21 replies to this topic

[lapider]

okay so ive just bought a 12g295 cylinder head for my 998. my intentions are to refurbish it and clean it up but reuse the original componentsi have a few questions to ask about it.
1. will it be okay to use the original valve guides on there which i believe are leaded. if not where can i buy unleaded valves from as ive done a quick serch on mini sport and mini spares and cant find anything. plus are there any issues running leaded guides that may happen to my cylinder head in the long run.
2.the cylinder head comes with double valve springs would these be okay for a relativley standard 998. if not what ones should they be replaced with.

thanks in advance.
max

Edited by lapider, 10 May 2011 - 07:03 PM.

[MRA]

1) Not really, unless you can check that they have only 0.003" of clearance....
2) They will keep the valves closed, however any extra revs that the springs can cope with that you don't use will be a waste of power

Why are you fitting a 295 on a standard engine, the point of a 295 is more power ? if you intend to utilise that extra power and engine revs then there are a few things that will help....

[MiniLuke]

I think you are mixed up with leaded and unleaded valve guides. I think you mean valve seats, and not the valves themselves or the guides.

You also need to consider the compression ratio. I am sure someone can explain better than me though.

[lapider]

yeah thats what i mean realy what im trying to say is would i have to do the unleaded head conversion or would i be fine without. and by relativley standard 998 there are some power gains that i am thinking of running including uprated inlet and outlet manifolds and an rc40. i was possibly thinking of an uprated cam if i ever take out the engine but im not sure of that just yet.

[Cooperman]

The biggest issue with the 295 is that it is meant for engines which have raised D-top pistons - i.e. the original 998 Cooper. Thus the combustion chambers are too large for a basic non-Cooper 998 which normally has dished pistons or, at best, flat top ones. It is, therefore, necessary to measure what you have, including the 295 head, do the compression ratio calculations and have the head skimmed to give the correct compression ratio for your engine. I would suggest a comp ratio of 10:1.
Whilst the head is being skimmed, it could be a good idea to have unleaded exhaust valve seats fitted. You don't need unleaded inlet valve seats. If the guides are worn fit new ones as you will need to re-lap the valves in before fitting anyway. The steel guides are the best really. Fit valve stem oil seals to all 8 valves.

[wile e coyote]

If you're really lucky and got one of ebay very recently from seller up north, with new inlet valves - thats probably the best std. '295 out there other than NOS - has never been exposed to UL fuel - came off an austin 1100 parked up in the mid eighties- tax ran out in 84 - had almost no miles on - and basically dissolved in all the wrong places and was scrapped as an eyesore. If its the very clean red painted head I think it may be you'll need the new valves lapped in. It hasn't been skimmed or faced before - unless recently...

But play safe regardless UL inserts aren't too costly to do... £100 does it..

[bmcecosse]

Unless you are going to thrash it long and hard - don't bother with unleaded (or additives) - set inlets to 12 thou gap and exhausts to 15 thou. But do measure the head thickness - if you have flat top pistons it needs to be ~ 2.710" inches, if dished pistons then go for ~ 2.670".

[Bluecray]

Unless you are going to thrash it long and hard - don't bother with unleaded (or additives) - set inlets to 12 thou gap and exhausts to 15 thou. But do measure the head thickness - if you have flat top pistons it needs to be ~ 2.710" inches, if dished pistons then go for ~ 2.670".

Flat top pistons - would that be a standard 998 piston?

[lapider]

cheers for all the advice fellas. my plan is to restore the head with 8 new valve oil stem seals and then just clean up and decoke all the ports and chambers. unles i find any cracks in the valve seats i will just run it with the existing valve seats, and obviously as was mentioned replace the guides if needed. i will look into skimming the head at a local workshop but if i were to run the head without skimming it would there be any issues baring in mind they are just flat top pistons not copper pistons. oh and in terms of weather i thrash it it will be driven as a daily driver, but there will be the obvious country lane that i wont be able to hold back on.

Edited by lapider, 11 May 2011 - 08:36 AM.

