The good old 948cc 'A' Series

[philthehill]

For those who consider that the market price for a 1275cc engine whether it be the Marina/Ital or Midget/Sprite to be going through the roof it may be time to return to the good old 948cc but in an improved form.
There are plenty of cheap but useable spares out there ('e' Bay is a good source) to make the 948cc engine a bit more lively without loosing the flexibility or tractability of the original engine.
There is also plenty of information on the web and in old magazines like Car and Car Conversions regarding the tuning of 948cc Minor engines from lightly modified to full race.
I ran a 948cc bored to 998cc for many a mile and it was not slow in fact it was dam quick and the only visual difference was that it was fitted with a modified H4 on a Janspeed manifold.
Any thoughts out there on the above.

[panky]

There are plenty of quick 998 Minis out there so why not and I'm sure there where many tuned Minors around using the same ideas, we just dont seem to hear about them. It's a shame the 1098 has it's limitations otherwise I would be tempted to take it further.
You do realise you've just doubled the asking price for 948 tuning bits

[philthehill]

The only limitation with the 1098cc engine is the crankshaft.
It is possible with a little bit of machining to put a 1275cc Midget/Crankshaft into a 1098cc making it into a near 10CC prefix engine.

[panky]

Hmmm, spare engine in the shed

[RobThomas]

Phil. You'll know this. Can a 10CC crank go into a 1275 block?




So, 948 bottom end, 12G295 head and a Shorrocks? How long do 948 crankshafts last under 'trying' condition?

[philthehill]

Rob
A 10CC crank should fit into a 1275cc block as the main bearings are 2.00" for both the 10CC and 12V engines. You will have to fit the appropriate con rods though.
The machining required of a 10cc block to fit a 1275cc crankshaft is to reduce the width of the centre main web. So to fit a 10CC crankshaft into a 1275cc block will require thicker crankshaft thrust washers.

As regards the 948cc crankshaft - so long as it has been balanced, tuft-rided and cross drilled it will last very well even under trying conditions.
I never had any problems with my 948cc (998cc) crankshaft and that was used in sprints and speed hill climbs for many years until the class size changed to 1275cc (front and rear wheel drive) when I increased the engine size to 1275cc.
Having the supercharger is kinder to the engine even though the power is increased.
There are many instances of the good old 948cc fitted with supercharger being used in competition.
Below is my 948cc (998CC) fitted with 12g295 modified head 2A948 (997cc Cooper) camshaft and twin HD4 1.5" SU carbs etc etc and the crankshaft never gave any trouble.
Phil[frame][/frame]

[RobThomas]

Thanks, Phil.

Any ideas what it costs to get a crank and flywheel prepared that way?

[philthehill]

Rob
I cannot give a precise price but I would expect to pay at least a £300 plus to cross drill, balance (that includes:- crankshaft, flywheel, clutch cover and front pulley/timing gears) tuft riding the crankshaft should come last just in case any metal has to be removed to balance.
The rods and pistons should be done at the same time as well which would come at extra cost - you may well be able to do the rods and pistons yourself which involves getting the pistons all the same weight and the rods the same weight at the big and small ends.
Using a 1/2 gram digital scale is more than adequate.

Whilst the above may seem excessive against the cost of a base 1275cc engine - the 1275cc engine to give good running and longevity should have the same tasks carried out on it.
Phil[frame][/frame]

[amgrave]

As a matter of interest Phil what does "cross drilling the crankshaft" refer to and what does it achieve.

[liammonty]

Hi Phil,

I agree, though i suppose the cost of modifying a 948 can end up being more than if you are lucky enough to source a 1275 in good running order (which seems increasingly unlikely these days).

I wanted to keep the 948 in my wife's 1962 saloon, so decided to do a bit of work on it when it needed a rebuild in order to liven it up (the 948s are so much sweeter than the 1098s it seemed a shame to change it). We ended up boring it out to 1030 cc (998 + 40 thou) and using Nuralite flat-top pistons, which helped bring the compression up with the 12G295 head. I've got an AC Dodd 'RT' camshaft (the 'softest' he does, as I wanted to retain some of the already scant low-down torque) - this works extremely well and I would recommend it in that kind of spec engine. I previosuly tried an MG Metro cam, but it didn't produce enough low down torque. I have a Maniflow small bore LCB and exhaust, having previously tried a Marina manifold and freeflow system from the Birmingham Morris Minor Centre. The Maniflow system transformed the engine, despite the cam being tame. On the induction side, I've got an HIF 38 - I forget what needle, but it's immaterial, as it had to be modified.

It has resulted in an engine which is sweet like the original 948, but manages around 55 bhp if the rolling road I used is to be believed, which is a good step up from the original 37, and also more than a standard 1098. the cars runs on a 4.22:1 1098 diff with ease, and is good fun to drive. The cost of the machining work (including line boring the 948 block for cam bearings as well as balancing etc.) together with the cost of decent quality parts added up though. I'm very happy with the result though!

