25 replies to this topic

[stickycreambun]

Ok so I have a replacement A(not +) series 998 to fit in the mini. From the Guessworks list it is a '74 on engine.

I am aware there are three types of cam/oil pump drives, early models were spider drive, pin drive in the middle, later ones (all A+?) slot drive - is this about right? Happy to be corrected or told specific dates.

1) will the cam in my A 998 be a slot or pin drive?
2) is it only the cam drive and oil pump that are need to match? Or put it another way, as I plan on fitting a new oil pump could I pop in the MG cam, and a slot drive pump or is that too simple and do other things like the dizzy mean it needs to stay as is?

Thanks guys

[Cooperman]

Ok so I have a replacement A(not +) series 998 to fit in the mini. From the Guessworks list it is a '74 on engine.

I am aware there are three types of cam/oil pump drives, early models were spider drive, pin drive in the middle, later ones (all A+?) slot drive - is this about right? Happy to be corrected or told specific dates.

1) will the cam in my A 998 be a slot or pin drive?
2) is it only the cam drive and oil pump that are need to match? Or put it another way, as I plan on fitting a new oil pump could I pop in the MG cam, and a slot drive pump or is that too simple and do other things like the dizzy mean it needs to stay as is?

Thanks guys

So long as the cam drive and oil pump driven ends match it will be fine. An MG Metro cam - probably the finest production cam for the A-series - will be slot drive, so get a new 998 slot drive oil pump. Dizzy drive will be fine as will the front end timing gears.
Don't forget to get new cam followers when changing the cam.

[stickycreambun]

Woohoo!

Excellent thanks, just what I wanted to hear! As I have the cam, and was going to fit the new pump thought for the cost of some new followers be silly not too.

[Cooperman]

That's a really nice cam in a 998, especially if you have a head with slightly better gas-flow, higher comp. ratio and possibly larger inlet valves.
You can do the head as you go along if cash is a bit tight.

[stickycreambun]

well yes hopefully something half decent for a sensible price will come along before I fit the engine, just be simpler to do it all in one go then tune it to that.

[Cooperman]

I'm sure you already know, but the best head for a 998 is the 12G295 casting as used on the 998 Cooper. You may find one for a reasonable price, but you need to allow for it to be skimmed as the 998 Cooper had raised top pistons and if just fitted to an ordinary 998 the compression ratio would be very low. Measurement and calculation is needed to get this right.

[bmcecosse]

Just for info - the 998/1098 engines NEVER had the spider drive oil pump - they were only for the 1275 (and yes 970/1071S) engines. I much prefer the 940 head on a 998 engine - cheap to buy and no skimming required. You may have to sink the exhaust valves into the head by up to 40 thou - but that's easy/inexpensive to do, and doesn't damage the block. Measure the clearance first!

Edited by bmcecosse, 22 June 2011 - 04:46 PM.

[Cooperman]

Just for info - the 998/1098 engines NEVER had the spider drive oil pump - they were only for the 1275 (and yes 970/1071S) engines. I much prefer the 940 head on a 998 engine - cheap to buy and no skimming required. You may have to sink the exhaust valves into the head by up to 40 thou - but that's easy/inexpensive to do, and doesn't damage the block. Measure the clearance first!

Is that OK with the MG cam? I haven't got the lift figures to hand to compare it with the standard 998 cam but I'm sure you are right.
I think the 970 and 1071 (and Mk.1 1275 'S') cars had that nasty pin drive which could (and did) snap if started from cold with a sronger relief valve spring fitted. I bet you'll remember that rom our 'early days'!

[jaydee]

Rob walker in Witney does cylinder heads modifications for really cheap prices.
The exhaust lift o the MG metro cam should be 0,313" with std rockers while the standard 998 is 0,294"

[Cooperman]

Rob walker in Witney does cylinder heads modifications for really cheap prices.
The exhaust lift o the MG metro cam should be 0,313" with std rockers while the standard 998 is 0,294"

Rob Walker's work is absolutely top quality. Highly recommended.

[stickycreambun]

Yup I have my eye on a 295 head.. I had considered the 940 route, but would rather just stick to what is a definate.
Speaking of the heads, is there a measurement figure of a 295 head from the factory? Or put it another way, is there a figure for it when it is no longer able to be skimmed? The engine I have has dished pistons so would need a fair bit to raise the CR.

Thanks for the help

[Cooperman]

Yup I have my eye on a 295 head.. I had considered the 940 route, but would rather just stick to what is a definate.
Speaking of the heads, is there a measurement figure of a 295 head from the factory? Or put it another way, is there a figure for it when it is no longer able to be skimmed? The engine I have has dished pistons so would need a fair bit to raise the CR.

Thanks for the help

There is an oil feed hole which goes downwards from the top of the 295 head. You measure the depth of this hole with a digital vernier and subtract the depth from the thickness of the head as you also measure that. The difference should not be allowed to go below 0.050", although i've known it to go to 0.040". The problem now is that most 295's have been skimmed once or twice during the past 40 years, so standard it probably won't be.
It is almost always possible to get an acceptable C.R. even on a 998 with dished pistons. I once skimmed 0.070" from a 295 to get 10:1 CR.

[Cooperman]

Just measured a very original 12G295 which I have. The hole depth is 2.592" and the head thickness is 2.717". I don't think the head has ever been skimmed. So there is a thickness of 0.125" (=1/8") between hole base and head face. Thus a safe maximum of 0.075" could be machined from this head with safety to leave 0.050".

[Cooperman]

Just checked the paperwork for a 1061 cc engine (998 + 0.060") which I built 3 years ago. The hole depth was 2.56" and the head thickness was 2.65" after machining. I had chambered the head to suit and it had flat-top pistons, so it did not need too much skimming to get to a 10.2:1 C.R.
I don't have the paperwork now, but my son and I did a 998 historic rally car a few years back so it was only bored +0.020" and I do remember the that there was 0.0456" between hole base and head face and that was as little as I ever wanted to go down to.

[MiniLuke]

Ebay linky

Buy these, no skimming required. (OR find some that are sensibly priced!)