Mini Carb...

[Peetee]

You should use the gasket that ensures it isn't exposed in the combustion chamber. in other words if the head chamber is larger than the piston bore then use the gasket that matches the head. If the piston bore is wider than the head chamber (though heaven know why you would want to do this ) use the gasket that matches the block.
I agree on the 12G295 head. It was specifically designed for the performance derivations of the 1098cc engine and as such has no incompatability issues with standard minor engines of 948cc or 1098cc. Coolant and oil passages match up, bypass port is present, so too is heater valve. Thermostat is the same and it can use the Minor rocker assembly. The only possible issue is the inclusion of a temperature sender beside the thermostat and checking the compression ratio. If I remember correctly 60 thou needs removing to provide a near-standard CR for a 1098 high compression engine (ie not LCV).

[turbominor]

I agree with Turbo minor

cheers max


only problem with the 12g295 is working out if it has been skimmed before, as if you take 60 thou of and it has already been skimmed you can brake in to an oil gallery if i remember rightly

[picky]

Hi there, thanks to everyone for their help with this, I have been told by the person I bought my 12G295 head from (it has not arrived yet so I cannot check fit on engine) that because it has 33mm inlet valves (bigger than standard for the 12G295) the inlet valves will hit the block. But as I have gathered from various posts here, using a standard cam and 1.3 rocker ratio, the valves will not hit the block anyway. I will check this with plasticene just in case though I'll check the gasket matches up with all the oil/water holes in the head and block before fitting.
Not sure how much to have the head skimmed.. Cam reckoned enough to bring the CR up to 10:1, how many thousands do I need off then? I'll see if I can work the formula in Vizards bible wish me luck!!

Just found out to my horror that I can turn the ignition on with a screwdriver Might just buy a new barrel!! Although they'll have a hard time starting it without a cylinder head..
Tim

[Cam]

Cam reckoned enough to bring the CR up to 10:1

As a maximum. If I was building an engine to be quick and beat the Novas then I would go to 10:1, but over 9 will still be fine.

[Kevin]

Sorry to be a pain Tim can you remove your notes on anti theft, as this board can be read by anyone and we normally deal with these issues as PMs and dont wish to give others ideas

[turbominor]

that because it has 33mm inlet valves (bigger than standard for the 12G295) the inlet valves will hit the block.

I cannot see how a 12g295 head with big valves can hit the block as the combustion chamber is the same size as the bore on a 948/998/1098?

sure we are not getting mixed up with a 12g940?

If the valves where big enough to hit the block you would have to remachine the valve chamber much bigger.. but you would runinto the next chamber

[maxwoods]

Picky,

There is a danger of valves hitting the block if you change rocker ratios and skim lots off of the head. Lift cannot be more than the depth of the chamber.

Your engineer will be able to tell you how much the head has or has not been skimmed by measuring the residual height on one of the external reinforcing pillars on the head. The standard 295 head has a 28.3 CC chamber - if you have 60 thou removed ( from a previously unskimmed head) this will reduce the chamber volume by 5CC, with a corresponding rise in compression ratio, depending on gasket thickness and piston dish.
Up to 90 thou can be removed before any danger of breaking into the oilways.
If you have a 60 thou skim you should ideally put similar thickness packing pieces UNDER the rocker pillars - they can be made of 1.5 mm steel, this will ensure that the rockers strike the valve tips squarely, reducing valve guide wear.
My local engineer who races minis says that with modern petrol ( low lead) it is not advisable to go much above 9.5 compression ratio, or there is a danger of detonation.

I have specified the mods I have made earlier in the thread, there is more power than my car can handle. Ok in a straight line on the motorway, but there are implications for cornering and braking!!!

The increased power will also put extra burden on half shafts, diff , clutch and gearbox, depending on how you drive.

Good luck

Max

[turbominor]

There is a danger of valves hitting the block if you change rocker ratios and skim lots off of the head. Lift cannot be more than the depth of the chamber.

Max i cannot see how that can be a problem with a 12g295 as the combustion chamber area containing the valves is the same size as the bore on the small block engine?

[Kevin]

There is a danger of valves hitting the block if you change rocker ratios and skim lots off of the head.

Sorry but I am with Iain on this point and as stated you could not hit the block as the chamber is the same size but it might be possible to hit the pistons if you really overdid it, is this what you meant.

[Cam]

My local engineer who races minis says that with modern petrol ( low lead) it is not advisable to go much above 9.5 compression ratio, or there is a danger of detonation.

Yes 95 octane fuel is not a good idea anyway. I personally use Castrol Valvemaster Plus which raises the octane 2 points and I use Shell Optimax (98 to start with). This gives me 100 octane which is excellent with a 10:1 ratio.

To be honest I would not run a standard moggy engine just on standard 95 octane fuel anyway, I would up it to 97 with the Castrol Valvemaster plus.

I have specified the mods I have made earlier in the thread, there is more power than my car can handle. Ok in a straight line on the motorway, but there are implications for cornering and braking!!!

Very much so, ideally the suspension and brakes need sorting out BEFORE engine mods!

The increased power will also put extra burden on half shafts, diff , clutch and gearbox, depending on how you drive.

I would worry about the gearbox first as this seems to be the weak link. The axle would be next but they can handle 70 BHP in standard form, so should be OK with those mods.

