uprating 948 engine

Post by **RogerRust** » Fri Jul 21, 2006 6:33 pm

If I can get one how about 12g295 head and hif38 carb? and 256 or 266 cam.
Alternativley can I fit a 12g940 with pocketed valves?
If so how much do I need to get the valves recessed?

I've been reading Vizard and it seems that a 12g295 will need a big skim to get the compression back, thats why I wondered about 12g940 which will has a lower volume than the std head so will up the compression.

This lookes like a reasonable upgrade or should I go to 1098 with the above mods.

None of the above is very expensive so should be able to slip it in during the rebuild.

Can anyone give me any advise on this upgrade?

Post by **Peetee** » Fri Jul 21, 2006 8:03 pm

948c engine with:
12G295 head
alloy inlet maifold
HIF38
standard manifold
stainless standard bore exhaust

0-60 = 18 seconds (standard 30 seconds). Holds 70 on hills with throttle to spare.
Very smooth willing revy engine with good mid range umph. Bcause of this I would recommend not fitting a fast road cam initially because of the extra work involved.

Post by **bmcecosse** » Sat Jul 22, 2006 9:55 am

The cam in the 948 is very soft - even going up to the AEA630 cam from a 1098 will give good benefit. The 948 block has no cam bearings on the middle and rear sections - so do not fit anything hotter than the 630 cam - or strong valve springs! 1098 engine is better bet - mine has 940 head with 38 carb - goes like the wind!! Exhaust valves need to be sunk in the head by 40 thou - or make pockets in the block under the exhausts.

Post by **RogerRust** » Sat Jul 22, 2006 11:21 am

Thanks for the info BMC I'm sure a 1098 cam will less expensive and the 940 heads are easier to find Imm (40 thou) I'll remember that.

Is it possible to measure the lift on the cam and relate that to the valve lift -then all I'd need to do is measure the depth of the combustion chamber to ensure all was ok?

Post by **bmcecosse** » Sat Jul 22, 2006 5:11 pm

Yes - thats exactly what I did - and on the AEA630 cam the exhaust valves will hit the block by ~ 20 thou. The older 948 engine cam has slightly less valve lift and so you 'may' be able to 'get away with it' and not sink the valves. To be honest - the better job is to make pockets in the block - and if the engine is in bits this is easily done. It's not so easy in situ - and i didn't want to mess the block up anyway. The 940 head I used cost all of £5 and had 'worn' exhaust seats anyway - so I used a large 45 degree countersink in a pillar drill - let this self-align on the old seat and then set the top stop to give 40 thou travel - and slowly let the countersink cut the seat down. It made a fabulous job of the seats - only the slightest of hand grinding with fine paste was needed to get excellent grey seat all round each valve. I then 'unpocketed' the sunken seat using a v small grindstone in a hand held leccy drill. The head i took off was a well modified 295 head - the car goes very much better on the 940 head.

Post by **RogerRust** » Sat Jul 22, 2006 6:45 pm

ok 12g940 it will be then, but I've noticed as soon as you want one they all dissapear. Oh well no hurry.

Post by **bmcecosse** » Sat Jul 22, 2006 9:44 pm

Run a search under '1275' - they pop up all over the place with various descriptions. I would avoid the MG version which had bigger inlet valves - a wee bit too big unless going for 'full race' conversion!

Post by **rayofleamington** » Sun Jul 23, 2006 10:40 pm

Alternativley can I fit a 12g940 with pocketed valves?

On the Africa car I used the 12G940 on a 948 engine, together with a HS4 and marina manifold and large bore van exhaust.
If I'd had time to play with dozens of needles it would have been a dream but unfortunately with 3 needles to choose from I never got her running right. Cam estimated a new needle profile and that ran like a rocket when cold (with no choke) but far too rich when warmed up

After that I was 'on the ferry' anyway so we just chose the leanest needle for good mpg. In the soft sand the engine got a good thrashing and the extra revs/power may have been what kept me from needing to be dug out of the sand.

For headgaskets I forget which I used (I got 1275 and 948 to see which lined up on the waterjacket holes) - there was one recommended by you know who, but that didn't work and the other one was fine.

