12 G 295 Cylinder Head

Post by **Willie** » Fri Oct 03, 2003 7:48 pm

MOWOGGY......regarding the 12G295 head. According to
my info the following applies: No skimming=8.9-1 compression
and chamber volume of 29cc. Skim 0.035" =9.5-1 and volume
of 26.5cc. Skim 0.060" = 10-1 and volume of 24.7cc.
Skim 0.080" = 10.5-1 and volume of 23.2cc.

Post by **mowoggy** » Mon Oct 06, 2003 2:05 am

Willie

Thank you - you are a real treasure!! Where did you get the figures from?
I think the CR's you quote might be for flat top pistons like the Mini Coopers and later AH Sprites & MG Midgets which used the 12G295 head.
I did consider using these pistons, but I thnk that they used different rods
with floating gudgeon pins.

However the chamber sizes for each skim sound right. I measured the chamber volume and came up with between 28 to 29 cc. The head depth is 2.75" high which indicates that it has'nt been skimmed before (Are all A series heads 2.75 " deep?) I roughly measured the surface area of the chamber with fine graph paper and came up with 28.4 sq cm. I calculated that 0.030" (0.76mm) off would reduce the chamber from 28.3cc (Vizards figure) to 26.1 which is the Std (12G202) volume. So your figure of 0.035 for 26.5cc sounds good.

All I want to do is restore the Torque that the modified 12G202 gave.

Thank you very much for your help.

Post by **Peetee** » Mon Oct 06, 2003 1:12 pm

How has the 202 head been modified?

Post by **Willie** » Mon Oct 06, 2003 7:07 pm

I think that the accepted easy thing to do with the 12G202
is to skim about 0.060" and fit a later inlet manifold with
a 1 1/2" carb

Post by **rayofleamington** » Fri Oct 17, 2003 8:36 pm

would you pay for this???
I couldnn't work out which bit was 'polished'!!! lol

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... gory=43122

Post by **Cam** » Fri Oct 17, 2003 11:42 pm

Hmmmmm...... no pictures of the inlet / exhaust ports eh?

Bit suspicious.

Post by **rayofleamington** » Sat Oct 18, 2003 8:38 am

maybe he couldn't tell the difference between 'ported and polished' and rusted away...
I just struggle to see why someone would pay more than £33 (including carriage) for a 1098 cylinder head that is missing half of its valves and looks like it was sitting in the rain for 2 years.
Digital photo's normally make stuff look better, but I still chuckled at his description 'COMBUSTION CHAMBERS ARE ALSO NICELY POLISHED'
Even if they once were, they are now quite pitted with rust.

Post by **Kevin** » Sat Oct 18, 2003 10:24 am

I think he meant to say that it was a nicely polished advert from the Arthur Daley school of sales.

Post by **MikeTrav68** » Sun Oct 26, 2003 12:38 am

There was a 948. It was a predecessor to the common 850, with a longer stroke. There can't be many around now. Ignoring the 997, there's two groups - small bore 998, 1098 with the 12G295 being the performance head, although the standard kidney shaped head can be worked to a similar state; and large bore 970/1071/1275 with a common and decent 12G940 head. There's a rare Cooper S head with larger valves but the 12G490 is a great start. I thought the large bore head *always* needs valve clearance pockets cut in the block when fitted to the 1098, although above suggests otherwise. You need the large bore gasket. It's possible to drill and tap the last two bolt/stud holes on any large bore engine.

Post by **rayofleamington** » Sun Oct 26, 2003 10:12 pm

I thought the large bore head *always* needs valve clearance pockets cut in the block when fitted to the 1098, although above suggests otherwise

it works with an otherwise standard 1098. Maybe it the head is very skimmed or the valve lift is increased then I don't know that it will work without pockets.

