Topic: 750 pistons ina 500

[mwohlenhaus]

without doing any research, I am just wondering if the pistons are similar.  I am soon gonna have a set of good used 750 pistons, and a 500/4 motor sitting on the floor waiting to be molested.

[cafe75-550]

750 pistons are have a taller pin height (24 vs 23mm), a dome, and offset valve clearances (750 heads have slightly offset valves). So, if you plan on running a stock head, they won't work without a lot of machining. Now if you found a set of aftermarket 750 pistons with a generic valve clearance, you might be able to make it work (your compression ratio would be insane) but also keep in mind to run those pistons you would need new sleeves as well.More than likely, you'd need to run a 650 head to keep the compression in check and make any use of the additional displacement.  If you want an easier solution, see Ebay for either a 555 or 605 rebuild kit for the 500, all necessary parts are included (I'd post a link but there doesn't seem to be any posted right now...).  If you're planning a major overhaul, see this thread

http://www.sohc4.us/forums/index.php?topic=13519.0

for discussions on how to bore a 550 to various 600 and 700+ displacements. There are currently 2 of us (myself and Paul ages) working on a 718cc 550...

But keep in mind that project requires a different head, sleeves, crank etc...

[hymodyne]

I am having jugg and new sleeves bored to accept a 605 overbore kit in my '72 500. I had to bore the jugg to accept the larber diameter sleeve, and the sleeve to accept the larger diameter piston. I hear that 605 cc is about all this engine can handle. I replaced cam and primary drive chain inside the cases. I've heard that any larger, besides requiring a head and juggs from a 650, would probably mean replacing head studs with heavy duty ones.

hym

[bryanj]

You can use a "K" model 750 piston in a 500 if you machine the skirt BUT the liner gets incredibly thin and yoou need a master machinist to not crack the liners

[crazypj]

Machine the piston crowns to match 550 head.
 Been close done similar
 Did over 70,000 miles so dont tell me it doesnt work
What dia. pistons does 605 use?
stock CB750 gives 591.8cc
see avatar
PJ

[bwaller]

crazypj,

From what I can gather, that "605 kit" uses 61mm pistons, the same diameter as a standard 750 piston, so it's not 605 but actually 592 as you stated.

There was an old article from Cycle magazine that showed what machining was necessary to make a 750K piston work, and besides the skirt, they did machine a little from outside edge of the crown too.

[cafe75-550]

The 605 pistons I have measure 61mm at the skirt, which is the same as the 750's. Nice thing about them is they're flat tops just like the 550/500's so they only require boring of the jugs and the sleeves, maintain compression ratio ( maybe not a good thing for power junkies but at least you know what you're getting) and don't need any special machining themselves.

PJ- Did the dome have to be taken down for yours, or did you just redo the valve pockets and turn the shoulder of the pistons?

[crazypj]

The 605 pistons I have measure 61mm at the skirt, which is the same as the 750's. Nice thing about them is they're flat tops just like the 550/500's so they only require boring of the jugs and the sleeves, maintain compression ratio ( maybe not a good thing for power junkies but at least you know what you're getting) and don't need any special machining themselves.

PJ- Did the dome have to be taken down for yours, or did you just redo the valve pockets and turn the shoulder of the pistons?

I just machined it flat to match the new squish area of head with a one and half degree taper difference to combustion chamber. Compression was up a bit but it worked exceptionally well for a long time. I did not shorten skirts as such, relieved the sides for clearance to flywheel
PJ

[bryanj]

I agree with the flywheel clearance its just easier th machine the skirt straight