Minimum CCs in the head

Post by **Onne** » Fri Apr 13, 2007 6:56 pm

I have recently acquired a 12G295 for the princely sum of 20 pounds sterling.

It has been skimmed in the past, is flowed, but will need a reskim to get it rustfree and flat.

What is the minimum of cubic centimetres I can have before there are problems wit the compression?

Bearing in mind that my block has had a skim as well in the past, now have 180psi with slightly skimmed 202

Cheers

Post by **rayofleamington** » Fri Apr 13, 2007 9:19 pm

how do you know that it's not flat? If you use a big shet of wet n dry on a flat surface (expensive kitchen worktop

) to clean the surface rust off, this would give a better view on it.

Post by **Onne** » Fri Apr 13, 2007 10:29 pm

It is at a workshop at the moment, as it has the wrong exhaust valves fitted.

To pressure test it as well, and if it is not too skimmed, will be refurbished and put onto Liz' engine

Post by **picky** » Sat Apr 14, 2007 10:39 am

it depends how high your pistons reach, and if they are dished or not, as the size of the dish will effect the compression ratio aswell. I ran a 12g295 head on the standard 1098 dished pistons (not sure what cc's??) and the head had 40 thou off it I think. that setup was OK and ran well.

Post by **chrisd87** » Sat Apr 14, 2007 11:36 am

The 12G295 I'm having reconned and coverted to unleaded at the moment has had 75 thou skimmed off it, and the block has been skimmed too. I don't envisage any problems with that, especially as I run on super petrol whenever possible.

I think you'll be fine with your setup Onne. To answer your original question, the 12G295 has a chamber volume of 28.3cc and the 202 has 26.1cc.

Post by **bmcecosse** » Sat Apr 14, 2007 7:43 pm

Onne - measure the thickness of the head to determine how much has been skimmed off before. They all start at 2.750" - so anything less than this is what has been removed already. These heads can 'all' go to 80 thou off before endangering the oil way - I have taken 100 off but it was getting mighty close. Why do you think it's the 'wrong' exhaust valves ? They just use the standard 25.4 (1") valve same as the normal head and the 202/206 heads. If it has been fitted with larger diameter exhausts beware - they will hit the block if you have a high lift cam and/or lots skimmed off the head face. If you feel the chambers are too small (ie less than 21 cc) then you can of course just enlarge them around the valves out to (almost) the gasket line, and this of course will improve the gas flow even more.

Post by **Onne** » Mon Apr 16, 2007 11:08 am

Thanks for all the advice, sounds good

The exhaust valves sit very proud of their seats, and I was told that this is not right.

Post by **bmcecosse** » Mon Apr 16, 2007 11:17 am

Indeed - very not right. Measure the diameter - if more than 25.4mm (1") then they will smack the block unless the bores are considerably bigger than standard!

Post by **Onne** » Mon Apr 16, 2007 11:30 am

Bores are +0.030"

If the valves are the same as the 202 I'll get some of those. Might even make it unleaded with my current mileage

I used a full can of Valvemaster plus since early february, bearing in mind that I use it every other time I put fuel in, and a can makes up for 250 litres of fuel. I need to cut down on my miles...

Post by **chrisd87** » Mon Apr 16, 2007 11:57 am

Sounds like you're in a very similar situation to me, Onne. The previous owner of my 12G295 head had put 12G940 inlet valves in, which were sticking proud of the (original) seats. I'm also having it converted to unleaded - although I worked out that it will take about 50,000 miles of use to break even (I use standard valvemaster).

Post by **Onne** » Mon Apr 16, 2007 12:00 pm

50,000 miles is 2,5 years on the mileage I am doing at the moment, I do about 300 a week

Post by **bmcecosse** » Mon Apr 16, 2007 1:13 pm

I have a virtually new 12g940 head with the factory unleaded seats for sale. See the 'sale' section.