MTFCA Forum

Customer asked if he can run dome piston with or of the new Chaffin RAJO heads? I am not sure so I will ask here.
What about the original ones? Thanks Dan

Dan Hatch wrote: ↑

Thu Aug 29, 2024 2:14 pm

Customer asked if he can run dome piston with or of the new Chaffin RAJO heads? I am not sure so I will ask here.
What about the original ones? Thanks Dan

Why not ask Chaffin?

Yes. The new RAJO head utilizes 14mm plugs which are smaller in diameter than the original spark plugs. As such they were placed higher in the chamber to allow the use of domed pistons for higher compression.

Thanks Mark. So that means original ones can not use dome pistons. Dan

High compression piston can be used in both new and old Rajo heads.

Mike is correct. The only factor to consider is how much has already been milled off the bottom.

I have domed pistons with a Chaffin head and a C crank. Be prepared to run premium gas. I guessed at the piston dome value and came up with 7 1/2:1 compression. Again. There was some guesstimating involved. If I recall Mark was building a similar motor and may have better numbers.

The other factor is to keep an eye on valve lift, I have about .070 with a mild lift overhead cam.

Andy

Really only .070?

Yeah Walt. The dome comes up pretty high and I believe the chamber is only 1 1/8” deep. The duration of the cam may have something to do with it but I didn’t get a cam card. I checked with no head gasket so it’s probably closer to .100. I think a 280 cam might skin by but probably not a 300. Runs good.


Andy

AndyClary wrote: ↑

Thu Aug 29, 2024 7:49 pm

Yeah Walt. The dome comes up pretty high and I believe the chamber is only 1 1/8” deep. The duration of the cam may have something to do with it but I didn’t get a cam card. I checked with no head gasket so it’s probably closer to .100. I think a 280 cam might skin by but probably not a 300. Runs good.


Andy

The piston is not up to hit he valves when the valves are open unless you are over revving & floating the valves.
So, cam lift with rocker ratio isnt a factor either.

Your probably right John, but I have always checked valve clearance. Funny, it’s never an issue on flatheads.


Andy

One caution that I would say is don't go too high with the dome. A friend and was rebuilding his BB Rajo with domed pistion and we had a bad spark knock. This was 20 years age, maybe the gasoline has changed a bit. Tom

Combustion chamber size varies so much from milling for higher compression over these heads lifes and depending on models from commercial to racing heads.
I see where the claim of 1 1/8" has been made. I have a head that looks like no or minimal milling @ 1" I thought that was the normal size.
I have a head that was severely cut to 3/4" chambers.

I do have one running BBR running an A crank & 292 pistons. The combustion displacement appears to be close to a stock T piston.
The horsepower is incredible as is. I dont see a need for more compression. But always open to improvements.
This thread has got me thinking to do a compression test.

Are these hi dome pistons stock T type, with stock rods & crank?
Couldnt be running a T crank or even stock journals on a scat crank ?

The Chaffin head has the 1 1/8 chamber, original heads are all over the board. If I had it to do over I probably wouldn’t have used the high compression pistons, but what the heck.


Andy

AndyClary wrote: ↑

Thu Aug 29, 2024 7:10 pm

I have domed pistons with a Chaffin head and a C crank. Be prepared to run premium gas. I guessed at the piston dome value and came up with 7 1/2:1 compression. Again. There was some guesstimating involved. If I recall Mark was building a similar motor and may have better numbers.

The other factor is to keep an eye on valve lift, I have about .070 with a mild lift overhead cam.

Andy

I did a compression test on my BBR. I got 110# = 7.5:1 aprox. Thats with 292 pistons over an A crank.
You say run premium fuel?

I got a little rattle on mine with regular. I need to check my total advance, I might be pulling the lever too much. I haven’t driven it enough to find where it likes to be.

Andy

Yea John, but the chamber depth on a real BBR is well under 1" deep..........