12G295 head on a 948 block

[dukeminor1971]

Hi - I'm looking to improve the performance of my 948 engine without too much modification. I've been offered a 12g295 cylinder head. It was recently rebuilt with 33mm inlet valves (std size is approx 31mm) and 25.6 exhaust (std size). It has a new set of double springs fitted (along with new valve guides/seals). With some skimming (60 thou off from the original 2.75" I believe) would it fit ok onto my 948 block?

[liammonty]

In a word, yes

Skimming that amount off it should bring the chamber size down to roughly that of the 948 head (24.5 cc) from the original 28.3 cc of the 12G295. I’d measure it to be sure once it’s skimmed - or get the machine shop to do it when they skim it. It will fit without skimming, of course, but it would reduce the compression ratio enough to outweigh any benefit over the standard head. I’ve got a 295 head on my 948 (which has been bored out to 1030 cc) and it’s great.

[dukeminor1971]

Thanks. That's made my mnd up. Will I be able to keep the same rockers/pushrods? I have a larger SU carburettor I could use, but I'm not sure whether to change the manifold - if so are there any other better BMC/Leyland manifolds that will fit a Moggy?

[liammonty]

Thanks. That's made my mnd up. Will I be able to keep the same rockers/pushrods? I have a larger SU carburettor I could use, but I'm not sure whether to change the manifold - if so are there any other better BMC/Leyland manifolds that will fit a Moggy?

Yes - rockers and pushrods will be fine. Some people talk of shortening the pushrods slightly to preserve the valve train geometry on the skimmed head, but I found there was no need and was able to simply adjust the rocker clearances as normal and the valve train geometry is fine. So all standard. Re manifolds, it’s worth changing them to get the benefit of the carb and head - I used a Minispares (heated) inlet manifold with my HIF 38. For the exhaust, I’ve now got a Maniflow LCB and exhaust which is excellent, but I had a BL alternative before - a Marina exhaust manifold (I cut the inlet section of it off) - you can use that with a larger bore exhaust and it’s a cheaper alternative than an LCB. I’ve got the manifold kicking around still so let me know if you decide to go down that route and want one! Alternatively you could just use a standard combined inlet and exhaust manifold from a Marina if you could find one.

If you like books, get a copy of David Vizard’s ‘Tuning the A-Series’ - it’s packed with useful information.

[philthehill]

As regards the rocker gear geometry - if you skim the 12G295 head (or 940 head for that matter) I would recommend that you place a set of rocker pedestal top plates Pt No: 2A515 under the pedestals. This goes most if not all of the way to restoring the geometry. You may have to lightly trim one or more corner(s) off the plates to fit. This is a well used way of keeping the correct rocker geometry. There is no need to shorten the push rods or carry out any other mods.
Below are the plates mentioned above - L to R ....standard 2A515 plate, modified plate, modified plate. The amount removed from the corner(s) depends on the fitment and can be determined by trial and error.

Rocker piller plate.JPG (1.54 MiB) Viewed 1914 times

Whilst talking about the 2A515 plates - a useful mod to the plate holding the rocker shaft retaining screw is that it can be cut back to exclude the adjacent 3/8" head stud which then allows for the removal of the rocker shaft and replacement of the rocker shaft, rockers, push rods and cam followers without disturbing the head gasket. BMC incorporated this mod into later engines and the modified plate used is Pt No: 2A259.

If you have alloy rocker pedestals ditch them and replace with either forged or sintered items. The alloy pedestals squash and head gasket torque cannot be maintained.
Phil

[dukeminor1971]

Thanks for this information - especially the photo which shows exactly what I need to do.

[dukeminor1971]

Thanks. That's made my mnd up. Will I be able to keep the same rockers/pushrods? I have a larger SU carburettor I could use, but I'm not sure whether to change the manifold - if so are there any other better BMC/Leyland manifolds that will fit a Moggy?

Yes - rockers and pushrods will be fine. Some people talk of shortening the pushrods slightly to preserve the valve train geometry on the skimmed head, but I found there was no need and was able to simply adjust the rocker clearances as normal and the valve train geometry is fine. So all standard. Re manifolds, it’s worth changing them to get the benefit of the carb and head - I used a Minispares (heated) inlet manifold with my HIF 38. For the exhaust, I’ve now got a Maniflow LCB and exhaust which is excellent, but I had a BL alternative before - a Marina exhaust manifold (I cut the inlet section of it off) - you can use that with a larger bore exhaust and it’s a cheaper alternative than an LCB. I’ve got the manifold kicking around still so let me know if you decide to go down that route and want one! Alternatively you could just use a standard combined inlet and exhaust manifold from a Marina if you could find one.