[philthehill]

Cross drilling the crankshaft journal is done to ensure that no part of the journal is starved of lubrication.
At high rpm the centrifugal force acting on the oil throws the oil off the journal too quickly; resulting in a bearing having reduced lubrication which could lead to bearing/journal failure. Having the cross drilled gallery reduces the chances of that happening.

[philthehill]

liammonty
I agree with you in that finding a good 1275cc engine that you do not have to spend money on once you have purchased it is now getting difficult.
As regards cost re modifying the 948cc it all depends on what is want to be achieved.
A full blown 998cc engine will cost a fortune to put together but there are lots of things that can be done to the basic 948cc to improve its power without breaking the bank.
998cc engine spares seem to be plentiful and cheap on the likes of 'e' bay.
The Cars and Car Conversions booklets on Tuning The Mini and Tuning BMC Sports cars will give plenty of ideas and information on what can be done at very little cost.
The biggest cost to bring the engine size up to 998cc plus is the re-bore and pistons/rings though good S/H 998cc flat top pistons do sometimes come up for sale on 'e' bay. I recently paid £25 for a very good set of BMC 998c flat topped pistons which I will fit new rings to before use.
(Note: I am happy to fit good (and they must be good) S/H pistons into new bores but not S/H pistons/Rings into used bores)
I also purchased a good set of fully floating rods (Minor 1098cc/Cooper 998cc) for £8.50 for use with the above pistons.
The 'A' Plus 998cc gudgen pins are a press fit into the rods and whilst they are a better set up it needs special tools to remove and fit the pistons from the rods. (it is important that the rods are not heated to allow fitment of the gudgen pin as the heating can effect the metal composition).
The standard 1275cc Marina/Ital/Sprite/Midget camshaft will give a noticeable improvement in breathing and can be purchased S/H very cheaply.
Only if you increase the strength of the valve springs would the plain (block metal) camshaft bearings have to be supplemented with white metal ones
A 948cc engine can be improved in power at little cost and still look original.

[firehor5e]

Just got the bill for my 1275 rebore,new pistons/rings .re face block and head,regrind crank,new big and mains.unleaded conversion.Best part of £700. then gasket set,studs,engine and gbox mounts oil etc...No winter holiday abroad for me this year then!

[olderisbetter]

firehorSe, £700 is alot but i guess machining being a specialist job has to be right, and you can always get a warm of the engine when its running.

[ampwhu]

interesting subject this. I changed the 803 in mine to a 948 a few years ago. what I have is a 1.25 single su on a standard exhaust.

plans are to fit a 1098 head to it. I have been told I need to skim 0.060 off the head for compression???? and then it will slot on nicely. I currently have a 4.5 diff but am changing to a 4.2. I also am having a 803 box fitted with 948 internals.

sounds good eh?

[philthehill]

The 948cc Minor cylinder heads either 2A628/12A1456/2A629 have a combustion chamber of 24.5cc.
The 1098cc Minor cylinder head 12G202 has a combustion chamber of 26.1cc.
The 12G202 cylinder head will fit straight on - whilst you are changing the head make sure that the oil feed gallery to the rocker shaft through No:1 rocker shaft pillar is drilled and clear especially if the later rocker shaft is fitted with the shaft locking peg in No:2 pillar
It will do no harm to have at least 0.030" skimmed off the face of the 12G202 head to keep the compression ratio similar.
Fitting the 12G202 head without skimming will reduce the compression ratio and any real benefit gained by fitting the 12G202 head will not be maximised.
If the 12G295 head is fitted that will require a 0.060" skim to keep the compression ratio similar.
Phil

[David W.]

Early in this thread, (Oct 16) Panky mentioned " It's a shame the 1098 has it's limitations."
How about a brief summary of those limitations, as I have two 1098's.
Thanks

[philthehill]

The limitation with the 1098cc (unless it is a 10CC prefixed engine) is the crankshaft.
It is recommended that revs are kept below 6000rpm if you want the crankshaft/engine to last.
You can help crankshaft/engine longevity by fitting a crankshaft damper.
If you want to tune a 1098cc engine the 10CC engine is the one to go for as it has a much stronger crankshaft and bigger 2" mains.
For normal use and slight performance enhancement the 1098cc engine is more than satisfactory but major performance enhancement is not to be recommended.

[David W.]

Thank you
I won't worry with my around town driving.
David

[edd_barker]

So if 6000 is the limit on 1098, what would you limit a 1275 to? Surely only a pure race engine is going to be run over that figure?

I can't get over 4000 revs any way, gearbox whines too much to bear!