Post by **bmcecosse** » Mon Jul 24, 2006 7:25 am

You MUST use the 1275 head gasket with the 940 head. May also need to file a bit off the water pump top edge to allow the head to sit right down - and if you are using your existing rocker shaft and rockers then you will need to spend some time re-aligning them so they operate squarely over the valves in the 940 head. If you get a rocker shadft with the head - with the 'modern' sintered rockers - then these are wide enough to straddle across the new valve position. They are NOT the best rockers around - but perfectly ok for a road car. There is one water way towards the back of the 1098 block which has no matching hole in the gasket or in the 940 head. In the past I have left this open - but last time I did this I closed it with a blob of silicon sealer, just to be sure it wouldn't start to weep across the gasket face.

Post by **rayofleamington** » Mon Jul 24, 2006 12:42 pm

You MUST use the 1275 head gasket with the 940 head.

Yeah that'd be it - it let the water jacket holes sit open.
I tried that first, as suggested, then used the one that worked ;-)

Post by **bmcecosse** » Mon Jul 24, 2006 1:11 pm

But there is no matching hole in the head (I suppose you could drill one - did you ??) - and if you use the small bore gasket the edges of the gasket will not be supported by the head in the combustion chambers - and so I would say there will be high risk of the gasket burning. If you used small bore gasket in the engine to Africa I would say you were lucky to get away with it - and you obviously did! ALL the Mini lads use the big bore gasket with the big bore head - as indeed did I some 40 years ago when I did this for the first time putting an S head on a 998 engine for my race Mini.

Post by **pskipper** » Mon Jul 24, 2006 3:02 pm

948c engine with:
12G295 head
alloy inlet maifold
HIF38
standard manifold
stainless standard bore exhaust

So Peetee, did the head need a big skim and if so how much, Rosies currently running on a 948cc engine but once her original engine is reconditioned I have plans for it

Post by **Peetee** » Mon Jul 24, 2006 8:43 pm

So Peetee, did the head need a big skim and if so how much

60 thou took it to near to standard compression.

Post by **rayofleamington** » Mon Jul 24, 2006 9:31 pm

indeed did I some 40 years ago when I did this for the first time putting an S head on a 998 engine for my race Mini.

Which gasket have you used on the Minor 948 block with 12G940 head?

But there is no matching hole in the head

What hole and is this relating to the 998 Mini engine?
I didn't need to drill any holes when doing this on my Minor 948 engine

Regarding the compression ring in the head gasket, that is a valid point - a 'true' 948 cc gasket may probably sit too far in to be clamped properly by a 12G940 head. I used a generic Minor head gasket (fits small block A series engines - also fits 1098) and it was obvious that the compression ring would be clamped properly - I checked.

I would say you were lucky to get away with it

I would not have fitted a head gasket without making a good visual inspection of the obvious things like 'do the compression rings get clamped by the head' (I find that comment a little offensive but I'm sure that wasn't your intention). When I was young and stupid I would have done these things based on anecdotal advice without understanding the consequences... and then learned from it.

Head gaskets for the 948 minor block / 12G 940 head combo are definately not something for anecdotal advice. I first tried this 15 years ago when I was a bit more green, and followed the 'usual advice' only to find the water jacket didn't seal. The head had been used without problems on a 1098 so I thought it would be fine on a 948...

With the advice over the last few years that 'the Mini boys are always doing this' I gave it another try but certainly not without a proper inspection of how/where the headgasket fits (or actually doesn't fit - unlike what is often advised by people who have info second hand).

Next time I get a 948 engine apart I will try and remember to take photos of the various gasket missalignment issues - ready for the next time somebody asks. ;-)

Post by **bmcecosse** » Mon Jul 24, 2006 10:26 pm

No need to get on your high horse Ray ! We can surely have a sensible discussion about this without it getting personal.
The 1275 gasket IS the one to use - I have just checked with 3 small bore gaskets from my stock on a standard 940 head and they all overhang the combustion chamber at the exhaust valves by a significant amount - you must have got hold of a very unusual gasket if it fitted without this. So I stand by my advice to use the 1275 gasket. Whatever you used - it fitted for you and it was a success. If you can identify that gasket - then pass the info on to others - but the small bore ones I have checked should not be used with a 940 head. It's a long time since I played with a 948 engine - but the 1098 certainly has a water hole near the back which is provisoned for in the small bore gasket (and so is almost certainly present in the 948 too), but is not present in the large bore head or gasket. In the past I just used the 1275 gasket as is - in this case I decided to add a wee blob of silicone sealer just to make sure. The engine has had several good long hard thrashings in this form - in hot weather - and so far hasn't run hot or lost a drop of water.

