Cylinder Head Work

[Mini_Lund]

Hi there

I have just had a Cam 4180 (998) 'vanilla' head skimmed, as it looked to have been previously leaking on another mini.

Can someone let me know a little bit more about compression ratio's and if there is any performance increase with a skim? I dont know how much was taken off, but Im guessing as much as necessary and as little as possible.

What else can I do to get the best from my cylinder head? Can i sand the outlet ports with fine 1500 wet&dry? Or is it not that simple.

A little knowlegde would be greatful!

Thanks in advance

[Snake the Sniper]

Skimming on it's own will increase the CR, but how much depends upon the amount removed. It's generally not enough to make a big difference.

[Cooperman]

Those standard 998 heads really are not the best for any sort of performance.The best advice is to read either Vizards book or 'Tuning a Mini' by the late Clive Trickey. To do any sort of worthwhile improvement you really need a compressor and a die-grinder plus a good selection of grinding stones of a high quality. If you look directly down onto the face of the head, you'll see that the inlet valve is shrouded for about 170 degrees of its circumference by the head casting. This is a serious restriction of the ability of the inlet charge to flow into the head. The intrusion of the valve guide boss into the throat of the inlet port is the other limiting factor in inlet gas flow.
This is such a big issue with these heads that it's too much to go into in detail on here. My advice is to thoroughly read up on it, then come back here for specific advice.
With any 998, unless you want to have to machine the block itself, is to try to find a 12G295 cylinder head casting. This is the old Mini Cooper 998 casting. It's design is just so much better and by doing a bit of additional work you can really make it perform, especially if you fit a decent exhaust manifold (e.g. an LCB or original Cooper 3-branch) and an alloy inlet manifold.
In terms of skimming, with a 998 head you can easily remove over 0.025" without any problem, although whether you'll gain much just by increasing the comp ratio without gas flowing the head is another matter.
I hope this helps,

Peter

[Mini_Lund]

Cooperman said:

Those standard 998 heads really are not the best for any sort of performance.The best advice is to read either Vizards book or 'Tuning a Mini' by the late Clive Trickey. To do any sort of worthwhile improvement you really need a compressor and a die-grinder plus a good selection of grinding stones of a high quality. If you look directly down onto the face of the head, you'll see that the inlet valve is shrouded for about 170 degrees of its circumference by the head casting. This is a serious restriction of the ability of the inlet charge to flow into the head. The intrusion of the valve guide boss into the throat of the inlet port is the other limiting factor in inlet gas flow.
This is such a big issue with these heads that it's too much to go into in detail on here. My advice is to thoroughly read up on it, then come back here for specific advice.
With any 998, unless you want to have to machine the block itself, is to try to find a 12G295 cylinder head casting. This is the old Mini Cooper 998 casting. It's design is just so much better and by doing a bit of additional work you can really make it perform, especially if you fit a decent exhaust manifold (e.g. an LCB or original Cooper 3-branch) and an alloy inlet manifold.
In terms of skimming, with a 998 head you can easily remove over 0.025" without any problem, although whether you'll gain much just by increasing the comp ratio without gas flowing the head is another matter.
I hope this helps,

Peter

Okay thanks for the advice. I'll bolt it on as it is and use it for what its worth! Its unleaded, and is to go on my 850.

[Cooperman]

If you are fitting a 998 head onto an 850 you'll need to skim it to get the correct comp ratio. If you don't, the C.R. will be very low. I can't tell you how much to skim off, but if you let me know what pistons (i.e. the part number) the engine has, I can tell you what the volume of the combustion chamber should be in cc's. Then you'll need to put exactly that amount of paraffin oil, water or some other fluid into one of the chambers in the head and measure how much needs to be machined off the face of the head. It may sound very technical, but it's quite simple really. Send me a personal email if you wish.

Peter

[Mini_Lund]

Cooperman said:

If you are fitting a 998 head onto an 850 you'll need to skim it to get the correct comp ratio. If you don't, the C.R. will be very low. I can't tell you how much to skim off, but if you let me know what pistons (i.e. the part number) the engine has, I can tell you what the volume of the combustion chamber should be in cc's. Then you'll need to put exactly that amount of paraffin oil, water or some other fluid into one of the chambers in the head and measure how much needs to be machined off the face of the head. It may sound very technical, but it's quite simple really. Send me a personal email if you wish.

