Modding 250C cylinder heads for 350 A

[dvsrd]

I just bought a pair of 250C heads, that I intend to modify for my 350A. The plan is obviously to machine out the combustion chambers to a suitable volume and squish area/clearance for a 350. Maybe cut the heads to a sort of thoroid profile, keeping the spark plug threads as they are. 

Then either modify the barrels to accept 400 head gaskets, or convert to o-rings. Has anyone modded stock heads with o-ring groove and a spigot that centers the head in the originakl gasket recess? That would be ideal for me, as it would leave the barrels completely stock, while ensuring heads are centered and squish clearance the same all the way around.

[signal]

My initial thought about machining the head to use the head gasket recess in the barrel as a locating spigot is that it is a good idea.

I do have some reservations regarding the removal of additional head material considering that you are also going to alter the squish band and combustion chamber.


I have recently modified some barrels to take O rings rather than the heads.

[muttsnuts]

I've done part of this on a customers bike, made 250D heads fit 350A barrels

I machined the squish so that it was 1.2mm and I made the total CC's for each combustion chamber to 21cc's, which gives you the higher compression heads and peps the engine up a good bit.

I used 400 head gaskets with the pip/locator removed, I wanted to O ring and peg the heads for the customer but they didn't want the extra cost, but its a good idea if you have the time and skills to do it

HTH

[reedpete]

Feb 9, 2019 16:38:55 GMT 1 muttsnuts said:

I've done part of this on a customers bike, made 250D heads fit 350A barrels

I machined the squish so that it was 1.2mm and I made the total CC's for each combustion chamber to 21cc's, which gives you the higher compression heads and peps the engine up a good bit.

I used 400 head gaskets with the pip/locator removed, I wanted to O ring and peg the heads for the customer but they didn't want the extra cost, but its a good idea if you have the time and skills to do it

HTH

Dave, 21cc, what reduction over stock is that?

[muttsnuts]

Feb 10, 2019 12:15:42 GMT 1 reedpete said:

Feb 9, 2019 16:38:55 GMT 1 muttsnuts said:

I've done part of this on a customers bike, made 250D heads fit 350A barrels

I machined the squish so that it was 1.2mm and I made the total CC's for each combustion chamber to 21cc's, which gives you the higher compression heads and peps the engine up a good bit.

I used 400 head gaskets with the pip/locator removed, I wanted to O ring and peg the heads for the customer but they didn't want the extra cost, but its a good idea if you have the time and skills to do it

HTH

Dave, 21cc, what reduction over stock is that?

Pete - none, I made them identical to the last version of the 350B head, I had a set loaned to me from a friend and they where stock, so just made the 250 heads identical to them,

[reevo]

Cool 😎