13 replies to this topic

[zef]

I have a 998 standard motor in the Elf and a friend has given me a 12G940 head. I'd like to fit it and have read on here that it can be done by setting the exhaust valves 40 or 50 thou into the head. When I spoke to a very well respected engineer locally he seemed surprised it would work and thought it might leave a ridge in the combustion chamber that would lead to hot spots and further problems. He was willing to do the work but thought further work would need to be done as the valve springs wouldn't sit right, but his main concern was the combustion chamber.

So, after all that, has anyone fitted a 12G940 head to a 998 by sinking the valves and what further work was, or is, necessary? I just want to be able to put my and his concerns to bed before going for it.

Cheers.

[Pooky]

I've got a 12G940 head in my 1000 but I pocketed the block instead of sinking the valves. I used THIS kit from MRA Minis to solve the problem of waterway alignment between the small bore block and large bore head. You'll then need a small water pump pulley to avoid fouling issues and a 1275 rad bracket and top hose because your thermostat housing will point a slightly different direction.

Sorry I can't be of more help with the valves

[minimender]

I've fitted them lots of times just one off and the other on with no problems.
As Pooky says, you will need to change the water pump pulley.
The valves dont hit the block as long as you use standard rockers and I have revved the cods off em like that with no problems.

Why would you want to though, as all you will be doing is moving what power you have further up the rev band, you will loose low down grunt but it will pull high revs.
pays yer money and takes yer choice..

Edited by minimender, 17 December 2009 - 06:17 PM.

[bmcecosse]

It's an excellent swap - best 'Power for money' mod you can make to a 998. Obviously works best in conjunction with a large carb HIF38 or even 44 on a good alloy inlet, and best exhaust system you can afford. Yes - sink the exhaust valves by 40 thou - you MAY get away without doing this - but I can't tell you to try it. My advice - sink the valves. Yes - you then need to add a ~ 40 thou washer under each exhaust valve spring - on the top of the head - just to keep the compression same on all springs. Set the exhaust valve gaps to 15 thou too! Check the head doesn't foul the water pump casting - you may need to run a file across the water pump, and indeed you need the smaller (original) water pump pulley. Also use a 1275 head gasket - and ideally you should re-align the rockers so they act directly on the tip of each valve stem. However - the wide sinter rockers can be used as they are - that's all they did on the 1275 anyway! If you have the (better) pressed steel rockers, then they WILL need re-aligning - it's just 30 mins work. This mod will give good power right across the rev range - don't worry about losing power low down! It gains power low down - and gains even more up the rev range!

[minimender]

In my experience, that's running two 998s side by side, one in a saloon and one in a mini van, the one with the 1275 head was slower at getting away than the standard motor. Both with inch and a half carbs (Hs4)
After fitting a hif 38 to the saloon and a 1275 head to the van (still with Hs4) the saloon was slower than the van everywhere.
On the marked out quarter mile that we had on a straight flat piece of local road both cars were tested regularly. We got the heads to work after much tweaking of the dizzys and needles in the carb but they were still not a lot quicker than a standard 998 with (3.6 diff by the way. )
They would rev more though and they would pull over seven thousand revs with the right needle.
I found that the Hs4 was best but with the Hif38 you could pull more power high up the revs with a loss of low down torque.
It is possible to doctor the dizzy to get more torque lower down but I found it to be too much messing about for a small gain on a road car.

Talking of carbs, I had a 1275 with 276 cam and standard small valve 1300 head using a HS4 with a tweaked needle. The car was quick and torquey but we ruined it by fitting a HIF 38. It pulled a bit better top end but was slower everywhere else so not worth it for a road car.

I have found that the best budget tweak for a 998 is to reshape the combustion chambers and open up the ports a bit around the valve area, it will go very well with the standard cam and you will get lots of power for your money.

The valves dont open enough on a standard head to clout the block so not a worry I've found. I have been warned that I will wreck my engines but never did, maybe I was lucky.

It could be that I am useless at tuning engines but I've given many an expensive moter a good run down the strip and round the streets.