Peter

My 850 is a late 850 1979 - (stopped production in 1980) therefore my Mini has a 998 head on it already, (same as what I'm putting on) with a HS4 (1.5 carb).

[Cooperman]

The difference between the head casting on the 850 and the one on the 998 was/is the combustion chamber volume. If it's the original 850 head it will be fine. If not you might be wise to check the combustion chamber volume before fitting it. What you don't want to do is end up with an 850 with a comp ratio of something like 7.8:1.

[Mini_Lund]

Cooperman said:

The difference between the head casting on the 850 and the one on the 998 was/is the combustion chamber volume. If it's the original 850 head it will be fine. If not you might be wise to check the combustion chamber volume before fitting it. What you don't want to do is end up with an 850 with a comp ratio of something like 7.8:1.

What would happen if I had a compression ratio like above?

[Cooperman]

You would be lucky to get a bhp figure of 30 bhp whereas the standard 850 shopuld have about 34. To get any powert out of an 'A-Series' engine you do need a fairly high comp ratio. For example, my 1964 |Cooper 'S' runs 11.1:1 C.R. and a standard 998 Cooper runs 9:1. For your 850 about 9.2 would be good and with a little bit of head improvement around the inlet tracts and the inlet valve seats you shoud be able to get 38bhp to 40 bhp, especially if you use a 1.5" carb.

[Mini_Lund]

Cooperman said:

You would be lucky to get a bhp figure of 30 bhp whereas the standard 850 shopuld have about 34. To get any powert out of an 'A-Series' engine you do need a fairly high comp ratio. For example, my 1964 |Cooper 'S' runs 11.1:1 C.R. and a standard 998 Cooper runs 9:1. For your 850 about 9.2 would be good and with a little bit of head improvement around the inlet tracts and the inlet valve seats you shoud be able to get 38bhp to 40 bhp, especially if you use a 1.5" carb.

I just looked at my receipt for my cylinder head skim and it says head skimmed - Mini 1275

... but its a 850/998 head - hope he hasnt taken off too much, such as getting the right CR for a 1275.

[dogfather]

Mini lund, i hav a spare 850 head you can HAVE if hes skimmed it wrong but hopfully he hasnt.
good look its here if u need it.

[Mini_Lund]

dogfather said:

Mini lund, i hav a spare 850 head you can HAVE if hes skimmed it wrong but hopfully he hasnt.
good look its here if u need it.

Cheers, will see If its fine, I hope it is.

[photo_ed]

Check the combustion chamber volume before fitting, if its been skimmed the chamber volume will be less which would raise the CR which is not a bad thing on an A-series. Used to run my 998 at about 10.5/1 CR with no problems on optimax.

Once you've got it fitted I'd recommend finding a reputable rolling road - one used to tuning mini's too - to get it set up properly for you.

(If you're after more power the best route is to source a 12G295 head (fitted to 998 Coopers - but also MG Midgets) as the standard 850/998 head is VERY restrictive in both port and combustion chamber design and not really worth the effort of mucking around with when the 12G295 flows so much better.)

[Cooperman]

You really must measure this. Skimming to get the correct comp ratio is not the same as skimming to make sure the head is flat and suitable for use. If simply ebuilding a standard engine you would almost always give the head a light skim, say 0.003", to make the head surface like new. However, for, say, a 998 head on an 850, you could well need to skim a lot more off.
Turn the head upside down, with valves and plugs installed, get a spirit level to make sure it's absolutely level, then using a 5cc syringe from Boots the Chemist, fill one of the chambers with water until it's level with the head surface. Count the amount of water it takes to exactly fill one chamber and let me know what that is. I'll do a calculation to tell you what comp ratio that will give on an 850. Then you'' know for sure. if it isn't correct, I can tell you what capacity each chamber should be and you can put that amount of water in and measure how much needs to be skimmed off.
It's really the only way to be sure it's right.

