8 replies to this topic

[Tubsy]

I have a 998 block and want to fit a 12g295 head to it, do I need the block pocketing? I've had the block rebored to +.020 just to tidy it up, and the head skimmed to tidy that face too. The guys at the machine shop didn't think I needed it pocketing, but it's niggling me, any help would be great thanks in advance.

[lapider]

I'm sure someone will be along to back up or contradict what I've said but

 

The 12g295 was off early 998 coopers, it doesn't need pocketing but the engine also had raised D top pistons (non existent now). so it needs to be machined enough to ensure that it has a decent compression, ensuring you have flat top pistons is good. Taking too much off the head can expose the water jacket and make the head unreliable (due to cracks) so it is a fine art. 

 

I know this from experience, i have fitted a 12g295 to my 998 which has had a 20 thou skim from its standard Finnish (not enough to gain the compression ratio) as a result the car has had a minimal power increase, can't remember exact figures but was verified on the rolling road. 

 

So no you don't need to pocket it, but you do need to calculate how much volume the chamber needs to hold and how much needs to be skimmed off the head. i don't know how to work it out but it is on the forum somewhere. 

Edited by lapider, 14 November 2015 - 02:35 PM.

[Anthony30]

No need to alter the block. You will need to get the valve seats converted to unleaded, and have the head skimmed about 80 thou, if completely original. You will need to send it off to a mini engine specialist, to measure the the chambers, and see if the seats have already been converted. You will need to take the whole engine to a mini engine specialist to make sure the compression ratio will be correct, You can't unskim a head!  IMO of course.

[Swift_General]

As above no. You would only need to pocket if putting a large bore (eg 12G940) head on a small bore block. If I recall correctly that casting has 28+ cc combustion volume which is the largest of the standard heads. I don't know what spec the rest of your engine is, but if for example you were aiming for a CR of 10:1 you would need 27.7cc total combusion chamber volume, ie including head gasket etc so you can see the potential issue.

Edited by Swift_General, 14 November 2015 - 03:03 PM.

[coopdog]

If you have an iPhones there's an app called "mini head" which is good for working out CR :)

[timmy850]

If you put extra large exhaust valves in you'll need to pocket the block, but there should be no problem with standard ones. The best way to check it is to bolt the head to the bare block and see if the valves are anywhere near the bore. 

 

According to my sheet with a 28cc head volume:

998 overbored 020 with flat top pistons = approx 9.1:1 CR

998 overbored 020 with 6cc dished pistons = approx 7.8:1 CR

 

If you could get the head down to 25cc:

998 overbored 020 with flat top pistons = approx 9.9:1 CR

 

 

What people tend to forget is the heads work much better on the 1098cc engine (like in the MG1100 & Sprite), and with the extra stroke you get a much better compression ratio. E.G. with a 28cc head:

1098 overbored 020 with flat top pistons = approx 9.9:1 CR

[limby2000]

Yeah pick yourself up a 1098 crank and pistons,worth the extra grunt. Would like a look at that (sheet) you have there Timmy, sounds like a good read.

[Tubsy]

Thanks guys, I need to speak to machine shop and get some info off them, they did machine valve seats out and fitted hardened ones, any recommendations for a decent firm to check compression ratios around Shropshire or Midlands please? Thanks for your help and sorry for delay in getting back to you, trying to keep everyone happy with that mini / family balance.

[Cooperman]

Do your own C.R. measurements and calculations.

I say this because some years ago I bought a fully gas-flowed big-valve from a very well-known supplier of Mini Parts (it was not Mini Spares I have to say).

It was for a rally 1071 cc Cooper 'S' and they were fully informed of this. Luckily I did a check measurement of chamber sizes before fitting and the chambers were much too large as the head had not been skimmed to give the right chamber sizes. Had I fitted it the C.R. would have been about 7.8:1 which would have been NBG.

So measure & calculate. If in doubt we will help you with this.