• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Using a 12G295 head on a 948

Share this page

Hap Waldrop

Hap Waldrop

Yoda
Country flag
Offline
This is topic that has came up here from time to time, so since I was doing this today, I thought I would share what I'm working on today. I'm in my final stages of completing a rebuild on a stock 1098cc 12G295 A series cylinder head to be used on a 948cc engine with a Judson Supercharger running 5 pounds of boost. Ok with that being said, all I really want to do is get the head's combustion chamber volume back to what it would have been with the popular repalcement pistons with a normal 948 cylinder head on the engine, say about 8.3 to 1 compression ratio. Now the 12G295 is listed in Vizard's book to be about 28.5 CCs, but I came up with a bit more than that in what appears to be virgin deck on this head, I came up with 29.5, now I really don't care what is is now, but rather what I need it to be to get me to the 8.3 to 1. Now the original 948 heads, the 2A 628 and 2A 629, and the 12G202 all were around 24.5 to 26 cc range. Now to do this right I had to have the customer feed me his piston dish volume and his deck height. Now never assume, even if you know your engine is stock and factory original that these numbers will be text book, the factory is famous for rather large tolerences, so in the case of this engine with factory "never shaved" block deck and .040" pistons had a deck height of .012, with the given piston dish volume , the bore, the stroke , the compressed head gasket thickness, and the head gasket bore, I can use a compression ratio caculator to help me fiquire out what combustion chammber volume I need, I just keep plugging in lesser cc volumes to I see the number I want to get to my desried CR. Now there are many methods of how to determine once you know your deisred combustion chamber volume, how to know how much to cut off the head itself. While the head is still leveled on the head stands after buretting the orginal volume, I just set the burette up to put the volume of liquid into the combustion chamber that I need it to be when cut. Then I use my depth micrometor and skim the surface of the combustion chamber until I just see it touch the surface of the liquid in the combustion chamber, re-check a few times to make sure I'm right and there ya go, I know now how much has to be removed from the head to get to my desired CR, in the case of this head it was .055", with this head on a 948 engine with carbs and no forced induction and say wanting a CR of 9.0 to 1 it could be more like .080" or more.

Anyway, thought some of you gearheads might enjoy a day in my life.
 
I just rechecked my combustion chamber volume after decking the head, dead nuts on the 25CCs I needed it to be to put me at 8.35 to 1 compression ratio. So now it on to final assembly of the head.
 
Thanks Hap for the great documentation!!! It should help everyone making the jump to a great flowing small bore head.

The more fun (?) trip with the 12G295 is hanging that sucker on a 1275.

Keep'em on the Road

Steve
 
2091351 said:
Thanks Hap for the great documentation!!! It should help everyone making the jump to a great flowing small bore head.

The more fun (?) trip with the 12G295 is hanging that sucker on a 1275.

Keep'em on the Road

Steve
Click to expand...

I just use the 12G940 or 12G1316 (smog head) for the 1275s, they have one of the smaller volume any way (aorund 21.5cc), the 1275 heads are open chamber design, but that helps when you put the bigger valves in there for shrouding anyway. Vizard make mentions fo using the 12G295 head on the 1275, but I don't see the samne way he does on that. You have zero problem getting the open chamber head to some pretty big CR numbers for racing, most SCCA guys run in the 15.0 -15.5 to 1 range. The deal with the 12G295 when using it for racing is eventually you'll get into the horizontal drillings that help to make up the rocker arm feed hole, then you have do some brazing, rerouting of the oil path to use the head stud hole to carry hole to the rocker pedestal, but I think on any CR needed for the street you would never have to go quite that far.
 
Hap, of course you are correct, the 940 or 16 fits a lot better! And a good/great 40 is something special.

One of the advantages of the 95 on conversions is that the big chamber allows some high compression units to have another life. (Think Street) When you add ~5 CC's to the chamber, an old (but good) race engine might be able to run on the fuel of today?

The biggie I found out about {Don't ask!} pertains to the cooling holes and how they align with either head gasket. $%^&*^&^&

High compression, O-ringed heads are just in another A-Series world. Nice on the race track, but tough to have fun on the street.

Keep up the great technical inputs. I love em!!!!

Steve
 
Well said, Mine worked out to .072 off of the stock head. However, I wanted a bit more compression. That works to 9 to 1 on a 988 engine.

Oh forgot to say, my head had already been leveled or something several times in the past.

They are all a bit different so as Hap says, check and recheck then check again.
 
Back
Top