12 replies to this topic

[Bradfirj9]

Does anyone have to hand a measurement or specification sheet for the 12G295 cylinder head? Looking at a second hand one and since I've been invited to take a look before committing to buy I'd like to check if any skimming has been done. (The current owner doesn't know anything about the castings unfortunately.)

I have a pair of metric verniers but no dial gauge, and I already know that Inlet valves should be 30.93mm and Exhaust 24.4mm.

Many thanks!

Edited by Bradfirj9, 01 February 2011 - 04:28 PM.

[Cooperman]

Can't remember the exact as-new chamber capacity, but it's in the region of 26 cc. However, the critical thing is how much it has been skimmed as the oil feed hole can break through the head face. Check this with a vernier gauge by measuring the thickness of the head and subtracting from it the depth of the oil hole. You need about 0.080" remaining to allow a bit more skimming if required.
You will need to do a compression ratio measure/calculate before assembling so just check that oil hole depth and inspect for any cracking between valve seats.

[Bradfirj9]

Thanks, it's to go on an A+ 998 running flat tops and a 10.5:1 CR at the moment, so I'm concentrating quite hard on the CRs!

Before anyone jumps on me with regards to the 940 head instead, I'm looking to go to a +80 thou overbore (1062cc approx) with a MD276 cam, which unfortunately means I can't get the required clearance with exhaust valve recessing and I'm not keen on the idea of pocketing the block either, the 295 head looked like the better option with regard to my plan for the motor, unless anyone cares to contradict me?

[Bluecray]

Can't remember the exact as-new chamber capacity, but it's in the region of 26 cc. However, the critical thing is how much it has been skimmed as the oil feed hole can break through the head face. Check this with a vernier gauge by measuring the thickness of the head and subtracting from it the depth of the oil hole. You need about 0.080" remaining to allow a bit more skimming if required.
You will need to do a compression ratio measure/calculate before assembling so just check that oil hole depth and inspect for any cracking between valve seats.

I quote ""the critical thing is how much it has been skimmed as the oil feed hole can break through the head face.""

YUP it happened to me LOL read the full story here with a picture of the hole at the end.......

http://www.theminifo...howtopic=180755

I've acquired a 12G295 head and thought it would be interesting to compare the head dimensions.. (hole in head is a standard head)

Hole in head WIDTH - 68mm
12G295 head WIDTH - 69.5mm

Rocker oil way depth - Hole in head is 67.2mm approx
Rocker oil way depth - 12G295 is 64.5 approx

So those figures mean that the hole in the head only had approx 1mm of metal between bottom of oilway and top of head !!! whereas the 12G295 has approx 5mm

NOTE - I cannot be certain that my vernier is reading the true bottom or the beginning of the hole in the "hole in the head" one...........

I was told the gap should be around 1/4 inch which is 6.35mm.


I want to know if you can tell how much has been skimmed off a head if the head width is known?

Is there a figure for a standard head width or for a 12G295 head?

Thanks.

Edited by Bluecray, 14 March 2011 - 08:41 PM.

[tommy13]

The original head width (thickness) is 2.75 inches . I have a 12g295 with .120" skimmed off and still a fair amount of metal left before breaking into the oil passage. Heads obviousley varied in this area.

[Bluecray]

The original head width (thickness) is 2.75 inches . I have a 12g295 with .120" skimmed off and still a fair amount of metal left before breaking into the oil passage. Heads obviousley varied in this area.

2.75" thats 69.85mm so if my 12G295 head is 69.5, then its been skimmed? ???

[tommy13]

The original head width (thickness) is 2.75 inches . I have a 12g295 with .120" skimmed off and still a fair amount of metal left before breaking into the oil passage. Heads obviousley varied in this area.

2.75" thats 69.85mm so if my 12G295 head is 69.5, then its been skimmed? ???

Yes but not by much, .35mm which is around .014", obviousley only done to remove any slight warping rather than a serious attempt to increase the compression ratio.

[Bluecray]

The original head width (thickness) is 2.75 inches . I have a 12g295 with .120" skimmed off and still a fair amount of metal left before breaking into the oil passage. Heads obviousley varied in this area.

2.75" thats 69.85mm so if my 12G295 head is 69.5, then its been skimmed? ???

Yes but not by much, .35mm which is around .014", obviousley only done to remove any slight warping rather than a serious attempt to increase the compression ratio.

This C/R ratio business which seems pretty complicated, do I need worry about it? the pistons and block is a standard 998 cc, the only change is the cam, its a Kent 276

Can this slightly skimmed 12G295 be fitted without worrying about the C/R stuff???

[bmcecosse]

Not all 295 heads can take all the skimming being discussed here....... I have one that has broken into the oil way - and been brazed up, and it's only been skimmed ~ 100 thou. So take care! And yes - ALL heads start at 2.750" thick -so anything less is the amount that's been removed. And yes again - the 940 head is a MUCH better option - especially since you have that 80 thou overbore. Just use a cam that doesn't have that high lift. Large diameter valves trump extra high lift cams anytime.....

[Bluecray]

quote " Large diameter valves trump extra high lift cams anytime." ...... meaning???

I really wanted an answer about the C/R business????

[bmcecosse]

You should start your own thread then - not hi-jack someone elses.......
And we can't answer until we know what pistons you have at present - raised D, dished, flat-top???

Edited by bmcecosse, 14 March 2011 - 10:57 PM.

[Bluecray]

You should start your own thread then - not hi-jack someone elses.......
And we can't answer until we know what pistons you have at present - raised D, dished, flat-top???

But I did say earlier up in thread, the rest is standard, ie standard block, standard pistons, only difference is the 276 kent cam.

[Ethel]

If you're going to the effort & expense of tuning an engine, then skimming the head to get the right CR is one of the biggest power returns you'll get for the effort & money involved.