18 replies to this topic

[jim295]

So I've done as many variations on the possible content on this on tmf and looked in the vizard bible (he only mentions putting 295 head on)...

I know that this is possible and that you need to have the block pocketed (as in jetblicks infamous dremeling) but what else needs to be done? i've seen on mspd and med sites that they offer drilling for 11 studs... i assume that this is needed aswell because the 998/small bore heads are 9 stud? then you need to move some kind of plumbing for the cooling as well??

another reason for asking this is now if people do a search it will pop up with this one the moment they type 998 and 12g940 into the search engine (hopefully without getting flood control...) also if a concise explanation is posted then it could be a sticky in FAQ?

Sorry if this bugs anyone but i have tried to find the answers without just doing the newbie thing and had no luck so any help much appreciated..


Cheers, Jim.

[Ethel]

Why should it bug anyone? We live for the chance to show off our minspertise 'n start an A series barney

You may not even need to pocket the block depending on the amount of valve lift.

No need to add extra studs, most 1275's have no more than small bore blocks.

You ought to put some thought in to getting the compression ratio right most likely by skimming the head depending on any gas flowing piston dish etc.

[jim295]

Cheers I know that it is annoying sometimes though when you get the same question about 8 times a day... sometimes from the same person...

basically i'm looking at the small bore route because everyone knows they are the best....
looking at 12g940, SW5, etc.... would it be more worthwhile giving mspd or med a ring and talking over the build because i will need to go and get head skimmed most likely to keep compression ratio good and to keep it safe (dont want a jetblick incident with piston in gearbox) will get block pocketed too..

also going to try for a revvy one aswell so lift may be an issue.. will also be getting a centre main strap lightened crank.. blah blah..

Edited by jim295, 22 November 2007 - 08:55 PM.

[Ethel]

Cheers I know that it is annoying sometimes though when you get the same question about 8 times a day... sometimes from the same person...

basically i'm looking at the small bore route because everyone knows they are the best....
looking at 12g940, SW5, etc.... would it be more worthwhile giving mspd or med a ring and talking over the build because i will need to go and get head skimmed most likely to keep compression ratio good and to keep it safe (dont want a jetblick incident with piston in gearbox) will get block pocketed too..

also going to try for a revvy one aswell so lift may be an issue.. will also be getting a centre main strap lightened crank.. blah blah..

I've no data to hand at the mo but I think with any sort of extra lift valve pockets in the block would be needed.

A 998 will have proportionally more breathing for the same valve sizes in a 940 head the amount of time the valves are open is the issue at high revs.

Not sure about strapping a small bore bottom end (no doubt it's sensible, it's whether you can use the same bits as a large bore) but I'm sure a decent machine shop will know ask on the forum about machine shops in your local area. It will need to be a bit of a screamer if you want real power out of 1.0L so think about gearing too.

[samwell]

copying my spec too

[Wil_h]

The other thing that need doing is blocking up the waterways in the block that encroach on the firerings of the 1275 gasket that you will be using.

The ones that need blocking are to the top right of No. 1 and the top left of No. 4. If you don't do this the water tends to etch away the block, it will eventually get under the firering and the block will be scrap.

[JetBLICK]

^ i think thats one of those things that you can sometimes get away with not doing. I certainly didnt do it and had no trouble at all.

[1984mini25]

I've also been planning on doing this and what i have found out so far.

The block will only need pocketing for the exhaust vales, only if you plan to use anything other than the standard 998 cam,

Nothing will need doing to the head as the compression ratio as even though the 12g940s vales are larger than the ones in the standard CAM4180 head, the chamber volume however is smaller from 25.5cc on the standard CAM4180 head to 21.4cc on the 12g940 so will raze the compression ratio just bolting it on.

The larger water pump pulley of the 998 will often foul the head, so the smaller 1275 type is required. Also, if using a 'high capacity' water pump, there is a risk that the head will foul on the top edge of the pump and not clamp down properly. Check the clearance, and modify the pump if necessary.
And as the head is a 1275 head so you need a 1275 head gasket and top radiator bracket.

And that’s about it.

[Ethel]

I've also been planning on doing this and what i have found out so far.

The block will only need pocketing for the exhaust vales, only if you plan to use anything other than the standard 998 cam,

Nothing will need doing to the head as the compression ratio as even though the 12g940s vales are larger than the ones in the standard CAM4180 head, the chamber volume however is smaller from 25.5cc on the standard CAM4180 head to 21.4cc on the 12g940 so will raze the compression ratio just bolting it on.

