Topic: dohc750 rods with cb350 twin pistons

[Mitchell]

Ok so i got all these motor parts for 750 sohc and i wanna do somthing with. So one of many ideas i had running through my head was could one get away with using the 350 pistons with the shorter dohc rods? What are the chances having valve to piston clearance issues and how could you figure the compression ratio.

[754]

You will have to cut the skirts , not sure about the top.

[Mitchell]

Cutting the skirts should be doable but i really wouldnt want to mess with the tops.

[gschuld]

You will also have to widen the pin boss area inside the piston where the rods fit in.  The 750 sohc and dohc rods are wider at the pin end than the 350 rods.

[Mitchell]

That doesnt seem so bad so i guess if you were to trim the skirt and widen the inside boss for the rod to fit i would need to keep weighing them through the process to keep them the same weight. I have a set of dohc rods and awhile back i compared them to a sohc rod and noticed the dohc rod bolts were noticeably bigger i guess that could be a good thing too right?  Do you guys think if everything cleared it would run on pump gas? I know the 350 pistons have a mean dome on them so i wonder how much the 1.5mm shorter rod would drop compression and say maybe use the thickest head and base gasket from cometic

[gschuld]

Before you get too carried away, what exactly is the point with this process.  In the very early 70s, cb350 pistons were modified to create the first 836 kits.  But once aftermarket piston manufacturers started making dedicated pistons for cb750s, it simply wasn't cost effective to modify off the shelf pistons one at a time anymore.  Same is true today.  If you are doing it just for the challenge, that's fine, but cost or time effective...

You can get pretty nice 836 cast pistons, rings, clips, and head gasket for barely over 100 bucks. 

As long as you are aware of the good options out there...

Stock rods, especially with better bolts, can handle just about anything a cast piston mid compression 836 can handle.

George

[Mitchell]

Right i understand there aftermarket pistons ready to go but i wanted more or less to do it just because, and i like the idea of an 810 motor with 64mm pistons. If you could get away with doing it i have all the parts minus the pistons but from looking around i could end up with used 350 pistons for 30 -50 bucks  the only money i would need to spend is getting the jugs bored but ive got a guy that will do that for the hook up too ao all in all it would be pretty cheap for me and if it poped it really wouldnt be a that big a deal. Lets say its more or less experimental and its not like its set in stone just a crazy idea i had running through my head and was wondering what you guys thought

[gschuld]

Oh sure, it's perfectly doable.  Just making sure you are aware of the alternatives.  I recently fly cut the valve pockets on a set of new Cruzin image 836 cast pistons for more clearance for a higher lift cam.  They've basically stock OEM style pistons.  Not really worth it, but I did it anyway .

And yes, the polish job was total overkill, but it entertained me

George

[Mitchell]

Damn! They look good that for sure. In the case of the 350 pistons i was hoping the shorter rod would allow enough clearance for the valves and drop the compression ratio enough to work with no plans for an aftermarker cam i guess the only real answer here would be to try it and see what happens

[gschuld]

I'm no expert in these matters, but yes, I'd think it's doable if you are into the project.

George

[Mitchell]

How are those cruzing image piston performing for ya?

[gschuld]

How are those cruzing image piston performing for ya?

On my shelf,  fantastic, still shiny.  Zero carbon deposits

Just recently modified them.  I honestly have no immediate plans for them.  They have a bead blasted cylinder ready for them if/when a need arises.

So sorry, nothing useful to report.

[mystic_1]

Dayum, those pistons look so tasty that I'd be hesitant to run them and tempted to build a brass, oak, and glass display case for them.

mystic_1