Post by **rayofleamington** » Mon Jul 24, 2006 11:55 pm

We can surely have a sensible discussion about this without it getting personal.

yeah hopefully - like I said, I didn't think you meant it that way on purpose ;-)

Post by **bmcecosse** » Tue Jul 25, 2006 7:26 am

Ray - I have to point out you started all this by suggesting I gave incorrect advice. I did not - you must use a 1275 gasket with a 940 head. It doesn't matter what block it is going on - be it 948/998/1098 - these all use the same 'small bore' gasket which has slighly egg shaped openings in the gasket for the combustion chambers. The 940 head has wider combustion chambers at the exhaust valves and a small gasket overhangs into the chamber and will very quickly be destroyed. the 1275 gasket has round openings to suit the wider combustion chambers. I can only surmise that you either did not use a 940 head (but you are sure you did) or the 'aftermarket' gasket was in fact a 1275 gasket. If you had problems sealing the water ways - this could have been due to the 940 head sitting on the edge of the water pump, which usually needs filing down to let the head sit right down squarely on the block.
Whatever happened - it's importamt for anyone doing this conversion to know that you MUST use a 1275 head gasket - you must check water pump clearance - you must use rockers that correctly align with the valves - and unless using an early 'low lift' camshaft, you must either grind little pockets in the block under the exhaust valves OR sink the exhaust valves into the head by 40 thou.
Don't let any of this discussion put any of you off - this is a great conversion for a small bore engine at very low cost and gives a big boost in engine power right through the rev range when used with a decent alloy inlet manifold and a 1.5" carb.

Post by **rayofleamington** » Tue Jul 25, 2006 12:20 pm

I have to point out you started all this by suggesting I gave incorrect advice. I did not - you must use a 1275 gasket with a 940 head.

I stand by my comments as they are tried and tested - experience not conjecture from a different engine.
Please feel free to try this combination yourself on a Minor 948 block, and I expect that you will also find a leaking water jacket as the 1275 gasket is not compatible for the waterjacket holes in a 948 block.

This was not caused by the head sitting on the water pump (especially as the water pump was fitted afterwards).
When I did a line up test with the 1275 gasket on 948 block it was clear that one of the waterways was barely sealed at all.

Don't let any of this discussion put any of you off - this is a great conversion for a small bore engine at very low cost and gives a big boost in engine power right through the rev range

Completely agree! The 948 engine is lovely and smooth and plenty of power when used with a big valve head.

If you get a high mileage 940 head that has had a couple of vlave grinds, you may well find that the valves are already recessed enough. This needs to be measured but that is simple enough and as mentioned already - there are easy ways to rectify it.
The valves in my head didn't need to be sunk because they had been used and ground enough over the last 40 years

Post by **bmcecosse** » Tue Jul 25, 2006 1:15 pm

The 948 block is no different to the 848/998/1098 as far as waterways are concerned - they all use the same 'small bore' head gasket. Yes - it's a close call on waterways with the 1275 gasket - and maybe some makes will be better than others - but you cannot use a small bore gasket without it overhanging the combustion chambers.
This whole conversion works even better on the bigger 1098 engine - which benefits even more with the breathing boost. And if you are lucky the 940 head may even be unleaded - for ~ £10 !!

Post by **chrisd87** » Tue Jul 25, 2006 3:08 pm

But surely if you have to sink the exhaust valves on an unleaded 940 head by 40 thou, that's going to cut away at the hardened seat and possibly 're-lead' the head?

I've just checked vizard's book and he says:
"Incidentally, always use the gasket which corresponds to the cylinder head in use, not the block."
I must admit I've never tried it though.