Peter

[photo_ed]

Cooperman said:

You really must measure this. Skimming to get the correct comp ratio is not the same as skimming to make sure the head is flat and suitable for use. If simply ebuilding a standard engine you would almost always give the head a light skim, say 0.003", to make the head surface like new. However, for, say, a 998 head on an 850, you could well need to skim a lot more off.
Turn the head upside down, with valves and plugs installed, get a spirit level to make sure it's absolutely level, then using a 5cc syringe from Boots the Chemist, fill one of the chambers with water until it's level with the head surface. Count the amount of water it takes to exactly fill one chamber and let me know what that is. I'll do a calculation to tell you what comp ratio that will give on an 850. Then you'' know for sure. if it isn't correct, I can tell you what capacity each chamber should be and you can put that amount of water in and measure how much needs to be skimmed off.
It's really the only way to be sure it's right.

Peter

Good advice, but one small point, DON"T use water - it doesn't give a proper reading - a light oil such as 3 in 1 or sewing machine oil will give a more accurate reading because of the way it sits in the chamber - all to do with the meniscus if I recall correctly.

[Fatboy]

photo_ed said:

Cooperman said:

You really must measure this. Skimming to get the correct comp ratio is not the same as skimming to make sure the head is flat and suitable for use. If simply ebuilding a standard engine you would almost always give the head a light skim, say 0.003", to make the head surface like new. However, for, say, a 998 head on an 850, you could well need to skim a lot more off.
Turn the head upside down, with valves and plugs installed, get a spirit level to make sure it's absolutely level, then using a 5cc syringe from Boots the Chemist, fill one of the chambers with water until it's level with the head surface. Count the amount of water it takes to exactly fill one chamber and let me know what that is. I'll do a calculation to tell you what comp ratio that will give on an 850. Then you'' know for sure. if it isn't correct, I can tell you what capacity each chamber should be and you can put that amount of water in and measure how much needs to be skimmed off.
It's really the only way to be sure it's right.

Peter

Good advice, but one small point, DON"T use water - it doesn't give a proper reading - a light oil such as 3 in 1 or sewing machine oil will give a more accurate reading because of the way it sits in the chamber - all to do with the meniscus if I recall correctly.

It's the surface tension - water has a very high surface tension, meaning you can get a bit of a dome (the meniscus) on the water as you put it in, which could throw your calculations out a bit - whether it would be enough to be relevant at the accuracy of the measuring syringe is another matter, but light oil has a far lower surface tension and would not suffer this problem...

Edited by Fatboy on Tuesday 27th May 16:09

[Cooperman]

I've used both air-tool oil and water and if you use your judgement it doesn't seem to make any real difference. Oil ruins the seals in the Boots syringes quite quickly and water seems to work for me, especially if you run your finger around the edge of the combustion chamber when it looks full. You can never be totally accurate, but remember, unless it's a race engine, +/- 0.5 cc really won't be critical.

[Mini_Lund]

Cooperman said:

I've used both air-tool oil and water and if you use your judgement it doesn't seem to make any real difference. Oil ruins the seals in the Boots syringes quite quickly and water seems to work for me, especially if you run your finger around the edge of the combustion chamber when it looks full. You can never be totally accurate, but remember, unless it's a race engine, +/- 0.5 cc really won't be critical.

I've got myself a 5ml syringe now, so will be giving this ago sometime soon, will let you know how much ml goes in one chamber!

[Cooperman]

Let me know and I'll do the C.R. calculations for you. Measure also how far down the bores the top of the pistond is from the block deck level. You can do this with a steel straight edge and a set of feeler gauges. Also, if the pistons are dished, I woill need to know the capacity of the dish, which again you can use your syringe to measure.

Peter

[Mini_Lund]

Cooperman said:

Let me know and I'll do the C.R. calculations for you. Measure also how far down the bores the top of the pistond is from the block deck level. You can do this with a steel straight edge and a set of feeler gauges. Also, if the pistons are dished, I woill need to know the capacity of the dish, which again you can use your syringe to measure.

Peter

Okay I'm going to do it now. flat valve and the amount of water in the chamber in ml. Cant do the piston as of yet as i need the car.

Edited by Mini_Lund on Monday 2nd June 17:04