearance of valves to the pockets in the pistons is a concern but the lower seat insert would solve that I guess.
6. My cam shafts have full vernier sprockets so timing will be carefully done allowing for the head height affecting chain length.

All comments and assistance would be greatly appreciated. Smile Smile Smile
Retaining this classic motor in my very special original Birkin is very important to me.
I am aware that replacement heads are available from Burton and other manufacturers but these are way outside my budget.
Finding a Twink head sitting on a shelf or residing in a scrap yard would be unbelievable.

Stuart Donkin
Quote:Stuart you can have the head metal sprayed with aluminium back to standard thickness - speak to Richard Twell - 021 558 3713.  Mention that I gave you his details.  He is a superb engine builder / machinist / engineer - he built the Climax motor in my Lotus 14. My Ph is: 082 924 9616 if you need to discuss.  Hope this helps.

Thanks for your response - much appreciated. I will collect the head from the engineers tomorrow after welding up and fitting new locally machined valve guides.
If the skimming is a problem I will consider the option of having the head built up with aluminium as you suggest. Thanks again, regards Stuart
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