Post by **Peetee** » Mon Oct 27, 2003 10:17 am

I thought the large bore head *always* needs valve clearance pockets cut in the block

I don't think this is the case technically. The valve lift isn't enough to cause contact. At least I have read about this mod in several sources but not all have specifically stated that the block needs pockets. However, as we are talking performance here, it's always possible that high lift rockers fitted later or an over enthusiastic right foot could cause the revs to rise to the point that valve bounce causes problems.
The other thing to think of is that without the pockets the ports are shrouded to a greater extent which in itself is not condusive to performance.

Post by **MikeTrav68** » Sun Nov 02, 2003 12:31 am

Ray/Petee
Evidence in your favour. Whenever I put a 12G940 on a small bore I always had a sports cam and/or 1.5 rockers, never tried it otherwise. Hence my comments. According to the Mini Miglia boys (998cc), the 12G940 was worth 2-3hp over the 12G295 in-spite of the valve shrouding (both ported of course). The gasket should match the head. I have used and had fail a couple of copper gaskets on small-bore, and on inspection they were two metal layers with no 'filling' sandwich layer. Serves me right for going cheap! I have drilled and tapped the extra bolt/stud using the gasket as a template on large bore with no problems, but unless you're above 100hp I wouldn't think it necessary.

If the 12G940 fits a 1098 without valve pockets required, this seems like an excellent bolt on mod. Still available on old metros. Significant flow increase; maybe half point CR increase (subtracting the additional gasket volume). Since my 1275 engine fell though I might try this myself...

Post by **turbominor** » Wed Nov 05, 2003 12:48 pm

rod thomason stated in this months minor matters that you must pocket the block to use the 12g940??

Any one got the opertunity to try this on a standard 1098 cam and rockets.

According to vizzard the 1098 cam was soo good it was used in some factory coopers..

Post by **MikeTrav68** » Tue Dec 09, 2003 1:26 am

Just had a go, and I think it’s an each way bet.

A summary of the measurements: 1) there’s cam lift. 2) This is multiplied by the rocker ratio. 3) Subtract from this the tappet clearance. 4) The valve face then moves down to the cylinder head face. 5) Then there’s a little more clearance from the gasket thickness. Then it hits the block!

1) The cam lift is supposed to be 250 thou but I measured a consistent 260 thou (6.60mm) at the top of the pushrod with a pretty new gauge. I believe it to be standard, not bad as you say but better profiles out there, mostly with more lift!
2) This is multiplied 1.23-1.28 through the rockers to give ~8.4mm valve drop. I didn’t measure this.
3) Tappet clearance was set to 12 thou (0.3mm)
4) The big difference in heads is the depth of the combustion chamber - about 8mm in 12g940 and 10mm in the small bore 1100 head. This puts the valve face about 7mm from the face for a good head. The main variability is valve recession: I measured head face to valve face after recutting the seats as 7.4mm-7.7mm. The valves were a bit recessed to start with but not as bad as other heads I have!
5) The old head 1100 gasket was compressed to 0.65mm. I think the asbestos filled large bore gasket might be a little thicker, but it's 1.1mm uncompressed so it's not much different.

Add this up on my head and it’s 0.03mm negative clearance

.

Options:
a) Cut the valves into the head. I put a 30 deg cut on top even with the valves at 7.4mm. Detracts from flow but still better than 1100 head.
b) Fit two head gaskets. Volvo give this as the mod for unleaded on B230 engines, but I can’t see it working on A-series. The gap between cylinders 2 & 3 is very narrow and my heads often fail here.
c) Open up tappets. I wouldn’t go more than 20thou (0.5mm). Otherwise, you’ll get reduced cam timing and increased valve train stress and noise.
d) Cut valve pockets in the block.

The nice bit is that block pockets are quite easy. About an hour with a flat grindstone, finishing with a dremmel thick cutoff disk (#420) used flat. Care is required to avoid skidding over the gasket seal line. I put plasticine on the block and the large bore head fitted with exhaust valves and light springs (inners from a double set) and no gasket. Press the valves down by hand, take the head off and it's easy to see what needs cutting. The marks on the block from the small bore gasket are a good template since they are inside the large bore gasket seal ring. There's plenty of depth to the top piston ring.