The larger water pump pulley of the 998 will often foul the head, so the smaller 1275 type is required. Also, if using a 'high capacity' water pump, there is a risk that the head will foul on the top edge of the pump and not clamp down properly. Check the clearance, and modify the pump if necessary.
And as the head is a 1275 head so you need a 1275 head gasket and top radiator bracket.

And that’s about it.

Very thorough research

I'd still calculate your compression ratio for example if your 998 has flat top pistons you may find you it's too high. You're right about the water pump pulley issue but I think the small bore block has the smaller pulley.

I thought the radiator mounting differences are down to the 998 block being a bit shorter, not anything to do with the head. Depending where the 940 head originated you may have to think about the bypass hose and heater take off too.

[samwell]

jim, if you want any practical experience, I'm in the process of finishing this conversion.

i required the smaller top pulley, may also need a shorted aux belt as a result to stop the alternator fouling the front when the engine moves under stress. I have pocketd my block as i am using an SW5 cam. This may counter the compression ratio issues that ethel mention but to be honest i haven't done many proper calculations. U do need a different rad bracket and also a specific rad hose. Don't forget the heater take off for you new head (assuming that's how ur 998 is atm). Don't forget, 1275 gasket to match the head. And you will probably need to cut down the mounting plate of your water pulley as it will foul the head and not let it sit flush.

And that my friends is about your lot. Unfortunately that is in the order that i remembered it so it may be dificult making sense of all that.

Sam

reason for edit:error 404

Edited by samwell, 23 November 2007 - 01:44 PM.

[JetBLICK]

Nice one 1984mini25.. maybe i can clear a few things up, its these kinda little things that people tend not to metion or forget about when planning mods like this.

If you have a late high compression 998 (with flat top pistons) then with a 12g940 you'll have a CR of around 10.5-1, which is perfectly manageable
If you have an early 998 with dished pistons then the CR will be far too low and the head will need skimming.

Don't quote me on this but i believe the high compression 998s came in with the introduction of the A+ engines.

You need a 1275 rad bracket and hose because the 'stat housing faces in a slightly different direction.

You need the smaller pulley for the pump. I dunno if some of them came on 998's but mine was a big one and it was in the way.

Some of the late 12g940's did not have a connection for the bypass hose between the water pump and the block. This is not a problem, you simply have to drill a small hole on the thermostat opposite the one thats already there (this will make more sence if you've seen one)

Theres something about the pushrods too, i think they're slightly different lengths, annoyingly i cant remember which set ur suppose to use.

If your making the pockets yourself and are not using the 'scientific method' of measuring the valve lift then working out how deep the pockets need to be, genrally speaking 5mm is deep enough for any application. And if your valves are opening and still hitting the block at 5mm then i'd imagine theres something wrong and you will also probly have to machine the piston to avoid the valve hitting it.

Also be careful when timing the cam if using anything with high lift. I used a 286 and timed it in at 107deg (should be 106 :S ) That, along with the stretch in the duplex chain ment that the exhaust valve was still open as the piston reached the top of its stroke, resulting in a big boom and a scrapped block and damaged gearbox.

[Ethel]

The 998 Cooper had flat tops (and a 12G 295 so not likely you'd be up for this swap) apart from that flat tops didn't reappear on factory lumps until the A+ and then only on the Metty HLE or equivalent spec engines.

I think older A's had the smaller pulley, this is also an issue if fitting a 12g 295.

Small bore pushrods are about 10mm shorter, again I think this is down to the block not the head but I was mistaken about the rad bracket

[Pie]

I think dans covered most of it but check this out; (its 1984mini25's topic)

other topic

[1984mini25]

Theres something about the pushrods too, i think they're slightly different lengths, annoyingly i cant remember which set ur suppose to use.

If your making the pockets yourself and are not using the 'scientific method' of measuring the valve lift then working out how deep the pockets need to be, genrally speaking 5mm is deep enough for any application. And if your valves are opening and still hitting the block at 5mm then i'd imagine theres something wrong and you will also probly have to machine the piston to avoid the valve hitting it.

The push rods are indeed different lengths and after looking at a 1275 block it appears that the 1275 blocks are in fact 10mm (ish) taller than the 998s, the only reason I could work out why is to ovoid the water pump fouling issue. And as it’s only the blocks that different I think you can still use the 998 ones.

And I did come across a technical drawing / picture on the pockets that need machining; will see I can go find it again.

Edited by 1984mini25, 23 November 2007 - 02:23 PM.

[samwell]

im sure dans got a picture of his hashed ones. I will take a selective photo of the one nice and even one that looks the ********