17 replies to this topic

[mark the diver]

Is it possible to remove 80thou from a 12G940 Spi head without running into problems. Tried measuring down oil way hole under rocker pedestal but the hole doesn't go straight down so can't measure, thanks.

[nicklouse]

hole should go straight as they have not made drill bits that can do corners which is why you have the other drilling to meet up with this one. as to your question no idea.

[DeadSquare]

You are measuring down the wrong hole,  the hole you want is the one in the face of the head in the corner by the water pump bypass hose.

 

If you look at the corner of the head by that oil hole that matches the one bringing the oil up the block, about an 1/8 inch from the surface of the head, the Oil Gallery is cross drilled to the rocker pedestal, and plugged with a small brass plug.

 

This will tell you how close the cross drilling is to the surface of the head.

[ACDodd]

The oilway drilling only refers to the small bore head. The real question is here, why do you want to skim this massive amount from the head?

Ac

[mark the diver]

The head has had work done to the chambers and they now have a volume of 22.4ccs. I want to get a compression ration of 10 to 1 and doing the calculations means I need to get this down to about 16ccs. Measuring with a burette and it seems I need to remove about 80thou, if that is possible.

[DeadSquare]

Does anyone here know if "raised crown " pistons are still obtainable ?

 

Once the head (or block) is shaved, it introduces changes in the rocker angles.

[ACDodd]

Is this engine featuring 21251 pistons?

I would strongly recommend changing them if this is a performance build. Or limit the compression to 9.25:1 maximum.

Ac

[Retroman]

Looks like AC pressed repeat....

 

Raised crown pistons (raised D) have not been available for many years and never for a big bore block

 

Only fitted to the 998 Cooper Minis with the big 28.3 cc combustion chambers

 

Are you matching the cam to compression ?

[Spider]

Is it possible to remove 80thou from a 12G940 Spi head without running into problems. Tried measuring down oil way hole under rocker pedestal but the hole doesn't go straight down so can't measure, thanks.

 

Is this going on a small bore or big bore engine?

 

Flat Top Pistons are available for either and that's where I'd start, not only to raise the reduce to volume of the combustion space, but as a side benefit, they increase the squish area and you'll get slight better burning and power.

 

Beware skimming these heads as much as you are looking to do, the old 12G940 you could go to about 80 thou but the later SPI and MPI heads have a thinner deck, off hand and generally speaking, I think you're sensibly limited to about 40 thou with these.

[mark the diver]

Hi guys, thanks for the help. It is a big bore 1275 Spi unit. It is going to have a 266MD cam and be a road only car running on twin 1.25 S.U.s.  It is the standard low compression unit fitted with the dished pistons, which I think are a 9cc dish? The pistons come to 60thou shy of the top of the block and the work done to the head has giiven a chamber volume of 22.4cc From my calculations and measurements I came up with the 80thou skim to give a 10 to 1 C.R.  Any advice greatly appreciated if this sounds like a bad idea, or my calculations seem wrong. 

[Rorf]

Then you need to skim that amount off the block, not the head

 

Hi guys, thanks for the help. It is a big bore 1275 Spi unit. It is going to have a 266MD cam and be a road only car running on twin 1.25 S.U.s.  It is the standard low compression unit fitted with the dished pistons, which I think are a 9cc dish? The pistons come to 60thou shy of the top of the block and the work done to the head has giiven a chamber volume of 22.4cc From my calculations and measurements I came up with the 80thou skim to give a 10 to 1 C.R.  Any advice greatly appreciated if this sounds like a bad idea, or my calculations seem wrong. 

 

That is skim 60 thou off the block deck. Having a high CR on a road engine is not recommended; I would go with ACs figure.

[DeadSquare]

It all depends on the length of your piece of string.

 

In the far off days of 5* leaded petrol. many 'sporty' road going heads were sold as having a CR of 11:1,

 

Super for the boy-racer at the pub, but they came with a more substantial head gasket "to cope with the extra power", which reduced the CR to barely 10.5:1.

 

An engine can probably achieve its theoretical CR, as it is started. 

 

From then on it is down hill, what with friction through the air filter, the carb and negotiating the valves, the charge will not be at atmospheric pressure, before, in its nice new warm cylinder, and meeting the remains of the previous explosion, it expands ...................Need I go on? 

 

To know how much, how much of your swept volume of charge is compressed at the moment that the spark plug sparks, you need to be a very clever chap.

 

Why not make use of all that lovely volume in the head by using a supercharger to fill it ?????

[mark the diver]

A supercharger would be lovely, anybody care to donate one in my direction!

[nicklouse]

Then you need to skim that amount off the block, not the head

 

Hi guys, thanks for the help. It is a big bore 1275 Spi unit. It is going to have a 266MD cam and be a road only car running on twin 1.25 S.U.s.  It is the standard low compression unit fitted with the dished pistons, which I think are a 9cc dish? The pistons come to 60thou shy of the top of the block and the work done to the head has giiven a chamber volume of 22.4cc From my calculations and measurements I came up with the 80thou skim to give a 10 to 1 C.R.  Any advice greatly appreciated if this sounds like a bad idea, or my calculations seem wrong. 

 

That is skim 60 thou off the block deck. Having a high CR on a road engine is not recommended; I would go with ACs figure.

as above that is a block skim not a head skim. also why the twins a single HIF44 would be way better.

[Spider]

In regards to CR I prefer to work it out accurately by the DCR (Dynamic Compression Ratio) method.

 

For road engines, regardless of Camshafts and all else and for fuels 97 RON or better, a DCR of 8.4 is the number to aim for.

 

While that may at first glance appear low it could translate to static CR numbers of anything from 9:1 to 13:1 and be fine, though, generally shorter duration Cam need less CR, depending on how they are timed in and exactly how you'd like the engine to behave and perform. 

 

Oh and as an FYI, just about all the HC engines from BMC & Leyland all had DCRs between 8.4 and 8.6 from the 850 to the 'fire breathing' Cooper S engines from the 60's (from memory)