4 replies to this topic

[tom.mh]

So I have a 1990 998 a plus checkmate with a few extras and like everybody else it must go faster!

I've read so many threads but still unclear on a few things

 

It's worth saying this is my first mini and I'm not a mechanic or engineer but I pick these things up pretty quick

Putting a 12g295 head on the 998 block is mostly making sense to me, apart from a few things.

Skimming it to improve the compression ratio makes sense, and from what I've read its pretty much- take the old head off and stick the old cooper one on.

 

Where I am getting confused is-

1 - some people are saying it needs to be pocketed,

2 - some people say just set in the valve into the head by a couple mm.

 

Someone enlighten me about the second point because to me it seems totally pointless as the camshaft lift, followers and rockers would make the valve not sit properly if you do this, unless there is some play in the followers or rockers...

And would cam followers of the same length as the standard 998 work alright?

 

With the pocketing, Since the valves point straight down the cylinders how can they hit anything apart from the piston??

Surely if this happens surely too much has been skimmed off the head?

 

When people say pocket the block, do they really mean reshaping it so there are no aerodynamically nasty sharp edges in the combustion chamber where the two surfaces meet?

 

I may answered my own questions here but I'd be grateful for some reassurance before I spunk all my money on some bits of metal

 

Tom

[Dusky]

No need to pocket or set the valves into the head.

Thats only needed on a 12G940, wich is a 1275 head.

Pockets are then made because the valve hangs over the side ( deck) of the engine.

 

The 295 WILL need skimming (unless its already skimmed) to get the R high enough. If you got dished pistons this might be a problem.

Edited by Dusky, 17 November 2015 - 09:25 PM.

[Cooperman]

The 12G295 head was designed for a 998 engine, the Mini-Cooper 998, and, therefore, the block doesn't need any pocketing. After all, the production 998 Coopers weren't pocketed.

However, the 998 Cooper had raised 'D-top' pistons to achieve the correct compression ratio. In order to fit the 295 to a 998 engine it is necessary to skim quite a bit of metal from the head surface. How much is determined by 3 things;

1.  The type of piston fitted - dished or flat top

2.  The distance from the piston top to the block deck level with the pistons at TDC.

3.  Whether the combustion chambers have been enlarged as part of a gas-flowing process.

 

So it is necessary to measure and calculate to work out the size the combustion chambers need to be. There is absolutely no other way of doing this, but here is a thread on here which tells you exactly how to do this and what you need to do it.

 

Once you have determined how much needs to be skimmed from the surface of the head there remains one more check to make. There is an oil-way hole which goes through the head and stops short of the head surface. Measure the depth of this hole accurately and deduct this dimension from the total depth of the head. After skimming there must be at least 0.050" of metal remaining between the base of the hole and the head surface. If the head cannot be skimmed enough to still leave 0.050" there are two options;

 

1.  Accept a lower compression ratio, or,

2.  Dismantle the entire engine and skim the block so that the piston tops protrude a maximum of 0.005" above the block deck level at TDC

 

This will only happen if the head has already been gas-flowed & skimmed and if you are trying to fit it onto a 998 engine with dished pistons. So flat-top pistons are what is really needed to fit a 295 onto a 998 block and it is best done when doing a re-bore and fitting new (flat top) pistons.

 

I hope this helps.

[andyt]

Hi Tom I too have a 295 head on a 998, basically you don't need to pocket the block unless you put massive exhaust valves in this head, I have 33 mm inlet valves and they don't touch the block,  most pocketed blocks will be used when you fit a 12g940 head from a 1275 engine. Setting the valves in the head also applies to fitting 12g940 heads to 998 blocks but is a way with careful measuring of fitting the head without the need for pocketing the block as the valve is moved far enough away from the block to touch anything. A pocket is a shape which is machined into the block by a machine shop not something that you would do at home without proper equipment.

Hope this helps Tom.

 

Andy.

[timmy850]

There's a very similar thread here from last week.
http://www.theminifo.../310462-12g295/