[picky]

I think turbominor must be right, It is a 12G295 head, and I dont see how the valves will hit the block because the 12G295 is the same bore as my cylinder block. Good detective work there Kevin! I will be using unleaded fuel with valvemaster plus, so a CR of 10:1 could result in detonation? Maybe I will go for 9:1 CR then to be on the safe side. As I will be using the standard cam profile and rocker ratio I dont think I will be hitting the pistons, thanks for that suggestion anyway Kevin. I've read in tuning the A series engine that the normal deck height is 2.750in., so I could work out how much a head has been skimmed by measuring how high the head is.

[Cam]

I will be using unleaded fuel with valvemaster plus, so a CR of 10:1 could result in detonation? Maybe I will go for 9:1 CR then to be on the safe side.

Not really. But anywhere from 9:1 to 10:1 should be OK. 10:1 gives more power and sounds nastier though!

[maxwoods]

Valves hitting pistons - I am not talking about valves hitting block due to overlap - I think we have already established that one. What about inlet and exhaust strokes when valves are fully open and piston approaches TDC?? I may be missing something, but you must be sure that the valve is not opened so much that it intrudes into the cylinder volume. I don't know that much about engine theory, but there must be a limit to the amount of valve lift you can use. Is this not what changing the rocker ratio does??

[Cam]

there must be a limit to the amount of valve lift you can use. Is this not what changing the rocker ratio does??

Yes, common ratios are 1.1:1, 1.25:1, 1.5:1, 1.7:1 I think over that and it's pretty pointless, but from the mechanical aspect as opposed to the gasflow aspect.

[picky]

So as a conclusion, the valves only hit the block if the lift is too high, or the head skimmed too much. I'm slightly worried about how the rest of the car is going to cope with my engine mods, how much can the standard gearbox cope with in terms of BHP? I've heard elsewhere that the halfshafts are the weak point in the minor transmission.

[maxwoods]

Picky

Valves would strike block if mismatched heads used( this is where pocketing the block would be required) - from your discription this is not a problem as the 295 head is compatible.

Valves could in theory strike pistons if lift ratio is changed excessively and lots skimmed off the head - I have been looking into this - With std rocker ratio and 60 thou removed it is definitely not an issue - as this is the combination I am using. I a hoping to fit 1.5 ratio hi lift rockers but need to make sure there is adequate clearance at TDC with the valve fully open. The standard came lobe is 3/8 inch- approx 9mm, with 1.25 rockers this translates to approx 11mm lift from the valve seat, a 1.5 rocker would lift approx 13.5mm. I know that the 'Cooper S' had larger cam lobes and flat topped pistons, so I believe there should be clearance. Obviously combustion chamber depth, gasket thickness, and how far up the block the pistons travel ( as well as any piston dish need to be assessed to make sure the numbers work. Plasticene may be the answer again!! Tappet clearances need to be changed as well - Vizard has all the answers, although sometimes it is a little difficult to dig them out.

With regard to the rest of the car I am still on drum brakes all round - all new and well adjusted, but for safeties sake will be upgrading to discs v. soon, swiftly followed by an anti roll bar. Incidentally I have replaced all the suspension bushes with super flex items - gives a much crisper and fimer ride, and has stopped several squeaks. The standard clutch, gearbox and haalfshafts are OK so far, but I am not using heavy acceleration. The extra power was for motorway driving. I added a 3.9 diff to help reduce the revs that the upgraded head is capable of . The noise reduction is noticeable with very little loss of acceleration compared to the 4.22 diff.

[picky]

Thanks Max,
One or two questions though - when you say that the super flex bushes give a crisper firmer ride do you mean that it handles better AND is more comfortable? I am also wondering what needle to use with the spec I am now going to use. I have gathered from several people that there is a bit of guess work involved, but I have acquired a copy of WinSU, a very good program for selecting needles and costs £5 to register. I dont think will be very accurate though as it lumps together the 12G202 head with the 12G295, which is a far better head. Go to http://www.winsu.co.uk/. So I will try what winsu suggests and see how it performs. About braking... I would like to fit the discs but at the moment ££ only allows me to fit a brake servo. Any comments on this?
Tim

[Cam]

Servos are all about preference to pedal pressure and don't really do anything for the braking effectiveness.

I personally prefer a firm pedal feel and so I don't use a servo but if you want the brake pedal softer to press (for the same braking effectiveness) then a servo is the way to go.

Remember a servo just helps you press the pedal - nothing more, which is why it's called a servo!

[Peetee]

Valves would strike block if mismatched heads used( this is where pocketing the block would be required) - from your discription this is not a problem as the 295 head is compatible.

However the valves on Picky's 12g295 head are not a standard size. If the oversize valves sit in the standard combustion chamber profile then there won't be a problem as there is no area on a standard 12G295 chamber that is wider than the bore of a 1098. The larger valves may have necessitated relieving the combustion chamber walls to allow room.
The easy way to establish whether is has been worked or not is to place a 1098 gasket on the head and see if it hides the edge of the combustion chamber at any point.

[picky]

Just to confirm, I have now received and fitted the 12G295 cylinder head, and I've turned the engine over with the starting handle and the plugs out and the valves are definately not hitting the block. I'm wondering what to set the valves clearences to though? I'm using MG metro turbo valve rockers. Im hoping to get it running tommorrow, when I have finished fiting it all back together. Any help with the valve clearences would be helpful. Thanks,
Tim