12g940 head on 1098 questions

[MoggyBlue]

Heya guys

So finally found and bought a 12g940 small valve head to go on my traveller. Got a few questions though

1. I've done my clearance checks and all valves without headgasket gives me 265 thou clearance between piston and valve. I just wondered why the valves achually need sinking as that is still quite a gap and nothing touches is it more to clear the block?

2. Any one know the compression ratio I would get fitting the head?

Last but not least
3. I am now on the look out for some mini sintered rockers to go on but can the original pushrods be used?

I've looked at just about every possible thread about the conversion but having found these answers

Hope someone can help
Thanks
Jordan

[Monty-4]

1. You might get away with that clearance but I wouldn't like to be held accountable! Test assembly and blue-tac?

2. If the chambers are standard 21.4cc then you can calculate this accurately - 9:6:1 (okay 9.59:1). Assumes standard bore etc.

3. That pushrods are fine, I've tried several different cheap eBay sets of rockers before settling on the quietest.

Good luck! It really give the motor some poke once you've re-tuned the carb and nailed the timing.

[amgrave]

From memory previous posts on the subject have said you need 320 thou clearance on the valves but you might get away with 300. You don't need sintered rockers the old ones can be used but will need aligning with the valve tops by re-arranging the rocker springs and spacers on the rocker shaft. As said the push rods will be fine.

[panky]

Just remember it's .320" clearance between exhaust valve face and block - without the head gasket. And make sure you use a 1275 head gasket when you come to fitting.
I did look at the chamber volumes when I did mine and the difference between the Minor head and the 12G940 was, I think, 1cc less on the 12G940 so CR will be almost the same.

[MoggyBlue]

Heya Monty
Yeah I'm sure it would need sinking to make the correct clearance but no one seems to say why it needs doing... many engines seem to work to smaller tolerances so just wondered why it has to be done. the clearance results I got were from the bluetac tests

Oh right ok that's great thanks the head thickness suggests it's never been skimmed and if it has then by very very small amounts. And think the bores have bored by 20 thou max but don't hold me to that. But happy with a rough ratio.

Oh ok that's great then
Thanks yeah I've heard and the difference is very noticeable but would love to feel it myself Haha will have a hs4 to go on aswell when it goes back together:)
Thanks

[MoggyBlue]

From memory previous posts on the subject have said you need 320 thou clearance on the valves but you might get away with 300. You don't need sintered rockers the old ones can be used but will need aligning with the valve tops by re-arranging the rocker springs and spacers on the rocker shaft. As said the push rods will be fine.

Heya amgrave
Thanks yep it is 320 thou without the headgasket fitted. I did think about keeping the old rocker shaft but think I'd rather get the sintered shaft for ease. I have a few friends with minis so may even get one cheap ;)

[MoggyBlue]

Just remember it's .320" clearance between exhaust valve face and block - without the head gasket. And make sure you use a 1275 head gasket when you come to fitting.
I did look at the chamber volumes when I did mine and the difference between the Minor head and the 12G940 was, I think, 1cc less on the 12G940 so CR will be almost the same.

Heya panky
Yep it is 320 I have also found Roy's big right up on the conversion but no where and no one (not even mini guys) achually say why the valves sinking is needed. Yep will be ordering a new headgasket soon. Oh right ok i would have thought CR would have been more with more power

[panky]

If it's much less than 320 then the exhaust valve will hit the block on high revs. I needed a little extra clearance and achieved it by skimming the valve faces, much easier than sinking the valves. and the later heads have loads of meat on the valves. Compared with the valves on the stage three head on my Mini there is still loads more metal.
Stage three valves



Valves fitted to my Traveller after a 0.020" skim

[philthehill]

The reason it is recommended to sink the valves into the head is because there is a good possibility that the valves in the 940 head when fitted to a small bore block will make contact with the top of the small bore block.
Sinking the valves into the head ensures that the valves and block do not come into contact.
Not only are the valves in the 940 head larger but the valve centres are further apart.
The only large valve head that can be fitted to a small bore block without the possibility of the standard valves hitting the top of a small bore block is the early Mini Cooper 'S' 12A185 head as the valve centres are the same as those on the small bore heads.

