32 replies to this topic

[Bill USN-1]

I have done some searches on swapping a 12G940 onto a 998/1098 block but I am coming up short on some important points.

I have not been able to find any actual examples of how the pocketing was performed and maybe some pics.
I understand the marking with clay or such, it's the actual cutting of the pocket I'm interested in.
I can scripe a line and use my die grinder if need be.....

I currently have a well used but running 1098 in my Inno with the 202 head.
I also have a 12G940 head in good shape already skimmed .020 and larger valves..


So my plan was to just swap on the 12G940 head and take advantage of the better compression ratio.
I have bought and compared the the 1098 and 1275 head gaskets. and measured my valve depth in the head.
I am .260 from the deck on the 940 head and figure a gasket thickness of .030 for a .290 total.

If I figure on a .281 lift for a future SW5 at 1.25 rockers and .015 clearance I get .336 lift.

This leaves me with a deficit of .046. So I figure I need about a .075-.100 pocket.

It appears i can use the 1275 or the 1098 gasket with the 1275 reducing compression and increasing squish area.

But the 1098 gasket looks more form fitting around the chambers/water passages.

Thoughts?

Another area of concern is the bypass hole on the underside of the small bore heads.
Can I run with out this with the large head? should I just drill and tap for 1/4 pipe?

Any write ups on using the small bore rockers on the large bore head? Shim spacers?


Immediate future plans are to swap a SPI manifold on and run a GM TBI FI system with LCB.

Long term is to use the 940 head on the 96 MPI 1.3 block with a DIS and the TBI or GM MPFI system.
But until I have time to work out an affordable DIS, this will have to wait.

I have documented with pics, all the head/gasket comparisons if you would like to see them. But I'm sure most have seem them.
If anyone has any pics or input it would be helpful...

Thanks,
Bill
USN-1

[miniboo]

the waterways and oil gallerys are slightly different and need tapping and blocking and re tapping etc to get right,or something along those lines.



when fitting a 12g940 head onto a 998 block you have to use the 1275 head gasket.

scribing a line and using a die grinder should be ok to be honest.

Edited by miniboo, 05 October 2006 - 08:47 PM.

[mad4mins]

just had a 12g940 head put on my 998 at Slarks Race Engineering, and like miniboo says they had to pocket the head and block off some of the water ways..not sure exactly which ... your best of ringing a mini specialist like Slarks (but depends where you are) in Amesbury, they did all the work for me..

i had to use a 1275 head gasket! and they suggested i should upgrade the camshaft, which was changed for a 266 Kent cam .. its pretty nippy now..

and make sure you get it properly tuned on the rollers...

Slarks tel: 01980 624477

hope this helps

M4M

[miniboo]

I think Bill is in Italy!! but it wouldn't hurt to contact somewhere like SLARK

[Bill USN-1]

I think Bill is in Italy!! but it wouldn't hurt to contact somewhere like SLARK

Yep I'm down in the ball at the end of the toe!!


Here's a few pics of what I have.

Here is the 940 head with the small and large bore gaskets.
The copper is the small bore.







Here are the gaskets mocked up on an old 998 block. I assume the passages in the block will all be the same on the 1098.






Here are the 2 gaskets setting on top of each other.





Mad do you have the small bypass hose on the bottom front of your head?

Based on the pics and looking at the block and heads, i'm trying to determine exactly which passages need to be changed and why the 1275 gasket is preferred over the small bore.

Edited by Bill USN-1, 06 October 2006 - 05:43 AM.

[mad4mins]

if you mean the really small rubber hose (2 inchs long), then they did block it off at the cylinder block..

not sure exactly which passages they blocked of sorry..

im not running a heater so they advised me to leave the thermostat out and block of the plate on top left of the cyl head...

http://i113.photobuc...ns/DSC02409.jpg

i tried to take some pictures of the blocked of hose but cant really get my camera in the gap..

http://i113.photobuc.../DSC02412x2.jpg

hope this helps anyway...

M4M

[Bill USN-1]

Thanks.
It was the small hose I was referring to.

[fikus01]

hello i think they suggest a 1275 head gasket because it has to seal round the same area as it would on the head!! there is metal in the same placeon the block the thing you are changing is the head!! does this make sense??

im just curious but why are you trying to fit the 940 head?? i know it has bigger ports and valves but it also has bigger chambers which should in theory lower you're compression ratio and so would the pocket in the block!! can you not find a suitable 998 cooper head?? they have the same ports as a 940 and the inlet valve can be made the same size as a 1300 inlet!! you dont loose as much through a smaller exhaust valve!!

EDIT: to get examples of this work check out the david vizzard bible

Edited by fikus01, 06 October 2006 - 07:22 PM.

[Bill USN-1]

hello i think they suggest a 1275 head gasket because it has to seal round the same area as it would on the head!! there is metal in the same placeon the block the thing you are changing is the head!! does this make sense??

im just curious but why are you trying to fit the 940 head?? i know it has bigger ports and valves but it also has bigger chambers which should in theory lower you're compression ratio and so would the pocket in the block!! can you not find a suitable 998 cooper head?? they have the same ports as a 940 and the inlet valve can be made the same size as a 1300 inlet!! you dont loose as much through a smaller exhaust valve!!

EDIT: to get examples of this work check out the david vizzard bible

Thanks,
I have the Vizard bible!! It's my nightly throne reading material!!

The 940 is actually one of of the smallest chambers which is why I chose it. IIRC it is 21cc but I have not cc'd mine yet. The 295 is supposed to be about the same ports but smaller valves and a larger chamber. So you gain in port velocity but lose compression unless you skim the head a lot to reduce the chamber size!

If I did all my reading correctly, this is how I understand it. I may have missed the section on pocketing the block or specific mods to the large bore head when mounting it on a small bore block...so if someone has a page number please pass it along.

[sqweaka]

my first mini was a 998 pocketed block with 649 cam and a 1275 s 11stud head ran on a 40 weber went like stink 10mile to the gallon did'nt com on cam till about 4,000 reved to 10,000 think i used 998 copper gasket and it had the bypass hose

[Retro_10s]

did'nt come on cam till about 4,000

god i bet that was a useful cam.... lol

[Bill USN-1]

my first mini was a 998 pocketed block with 649 cam and a 1275 s 11stud head ran on a 40 weber went like stink 10mile to the gallon did'nt com on cam till about 4,000 reved to 10,000 think i used 998 copper gasket and it had the bypass hose

Did you install the head yourself?

Was there any other water passage issues?


I look at the A+ bible again and the only reference to pocketing I found was in the 998/1098 fast road engine build section. There was 1 picture of a short block with the mod performed. No mention in the text on specifics.

I have read numerous references on the net to the mod being performed and would have thought more people on this forum would have already done it themselves?

[miniboo]

there was an article in one of the mini mags not long ago. i will dig it out and see if i can scan it in or do something with it

[sqweaka]

sorry cant remeber moding the head it was used as a rally car so the cam was not a prob realy this was about 25 years ago

[miniboo]

http://www.minifinit...5_head_to_a_998