12G940 Head questions

[chrisryder]

Hi there, me again!

I picked up a cheap square profiled (ie: pre-turbo) 11 stud (not sure if it's been drilled post-prod) 12G940 head today, as it had bigger valves than the one i've just taken off the midget engine i've got.

Inlets are ~36mm instead of ~33mm, and exhaust is ~31mm up from ~30, so not a huge leap up, but heading the right direction.

The funny things with it are:[frame][/frame]
the valve tops on number 1 and 8 are both peened over. i think it must have been run with too big a valve gap, meaning the rocker is hitting the top much faster that it's designed to. might make it fun getting the valves through the guides

Next funny this is:[frame][/frame]
all the inlets sit higher than the exhausts. is this normal? have the exhaust seats suffered with recession?

Final thing:[frame][/frame]
anybody know what the 'X' above the 12G940 denotes? if anything! there wasn't an X on the other 940 head i've got.

as ever, any help much appreciated!

[chrisryder]

bumpety bump...

[Alec]

Hello Chris,

you're back to front I'm afraid, it's the inlets that have sunk, maybe to fit it to a 1098?
To get the valves out, remove the springs and then you can grind the bit of mushrooming off to let them through the guides. What are the rocker tips like?

Alec

[Alec]

Hello Chris,

my mistake, I've just realised, I went on what you said but it is the exhausts that are higher so possible recession?

Alec

[chrisryder]

there was no rocker assembly with the head, was planning on using the assembly off the head i took off the engine i've got. i'm going to strip the head down tonight or tomorrow, i am prepared that i may need 2 new valves to replace the two 'squashed' ones.

i've never actually had any experience of seat recession, so i don't know what to be looking out for! the exhausts could have sunk into the chambers by the same process that has squashed the valves (by the rockers hitting the valves too fast) could be that the rockers haven't be decellerating the valve, so the valve hits the seat too fast and has smashed the seat down. although it's only valves 1 and 8 that have the damaged tops, and all 4 exhausts are sunk...

i'll get out my spring compressor...

[bmcecosse]

Yes - does look like the exhausts have become recessed - perhaps by intention, perhaps by lack of lead! And then the valve tips have been reduced so they all sit more or less equal..........Show us a pic or two of the chambers before you pull the valves. Just carefully stone the valve tips if it's just localised damage, although may be through the hardening now. The exhausts were not increased in size in the standard head - although the original Cooper S heads (not 940) had larger exhausts. The rocker gear will be fine (but not the best!) if it's the sintered rocker variety........

[chrisryder]

thanks BMC. any ideas on the fact it's an 11-bolt head? any way of telling if it was born that way or has been drilled out since?

[chrisryder]

Pictures as requested, first up is the bigger valved 940 with potential seat recession:[frame][/frame]
compared to the 940 head that came off my midget engine:[frame][/frame]
on a side note, both heads have a ring around the valves, and both have bigger rings around the exhausts. i've just taken an exhaust valve out of the standard head, but i can't tell if it's unleadedized or not. i tried taking a valve out of the big valve head but it broke my spring compressor! ...worse, my dad's spring compressor!

[Alec]

Hello Chris,

the exhaust valve doesn't look as if it is noticeably recessed. I'm not familiar with all the variations in valve sizes of this head but could the intakes be too big for the port, making it sit up in the head?
Did you tap the valve spring cap before using the spring compressor, it does make it easier by unseating the collets momentarily.

Alec

[chrisryder]

i'll try whacking the cap once i've got a new spring compressor, thanks for the tip.

the faces of the 'sunken' exhaust valves are almost flush with the chamber, whereas the inlets are about 1mm proud, on the other head the exhausts and inlets are both about 1mm proud, so i'm fearing the recession has hit! i think getting the head unleadedificated would solve it, as the insert could effectively replace the missing material and bring the exhaust valve back in line with the inlet, but i think the valves are too big, thus too close together, to get a seat in there...

[bmcecosse]

The valves have been recessed - perhaps intentionally. It would be ideal for a small bore engine although many will say the inlets are too big! I would use it. But it can still be used on a 1275 engine - don't worry about unleaded - just run the engine and set the exhausts to 15 thou, and check/reset every 3000 miles.