15 replies to this topic

[NDT]

850 cranks (particularly the thin nose ones) supposedly fail with high rpm use.

This is despite them having a relative short stroke and a nice overlap between the big end pins and mains, which should make them relatively stiff.

Does anyone know whether a decent crankshaft damper improves things? I don't think these were fitted on the 850?

 

[DeadSquare]

"High rpm use" is a bit vague;  would that be sustained of peak ?

 

In 1960, with Formula Junior double valve springs, early thin tail cranks were revving to 7,000 rpm through the gears, without a damper.

 

Even with a damper, a sustained 5,500 rpm would break 1100 cranks at 70mph on motorways, if an engine and box were dropped into a Mini without changing the 4.1  Diff to accommodate the 10" wheels.

 

Downton used to fit the 997 Cooper damper to their 850 racing engines with Tuftrided cranks, and I often got the needle past the 8,000 mark.

[Spider]

My 850 experience is quite limited.

Do you know where they fail ? This might show if their failure is from torsional vibrations or simply being too spindly.

 

The correct Damper can't hurt, but be aware, for conventional rubber mounted dampers, they are 'tuned' to suit a particular bottom end, they are not 'one size fits all'. I'd suggest if you were to try one, go for a fluid filled type (I think the KAD ones are) and that would negate the need for tuning it.

 

[NDT]

"High rpm use" is a bit vague;  would that be sustained of peak ?

 

In 1960, with Formula Junior double valve springs, early thin tail cranks were revving to 7,000 rpm through the gears, without a damper.

 

Even with a damper, a sustained 5,500 rpm would break 1100 cranks at 70mph on motorways, if an engine and box were dropped into a Mini without changing the 4.1  Diff to accommodate the 10" wheels.

 

Downton used to fit the 997 Cooper damper to their 850 racing engines with Tuftrided cranks, and I often got the needle past the 8,000 mark.

 

That's a great question.

In reality, it's for a road car, so we're not talking sustained high rpm use, just the occasional thrash to high revs.

I've just got my freshly painted 62 shell back and want to get the powertrain sorted.

I've got a 1275 engine ready to rebuild, but I have an urge to see how far the original 850 could be taken, maybe with a supercharger.

I know the 1275 option will be much much easier and cheaper, while giving me more power, but keeping the original engine and box could be much cooler.

[NDT]

My 850 experience is quite limited.

Do you know where they fail ? This might show if their failure is from torsional vibrations or simply being too spindly.

 

The correct Damper can't hurt, but be aware, for conventional rubber mounted dampers, they are 'tuned' to suit a particular bottom end, they are not 'one size fits all'. I'd suggest if you were to try one, go for a fluid filled type (I think the KAD ones are) and that would negate the need for tuning it.

 

My 850 experience is close to non-existent. I don't know where they fail - good question.

And a good point about damper tuning.

[ACDodd]

Spider is right here, the conventional dampers are tuned to work at the rpm you most need the effect, so fitting one from a different engine is not likely to achieve the effect you want, however after further reading it’s likely you won’t really need one for the use you intend.

Ac

[Cooperman]

If you fit a supercharger, then you won’t need high revs.
I am old and have used 850 Minis with modified engines in competition. With rally engines giving maybe 55-ish bhp they would rev to 6,500 rpm regularly with no problem. We didn’t fit different crank pulleys.
For full race 850’s I seem to remember that a special crank was available, but I could not have afforded one back then on apprentice wages!

[NDT]

Spider is right here, the conventional dampers are tuned to work at the rpm you most need the effect, so fitting one from a different engine is not likely to achieve the effect you want, however after further reading it’s likely you won’t really need one for the use you intend.

Ac

Thanks AC - that's really helpful.

[NDT]

If you fit a supercharger, then you won’t need high revs.
I am old and have used 850 Minis with modified engines in competition. With rally engines giving maybe 55-ish bhp they would rev to 6,500 rpm regularly with no problem. We didn’t fit different crank pulleys.
For full race 850’s I seem to remember that a special crank was available, but I could not have afforded one back then on apprentice wages!

Great, thanks.

I will get the engine out and inspect it at the weekend and start planning...

[Cooperman]

I once gave a lot of thought to fitting a Shorrock supercharger on my 850 Mini. I did the calculations, but then I got a much better paying job and bought myself a brand new 998 Cooper instead. That was in 1964!

[DeadSquare]

I ran my Shorrock on a host of A Series engines and the one that responded best was the 1100, easily topping a 1275 S.

 

An 850 needed MG 1100 inlet valves and Cam number 948.

 

For a brief period, Shorrock sold a very good inlet manifold, which meant replacing the exhaust manifold, but in those days, a 3 branch S exhaust was easy to buy and not too expensive.

[Cooperman]

I do seem to recall that a 997 Cooper cam was recommended and I guess a 12g295 head with chambers ground to give the correct volume.
But it was a long time ago!

[DeadSquare]

I do seem to recall that a 997 Cooper cam was recommended and I guess a 12g295 head with chambers ground to give the correct volume.
But it was a long time ago!

Because no insurance company would think to look for the modification, back in the day, I fitted a lot of Cooper 997 cams in 850s.  It took 1/2 a morning and when the guys collected it at lunch time, lots of them rang in the evening to say that it lifted the car from struggling to flowing.

 

Many came back the next month for double valve springs.

[Cooperman]

Of course, the main performance limiting thing was the poor combustion chamber and inlet/exhaust tracts.
I do recall an 850 with a 997 Cooper cam, a gas-flowed 12g202 head, 3- into-1 ex manifold and twin HS2 carbs. The head was skimmed and I think the CR was about 9.5:1.
It went well, but stopping it with the single leading front brakes was-interesting’ even with Mintex AM4 linings.

[DeadSquare]

 

I do seem to recall that a 997 Cooper cam was recommended and I guess a 12g295 head with chambers ground to give the correct volume.
But it was a long time ago!

Because no insurance company would think to look for the modification, back in the day, I fitted a lot of Cooper 997 cams in 850s.  It took 1/2 a morning and when the guys collected it at lunch time, lots of them rang in the evening to say that it lifted the car from struggling to flowing.

 

 

 

Many came back the next month for double valve springs.

 

I soldered a hexagonal nut to a bit of wire, so that I could poke it through the plug hole, raise the piston and with luck, hold the valve in place while I moved the rocker to removed the valve cap.