[lapider]

Unless you are going to thrash it long and hard - don't bother with unleaded (or additives) - set inlets to 12 thou gap and exhausts to 15 thou. But do measure the head thickness - if you have flat top pistons it needs to be ~ 2.710" inches, if dished pistons then go for ~ 2.670".

Flat top pistons - would that be a standard 998 piston?

yes standard pistons for a standard 998 were flat top any others like dished or d-toped pistons as there called are either cooper of aftermarket pistons

Edited by lapider, 11 May 2011 - 08:51 AM.

[MiniLuke]

Unless you are going to thrash it long and hard - don't bother with unleaded (or additives) - set inlets to 12 thou gap and exhausts to 15 thou. But do measure the head thickness - if you have flat top pistons it needs to be ~ 2.710" inches, if dished pistons then go for ~ 2.670".

Flat top pistons - would that be a standard 998 piston?

yes standard pistons for a standard 998 were flat top any others like dished or d-toped pistons as there called are either cooper of aftermarket pistons

All the early 998's I have ever worked on have dished pistons as standard.........

[lapider]

Unless you are going to thrash it long and hard - don't bother with unleaded (or additives) - set inlets to 12 thou gap and exhausts to 15 thou. But do measure the head thickness - if you have flat top pistons it needs to be ~ 2.710" inches, if dished pistons then go for ~ 2.670".

Flat top pistons - would that be a standard 998 piston?

yes standard pistons for a standard 998 were flat top any others like dished or d-toped pistons as there called are either cooper of aftermarket pistons

All the early 998's I have ever worked on have dished pistons as standard.........

i thought early blocks had the same boar profiles but what distinguished them was different pistons that were in them. i was told about it a while ago and im not to sure if it is true or not. ever heard of this.

[Bluecray]

cheers for all the advice fellas. my plan is to restore the head with 8 new valve oil stem seals and then just clean up and decoke all the ports and chambers. unles i find any cracks in the valve seats i will just run it with the existing valve seats, and obviously as was mentioned replace the guides if needed. i will look into skimming the head at a local workshop but if i were to run the head without skimming it would there be any issues baring in mind they are just flat top pistons not copper pistons. oh and in terms of weather i thrash it it will be driven as a daily driver, but there will be the obvious country lane that i wont be able to hold back on.

If the head is unskimmed and you bolt it on, I dont think you'll gain any performance because the standard mini head combustion chamber is somewhere around the 25cc mark and the 12G295 has a combustion chamber of somewhere around 28.3cc , so bolting a unskimmed 12G295 head on you lose a little in compression, and hence lack of performance.

Feel free anyone to correct my figures they are only taken from what I remember in my head atm.

I've recently put a 12G295 head on I measured the chamber using water and a wine makers measuring implement and I got around 27cc, the head had been skimmed by 0.3 mm and it didn't give me the performance, so I may have it done properly at a machine shop to make the combustion chamber nearer to a standard 998 head to get my compression ratio back.

[bmcecosse]

0.3mm skim is only 12 thou....... you need a 40 thou skim for use with flat top pistons (yes all A+) and an 80 thou skim for use with the dished pistons that many many pre A+ engines used. Running the head without the skimming will negate most of the possible advantages of the head. I found the 940 head to be FAR better, and no skimming involved........... Although the exhaust valves may need sinking into the head slightly......

Edited by bmcecosse, 11 May 2011 - 08:22 PM.

[Joshua.w]

Hi sorry to jump on this thread I am new to the forum. I want a little more out of my mini and have been told that changing the cylinder head is probably the best thing to do on a standard 998. I am pretty good at using a spanner and have done plenty of work on my mini before but have never ventured that far just need a bit of advice as i'd pretty much like to do a straight swap. and only really have to readjust the carb. so just wondering which head i could use to just do a straight swap, rough price and how much power it would give? I do not want loads of power but just a bit more that the normal mini has. Ive recently fitted a stage 1 kit so that should also improve it.

any advice would be much apreciated. 

thanks Josh