[MoggyBlue]

If it's much less than 320 then the exhaust valve will hit the block on high revs. I needed a little extra clearance and achieved it by skimming the valve faces, much easier than sinking the valves. and the later heads have loads of meat on the valves. Compared with the valves on the stage three head on my Mini there is still [/img][/url]

Oh wow I never thought of shaving the valves. Think 55 thou is a little to much to take off the valves though I have the small valve with by pass hose but we have a valve seat cutter at work luckily ;) thanks for the help

[MoggyBlue]

The reason it is recommended to sink the valves into the head is because there is a good possibility that the valves in the 940 head when fitted to a small bore block will make contact with the top of the small bore block.
Sinking the valves into the head ensures that the valves and block do not come into contact.
Not only are the valves in the 940 head larger but the valve centres are further apart.
The only large valve head that can be fitted to a small bore block without the possibility of the standard valves hitting the top of a small bore block is the early Mini Cooper 'S' 12G185 head as the valve centres are the same as those on the small bore heads.

Heya Phil
Ah OK that's interesting. It's nice to know why the modification is having to be made. I'm guessing the 185s are like the 295 prices then as havn't heard many people talk abour them whilst researching.
Thanks for the help and clearing it up

[philthehill]

Because the larger valves left very little metal between the inlet and exhaust valve seats the 12A185 head was therefore prone to cracking between the valve seats.
I suspect that very few un-cracked 12A185 heads are still around though they do come up for sale occasionally but personally unless you know that you are purchasing a good one and the engine is not going to be thrashed within an inch of its life it is best to steer clear of the 12A185 head and aim for one of the later 12G940 heads.
What also helped in fitting a 12A185 head to a small bore block was that whilst the combustion chamber had the same capacity as the 940 head the chamber was deeper and not so wide across. The valves were also shorter so giving plenty of clearance between valve head and block.
As regards prices - 12A185 heads in good condition are usually cheaper than the 12G295 head but availability is poor but it is the AFG163 head which commands a very good price even S/H.
http://www.ebay.co.uk/itm/classic-mini- ... Sw32lYyrNU
I suspect that because of the problems and availability with the 12A185 head as mentioned above the head is discounted as a viable option for most people wanting to improve the performance their small bore engines so discussion will or may be limited.
A bit of background to the 12A185 cylinder head and a jolly good read regarding Downton Engineering/Conversions.
Note the reference to the Minor which started it all off.
http://www.aronline.co.uk/blogs/cars/mi ... nversions/

[les]

Interesting link Phil, brought a tear to my eye!

[philthehill]

At the Downton Cuckoo Fair there is always a good turnout of Downton modified cars plus other interesting cars and well worth a visit.

http://www.cuckoofair.co.uk/

https://www.bing.com/images/search?q=do ... &FORM=IGRE

The Bull serves a good pint too

[MoggyBlue]

Oh right ok so at the end of the day a 295 or 940 head would be better off anyway

I have the 940 head anyway so I'll stick to working on that

£700!!!!!!! Who buys that

So it sounds that it was the first of the 940 but they improved all the problems when it came to producing the 940.

Ooo will have to have a good read of that thanks

[dalebrignall]

just to add something else to the topic , i have got a 1098 with a mg metro 1275 head and hif 44 carb with maniflow exhaust and manifold its got lots of tourgue for overtaking , had it done by a series spares carnt recomend them enough great service .

[MoggyBlue]

just to add something else to the topic , i have got a 1098 with a mg metro 1275 head and hif 44 carb with maniflow exhaust and manifold its got lots of tourgue for overtaking , had it done by a series spares carnt recomend them enough great service .

That's good to gear. Is there any modifications you've done to the engine? How does it run on the 44. May end up getting one eventually if they are far better than the hs4 I have ready to go on. Always thought they were just a bit to big for the moggy but thinking abour it that may have been with a standard head

[dalebrignall]

i had this done in 2 stages i had the induction kit fitted 1st going through standard 1098 head it was too much for it the engine was restricted by the small valves .i then had the maniflow exhaust manifold and exhaust system and a reworked 1275 mg metro head fitted . the engine is a recon 1098 its on plus 20s . the results are very good . its not the power of a 1275 but it wont be far off.. i tend to cruise at 60 on the motorway but if you want to pass a lorry you can with confidance and there is a lot more power going up hills .its still on standard cam but the rockers have been upgraded to forged ones that give slightly more lift im happy with what i have now , but i will stress you need the bigger valves and exhaust system for it to work hope this helps

[Declan_Burns]

We had a discussion on this in fair detail back in 2013.
https://www.mmoc.org.uk/Messageboard/vi ... 40#p245538

Regards
Declan

[dalebrignall]

just read it declan very intresting