5 replies to this topic

[octagonalpaul]

1988 Mini City E
998 engine - stage 1

Hi, just wanted to know a few things about upgrading my cylinder head. I got a 12g940 head in really good condition recently fairly cheap - will it go straght onto my 998 and is it advisable? I've read before that it's ok but the exhaust valves can come into contact with the engine?
What sort of performance/driveabilty increase can I expect?
Once fitted, is there anything else that needs to be done like running it in or similar?
Also will it be ok to use the previous standard 998 heads rockers or springs with the new head (they are in good condition)

thanks in advance - Paul

[Pooky]

I have done this conversion adn would recommend it as a fairly easy mod. You won't get the most out of it without fitting a better cam though.

The most common method to obtain valve clearance is by the machining of 'pockets' in the face of the block where the exhaust valves would otherwise make contact. This is really a job for an engineer so would require the engine to be taken out (perfect time to change the cam) but some people have done this job themselves with a Dremel.

The other method is to 'sink' the valves into the head. I've never done this so you'd have to ask elsewhere about this method.

I am using the same rockers that came off my 998 head.

You'll also need to modify your water pump, you'll need to file a few mm off the top as the 12G940 won't sit on it without doing this. Then you'll need a small diameter water pump pulley as the bid one fouls the head. you can use the existing thermostat housing but you'll need a 1275 top hose and radiator mount.

I'd highly recommend the special MRA Minis head gasket. This gasket is custom made for this conversion and ensures that you're not over lapping waterways as you would with a 1275 head gasket.

[octagonalpaul]

thanks, thats really useful!
seems like more than I thought to get it working though

[bmcecosse]

You need to measure the space between exhaust valve head and face of the head. If you have 320 thou - then no need to do anything, assuming a standard cam. If any less - just sink the valves into the head until there IS 320 thou! It took ~ 10 minutes to do mine with a pillar drill and a 45 degree countersink. You will need to re-align the rockers so they act DIRECTLY over the tips of the valves - if using the ghastly 'sintered' rockers you may get away without doing anything - but the 'pressed steel' rockers are much more reliable. The rest of the conversion is easy - and the performance gain is very significant - best BHP for £ you can do! I just used standard 1275 gasket - no problems.

[l_jonez]

ive not had any issues with a standard 1275 gasket, every thing lines up ok

[Azbornmini]

Here is an 1100 block that I did years ago. Fitted a 940 head that was shaved down and ported with reshaped chambers.