Topic: reworked 836cc head on stock 750k jugs?

[bran-don]

i just picked up a nice box of parts, and included was a 750 head that had been shaved and given up-size valves and stiff springs for an 836 kit.   i don't see any reason why this would not work on my stock jugs, maybe a little trouble tuning/jetting.   what do you think?  is there trouble on the horizon, or will i experience some small hp gains?

 measurements coming soon...

[MRieck]

i just picked up a nice box of parts, and included was a 750 head that had been shaved and given up-size valves and stiff springs for an 836 kit.   i don't see any reason why this would not work on my stock jugs, maybe a little trouble tuning/jetting.   what do you think?  is there trouble on the horizon, or will i experience some small hp gains?

 measurements coming soon...

It will work with a stock cylinder but the oversize intake valve might be a problem with stock pistons (depending on the cam)

[bucky katt]

where in new england? i was born and raised in windsor ct myself.

[bran-don]

   could i have a few more details on the intake issue? i know my intake vales are big, what are the mechanics of that problem? is there another cam that could easily be sourced and function better?

i'm in jamaica plain, (part of boston) and i get up to portsmouth NH every once in a while.

[MRieck]

   could i have a few more details on the intake issue? i know my intake vales are big, what are the mechanics of that problem? is there another cam that could easily be sourced and function better?

i'm in jamaica plain, (part of boston) and i get up to portsmouth NH every once in a while.

The valve reliefs are small and shallow. With a stock cam and no head milling it very well may be OK. You get into larger lift/duration cams and it very well might not work. I can't say from an armchair....you would need to mock it up and see piston to valve at 10 degrees BTDC and 10 degrees ATDC. I live in Ashland MA....next to Framingham

[bran-don]

currently running stock cam so maybe it will work,   many thanks for the info!

[eurban]

Sounds like the head has been reworked for better breathing.  Likely, it will be counter productive to use a stock cam in such a head and with a stock displacement engine your low RPM performance (where one typically spends most of the time riding) will be softer than stock.  If you do install a cam designed for top end power, it may make use of the better breathing ability of the head and make gains in high RPM power, but will also reduce lower RPM power output and, with a street ridden stock displacement engine, you probably won't be happy.  Basically IMHO it doesn't really make much sense to install a stock cam designed for all around performance in a flowed head with larger valves and then slap this assembly on a stock size bottom end.  

[MCRider]

After addressing the problems that MRieck points out, I'd say if that's what you've got to work with go for it. I'm one of the few if only persons who has actually put a ported head on a stock bottom end, with a stock cam and 4-1 pipe. You didn't mention if yours was ported, just shaved and OS valves. Is yours ported too?

If so, it was really SWEET. Much more powerful than stock, and midrange was great.

The head was a castoff from a dragracer I picked up a shop where i worked. I assembled a rat from pieces and got the head for free. Now yours has OS valves? So that s the variable, I didn't have.

But to put a number on it I'd say a stocker with a ported head is equal to (or better than) an 836 without a ported head. Lots of latent HP in the head.

[bran-don]

AH-HAH.   i see.   well the thing was pretty dirty when i got it, so i'll clean it up and and mic the valves, and the ports, and take a photo next to my stock head and maybe we can come to a better conclusion here.    thanks again.

[MCRider]

AH-HAH.   i see.   well the thing was pretty dirty when i got it, so i'll clean it up and and mic the valves, and the ports, and take a photo next to my stock head and maybe we can come to a better conclusion here.    thanks again.

Great you have a stock head to compare. It should be obvious looking in the intake port, which should be clean anyway. If ported the passageway will be obviously different to the naked eye, more symmetric, less aluminum around the intake guide, the guide itself may have been ground on to improve flow.

[Sam Green Racing]

Check if the combustion chamber has been squished for the bigger pistons, If it has you might run into problems with the smaller pistons and head gasket.

Sam.

[bucky katt]

After addressing the problems that MRieck points out, I'd say if that's what you've got to work with go for it. I'm one of the few if only persons who has actually put a ported head on a stock bottom end, with a stock cam and 4-1 pipe. You didn't mention if yours was ported, just shaved and OS valves. Is yours ported too?

If so, it was really SWEET. Much more powerful than stock, and midrange was great.

The head was a castoff from a dragracer I picked up a shop where i worked. I assembled a rat from pieces and got the head for free. Now yours has OS valves? So that s the variable, I didn't have.

But to put a number on it I'd say a stocker with a ported head is equal to (or better than) an 836 without a ported head. Lots of latent HP in the head.

i have a 75 750f head i'm doing some mild porting on to work with a stock displacement bottom end, with good rods and a crank with the APE treatment.

[andy750]

Good luck with the head. If you need some parts or want to see a couple of 750s then I am in Roslindale. I have a 74 with a mild ported head/stock cam/rebuilt top end and a 72 thats getting a 812cc top end (courtesy of MRieck).

good luck
Andy

[bucky katt]

i also have a set of the cbx pistons. i'm going to figure out how much i can safely mill off the crown and how much needs to be taken off to get a streetable compression. i'll sacrifice one of the 6 to check the thickness of the dome.

[754]

no need to cut the piston, just measure it..