[Sir Cheat]

I will be doing this soon to a 998 with a sw5-07 cam and Twin 1.25 SU Carbs. Should be GOOD!

[zef]

Thanks for the advice and more food for thought. I should have mentioned i've a HIF38 already fitted and have noticed a big improvement over the Hs2 that was fitted.

I found the article keith calver did on fitting the 940 heads so I think I'll be measuring valve lift and valve to deck height before making a decision. All this hassle so I can fit my 998 head on my girlfriend's mayfair because hers has a sticky exhaust valve and I can't be putting a better head on hers now can I

[HARBER07]

You CAN fit a 940 using the larger W/P pulley, with a bit of grinding to the cylinder head. It's what i did, as I needed to get it running ASAP and didnt have a smaller pulley to hand.

[minimender]

Thanks for the advice and more food for thought. I should have mentioned i've a HIF38 already fitted and have noticed a big improvement over the Hs2 that was fitted.

I found the article keith calver did on fitting the 940 heads so I think I'll be measuring valve lift and valve to deck height before making a decision. All this hassle so I can fit my 998 head on my girlfriend's mayfair because hers has a sticky exhaust valve and I can't be putting a better head on hers now can I

Best of luck with it mate, hope it goes well for you.
I made a mistake in my post above as me memory is crap and I got the sizes mixed up, should have put my brain in gear first. I was using the HIF44 (inch and three quarter) and that's the one that loses you power
Hif 38 is marginally better they say but I've never tested them side by side, I did once do a quarter mile in a 998 and then swap the standard rockers for cooper 'S' ones and actually lost power on the next run. Food for thought.

[bmcecosse]

Cooper S rockers are no different (lift wise) to standard rockers ...................

A 940 head on a good condition 998 with AAA needle in an HIF 38 on an MG Metro inlet, and decent exhaust should be good for ~ 55 maybe 60 bhp. Go for it!

[minimender]

Cooper S rockers are no different (lift wise) to standard rockers ...................

A 940 head on a good condition 998 with AAA needle in an HIF 38 on an MG Metro inlet, and decent exhaust should be good for ~ 55 maybe 60 bhp. Go for it!

These were forged rockers and gave 1.5 to one.
I rather doubt the 55+ bhp but having never measured it after doing a similar engine to above I cant really argue. You are in 998 Cooper territory there though.
The valve overlaping the block face does restrict the gasflow in my opinion when using the 1275 head but the valves are huge I suppose which should compensate.
That MG manifold is also a good one, regarding the needle I have always doctored a standard needle to give best power all through as every engine is different. Needles are cheap enough now though so one could buy a few and try them out, tha AAA is quite a popular upgrade.
I've fiddled with 850s 998s since the sixties and have had successes and failures, the general rule is , head fuel sparks and exhaust first, the rest will only give minor improvements.
So as 'bmcecosse' says, head carb and exhaust but dont forget the timing, you will need to advance the timing low down so an MG distributer or similar will be required (that's a 10 degree built in dizzy) I got one off a Mini in a scrap yard once and it was an eight degree dizzy which was brilliant for the engine I was on at the time as it required 20+ degrees at tickover to run.

[Burnard]

Is there a guide, or template for pocketing the block?
Or does anyone have a picture of one that has been pocketed?

[Asphalt]

The valve overlaping the block face does restrict the gasflow in my opinion when using the 1275 head but the valves are huge I suppose which should compensate.

I guess nicely shaped pockets can help a good amount there!

Well - that's what I'm trying to do.
Not just a machined 'square', but smooth and rounded pocket.

@Burnard:
Not tried yet, but I tell you my plan: Sheet of paper, put the head on, press down the valves (to leave a mark on the paper, dirty valves will help ), mark the stud holes and put it on the 998 block.

[HARBER07]

i just painted the valves with anything i could find, put the head on the block and pushed the valves down till it hit the head (wasn't with a 940 but same idea & springs were out). Then just took out what i needed with a dremel.