Hi,
1098s are pretty rare in Australia. I have bought a very second hand one and want to know what the "standard" head is. Mine has 12A185 as a casting but has 1.156 inlets and 1.00 outlets, chamber capacity is about 26 ccs which doesn't line up with the head information I have.
So, what is the standard head, and valve sizes and do they have double valve springs as mine has?
Thanks,
Chris
1098 Head
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philthehill
- Minor Maniac
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- Joined: Thu Jul 19, 2012 12:05 pm
- Location: Hampshire
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Re: 1098 Head
The head that you have is an early Cooper S with a combustion chamber volume of 21.4cc
The standard 12A185 head will fit the 1098cc engine without any problems but it is very prone to cracking between the inlet and exhaust valve seats.
The standard valve stems are shorter and the combustion chambers deeper. The combustion chamber shape is very much like that of the 12G202 but fitted with the larger valves.
The valve sizes appear from the measurements you have given above to have been changed.
The correct valve sizes for a 12A185 head are 1.4" inlet and 1 5/32" exhaust.
The head may have had new undersize valve seats fitted to try and cure a cracked head. Examine the valve throats for signs of new valve seats being fitted.
New valves for the 12A185 head are very expensive compared to those fitted to the 12G940.
The standard 1098cc cylinder head casting number is 12G202 with a combustion chamber volume of 26.1cc.
The 12A185 head was fitted with double valve springs as standard whereas the 12G202 was only fitted with single springs.
In conclusion - if your 12A185 head (in standard form) is not cracked you have the best head for a small bore engine and I personally would continue to use it.
See also discussion/reference to 12A185 heads in:-
viewtopic.php?f=4&t=66340
The standard 12A185 head will fit the 1098cc engine without any problems but it is very prone to cracking between the inlet and exhaust valve seats.
The standard valve stems are shorter and the combustion chambers deeper. The combustion chamber shape is very much like that of the 12G202 but fitted with the larger valves.
The valve sizes appear from the measurements you have given above to have been changed.
The correct valve sizes for a 12A185 head are 1.4" inlet and 1 5/32" exhaust.
The head may have had new undersize valve seats fitted to try and cure a cracked head. Examine the valve throats for signs of new valve seats being fitted.
New valves for the 12A185 head are very expensive compared to those fitted to the 12G940.
The standard 1098cc cylinder head casting number is 12G202 with a combustion chamber volume of 26.1cc.
The 12A185 head was fitted with double valve springs as standard whereas the 12G202 was only fitted with single springs.
In conclusion - if your 12A185 head (in standard form) is not cracked you have the best head for a small bore engine and I personally would continue to use it.
See also discussion/reference to 12A185 heads in:-
viewtopic.php?f=4&t=66340
Re: 1098 Head
I have checked the head capacity with a syringe and it takes 26 cc and according to a page out of some Mini specs it should have huge valves. Anyway happy that its equivalent to the normal head fitted. It will need a full service as its pretty coked up but doesn't appear to have been skimmed so no problems with distortion. While I cant be certain that it hasn't had smaller valves fitted but the ports are very small compared to a 12G940. After its been cleaned up I'll try and post some photos of it but this wont be for a few weeks.
Thanks.
Thanks.
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philthehill
- Minor Maniac
- Posts: 11585
- Joined: Thu Jul 19, 2012 12:05 pm
- Location: Hampshire
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Re: 1098 Head
The 'huge valves' fitted to the 12A185 head are the ones listed and sized in my post above; they are the largest standard valves fitted to a 'A' Series head.
I suspect that your 26cc combustion chamber is a result of having work done on those valve seats, the fitting of smaller valves and possibly an attempt to keep the compression ratio near standard - as I mentioned above the standard 12G202 combustion chamber has a capacity of 26.1cc so only 0.1cc difference.
Having just compared the porting of both the 12G940 and the 12A185 heads that I have on the spares shelf there is not a lot of difference between the two.
The main difference between the two is the combustion chamber shape. The 12G940 combustion chamber is more open with less shrouding than the 12A185 but for 1098cc engine the combustion chamber shape of the 12A185 is more than satisfactory.
It may be that it is the coking which is giving the appearance of smaller ports.
I suspect that your 26cc combustion chamber is a result of having work done on those valve seats, the fitting of smaller valves and possibly an attempt to keep the compression ratio near standard - as I mentioned above the standard 12G202 combustion chamber has a capacity of 26.1cc so only 0.1cc difference.
Having just compared the porting of both the 12G940 and the 12A185 heads that I have on the spares shelf there is not a lot of difference between the two.
The main difference between the two is the combustion chamber shape. The 12G940 combustion chamber is more open with less shrouding than the 12A185 but for 1098cc engine the combustion chamber shape of the 12A185 is more than satisfactory.
It may be that it is the coking which is giving the appearance of smaller ports.