I cut about 0.9mm down. This is cater for the variables – unknown compressed gasket thickness, variable valve recession, tappet clearance, rocker ratio. Plus a margin for valve bounce or new head.

The larger bore gasket is required. On the small bore the seal ring around the chamber doesn't! Using the larger gasket on the small block, the top front near side small cooling hole is partly blocked, I will be cutting the 12G940 gasket to clear.

The head depth was 69.9mm face to face. It might have been ground, but a standard metro turbo head was the same. The two faces were not very parallel either head (0.3mm)! Obviously a refaced head would need more clearance.

The acid test will be whether it holds up back in the car. Will be with an HS4, Minispares inlet and larger bore 3-1 manifold. Will report later.

Post by **rayofleamington** » Tue Dec 09, 2003 10:23 pm

stated in this months minor matters that you must pocket the block to use the 12g940??

I've used one on a standard 1098 with no pockets, one headgaskets and correct tappet clearances.

When you look at second hand heads, there will be a difference between the height of the valve to the seat.
I guess if you are unlucky the valve may hit the block - especially according to MikeTrav68's dimensions

but seeing is believing....

Post by **MikeTrav68** » Wed Dec 10, 2003 11:02 am

I believe you Ray. I was only 0.03mm the wrong side with no mods. Rod is not correct.

[/quote]
I've used one on a standard 1098 with no pockets, one headgaskets and correct tappet clearances. [/quote]

That's options d), b) and c). So you're using a)

My exhaust valves have 1.5mm vertical height above the edge of the 45deg seat (only exhausts need clearance of course). As new this would all be proud of the combustion chamber floor. Yet the top of the face is only 1.0mm proud, so mine are 0.5mm recessed.

Had they been 0.7mm recessed they would have been clear. However I'd have needed another gasket to prove clearance and I'm too mean for that! Also I'd recut and reground seats, and refaced valves already to get to 0.5mm recessed.

I chose d) because I wanted >=0.5mm clearance. Call it a comfort zone for future head replacement, measurement error, valve bounce & heat expansion if you will. It also took less time than recutting/grinding.

However in hindsight, whilst I had the cutting kit out just cutting exhaust valves deeper in the first place would have been simplest. It would go like this:

1) clean gasket face of head and carbon off valves
2) measure from the top of the valve face to the gastket face
3) cut the 45 deg seat until the measurement is >7.8mm
4) cut a 30 deg angle to regain some of the flow lost in recessing the valve

The last step is not essential but good for performance.

Post by **Kevin** » Wed Dec 10, 2003 11:27 am

Just one thing Mike will this still work with the hardened valve seat inserts, can they still be cut back easily.

Post by **MikeTrav68** » Wed Dec 10, 2003 1:27 pm

Well it's a harder cutting job of course but feasible. The measurements would be the same but there will be more metal to remove (no initial recession). I'd guess the 30 deg cut would extend to the diameter of the insert. The insert probably has enough metal to stay in place and not crack, but I would speak to the manufacturer for his opinion, I haven't tried it. Or go back to plan d) and pocket the block.

Post by **turbominor** » Wed Dec 10, 2003 9:08 pm

i have test fitted on to my 1098 an nothing touched, decided to put a thin layer of bluetack on on the block and turn it over to see how close things are.

Has any one used two headgaskets togeather to lower the compression??

Post by **rayofleamington** » Wed Dec 10, 2003 9:37 pm

So you're using a)

not intentionally - like I mentioned, not all heads are the same.
One where the valves are further into the seats will be pretty low risk.
Mine seats were never cut to get it to work on Minor, although the head probably had some miles and therefore the valves may have naturally recessed, or a previous owner may have had new valves and the seats cut I can't be sure.
Just don't expect all BMC heads to be identical in used condition, as they probably varied a lot in new condition to start with.