If you like books, get a copy of David Vizard’s ‘Tuning the A-Series’ - it’s packed with useful information.

Thanks for your help. I have a copy of Vizard's book but it seems to be very technical, for what I want. I prefer the 948 to the 1098 engine and just want to increase it's performance a bit (particularly if I'm carrying passengers). It sounds like a standard MM exhaust won't fit onto a Marina manifold - do you know of any companies selling LCB exhausts that would fit onto one? Out of interest how much do you want for your Marina manifold?

[liammonty]

Agreed re the 948 vs. the 1098 - it’s a much sweeter engine.

I used a down pipe from the then Morris Minor Centre in Birmingham (now David Manners) who made a large bore one to fit the Marina manifold. They also did a free flow exhaust to match. I see ESM do a down pipe and exhaust system to match too. The term ‘LCB’ refers to something different - the ‘Long centre branch’ manifold, which is a manifold and down pipe in one, essentially, and while they’re better for all out performance with certain longer duration cams, they are pretty pricey too. Hope this makes sense!

I’d be happy to let my manifold go for £25 plus postage if you can put it to good use.

[philthehill]

If you are finding the Vizard book a bit technical I would recommend a book by Clive Tricky titled 'Tuning the Classic Mini'
It can be obtained as :-
'Tuning the Classic Mini' & 'More Classic Mini Tuning'.

The combined edition is ISBN 0 947981 61 6.

It covers more basic tuning of the 'A' Series.
Clive Tricky was an amateur tuner and racing driver and an avid contributor to the well known Cars and Car Conversion magazine back in the 1960s/1970s.
Well worth a read if nothing else.

Phil

[dukeminor1971]

If you are finding the Vizard book a bit technical I would recommend a book by Clive Tricky titled 'Tuning the Classic Mini'
It can be obtained as :-
'Tuning the Classic Mini' & 'More Classic Mini Tuning'.

The combined edition is ISBN 0 947981 61 6.

It covers more basic tuning of the 'A' Series.
Clive Tricky was an amateur tuner and racing driver and an avid contributor to the well known Cars and Car Conversion magazine back in the 1960s/1970s.
Well worth a read if nothing else.

Phil

Thanks - I'll give that a go.

[dukeminor1971]

Agreed re the 948 vs. the 1098 - it’s a much sweeter engine.

I used a down pipe from the then Morris Minor Centre in Birmingham (now David Manners) who made a large bore one to fit the Marina manifold. They also did a free flow exhaust to match. I see ESM do a down pipe and exhaust system to match too. The term ‘LCB’ refers to something different - the ‘Long centre branch’ manifold, which is a manifold and down pipe in one, essentially, and while they’re better for all out performance with certain longer duration cams, they are pretty pricey too. Hope this makes sense!

I’d be happy to let my manifold go for £25 plus postage if you can put it to good use.

I've seen the systems by ESM and an alloy Mini inlet - both of which would fit your Marina manifold - so I'd definitely like to give it go. How do we go about arranging payment via this forum?

[liammonty]

I’ll send you a PM

[RobThomas]

Mini MPI/SPI exhaust manifold looked an interesting option. I bought one with all the downpipes still on. Runs too close to the block so would need the tappet covers trimmed down a bit but gives large posrts, LCB (sort of) and a Lambda sensor boss. Cost £18, inc postage.

12G787 was the Mini manifold to fit the Morris exhaust bit the 1.5 HS4 with diagonal studs, IIRC. https://www.ebay.co.uk/itm/1-1-2-su-car ... Swo7Zeqa-L


Here's the MPI manifold. Just need to cut-n-shut the pipes round the corner for the Minor.

mpi.jpg (673.43 KiB) Viewed 1797 times

[RobThomas]

Mini MPI exhaust manifold fitted to a 12G295 (at last)

blo9.jpg (69.84 KiB) Viewed 1620 times

blo2.jpg (279.66 KiB) Viewed 1620 times

[ampwhu]

I like that. some more info on the other components would be great

[RobThomas]

948 Vedette Engine block and crank, early Mini tappet covers, A+ timing gear with duplex chain, 2-ring (2A948?) 1100 camshaft, 12G295 head, 1275 head studs etc