Topic: CB500 pistons in a Cb550

[MotoRico]

According to the specs, the original bore of the CB550 is 58.5mm.  Is the first overbore a 59mm overbore? 

I assume that it would be possible to get the cylinders bored to allow the use of 59mm pistons, unless there is too much missing from the cylinder wall already.

[crazypj]

First overbore is 0.25mm, second 0.50, 0.75 and 1.00mm
Go big, get a set of CB750 sohc pistons, they fit with minimum modification (small amount of turning on crown to fit 550 combustion chamber, makes it exactly 591cc) I did about 70,000miles on mine. Was a bit heavy on piston rings ( around 10,000miles to a set) but that was due to the Pirhana ignition allowing it to spin at 13,500 rpm (just over 82mph in second gear) picture at my yahoo group at 100,000+miles ( http://autos.groups.yahoo.com/group/StreetfighterBikes/ ) Was very tired engine when this was taken!
Still have bike, may rebuild it one day.
crazypj

[clarkjh]

crazy

do you have to mod the piston skirts any on the 750 pistons?  I might be doing this with my 550

James

[crazypj]

Sorry, I don t remember. I dont think so though. I have a set of CB750 0.50mm OS NOS I'm intending to fit into mine, if the liners will take another bore. I have some CB550 pistons laying about, I'll try and dig them up his weekend and take some measurements.
I remember its a lot easier to do 550 with 750 pistons than to do a CB400F with 500 pistons, have to change liners on 400 ( never got round to fitting 550 pistons to 400) CB350 Twin pistons are replacement for CB750, (the 836kit) easily available as they are still used for vintage racing.

[MotoRico]

So, the first overbore is to 58.75mm and the second is to 59mm.  

In essence the answer is yes, I can use the 59mm pistons.  

As far as CB750 pistons, would 'domed' pistons require more work?  I would think so as it would seem that the compression ratio would be higher.  In addition valve clearance may become an issue.  I know an article on this site - a daily ritual for me - mentions using CB750F pistons and I think those are the domed ones.

I have a second engine and I want to rebuild it and if I'm in there I may as well make it worth my time.  Especially given the price of gasket sets.

After looking at a website - which I can't find now - I saw a service that could offset the crank by as much as 6mm.  Oh the thoughts that went through my head.  I believe the site I saw I found from here. Now I have to look.

[clarkjh]

I scored my jug with a broken ring so I'm trying to figure out wether to go with the 605 kit for the 500 or go the 750 piston route.
Oh, so much to do and no moneys to do it.

James

[crazypj]

Cranks are probably done by Falicon, or Powerol (are they still in business?) Never liked the Powerol idea of shortening connecting rod.
Either way sounds expensive, have to weld up crank journals and re-grind to greater throw, or the cheap way is to just grind journals 3mm offset but that makes them really small diameter. The dome on the CB750 pistons isnt a problem, just machine a flat surface which acts as a squish band (or quench area if you prefer) or flycut the combustion chambers ( I've done both, did'nt notice any difference in performance) cutting the pistons is easier. A side effect was fuel economy, better combustion actually decreased fuel consumption, went from around 26mpg stock to 41mpg modified, even with larger main jets. Bike would pull slightly higher gearing and still outperform stock CB750 ( about 127mph with hard bags) If I can find a pic I'll find out how to post it(may take some doing since pics will be from 1980, I moved a couple of times!)

[clarkjh]

Would love to see the pics.  If you can't post em here, e-mail to me and I can put them up.

james@cryscom.nb.ca

James

[crazypj]

I'll see what I can find over the weekend
PJ

[moncb550]

Hi guys,

liked your discussions. I'm not that familiar with machining, but I seem to have a similar problem, and I was wondering why not take 650 pistons, which seem closer tahn 750s, at least in cc. Is the diameter different in size.
I just reinstalled my original motor on my 1974 550 and, althought it worked fine the first few days although i noticed compression was getting pretty low on all 4 cylinders ( I did a complete overhaul of the carbs, and checked the pistons, rings and cylinder) it suddenly started heating more than usual, and the kick start is harder than usual. I don't like the feeling. I have 3 other identical motors which have similar piston wear, but two of which have rusted cylinders that need to be rebored. My goal is to rebuild at least 3 motors in all, and I'm trying to find the best technical solution. Besides your exchanges in march, would you have any additionnal suggestions or recommendations.

P.S.: re article on home page, I tried finding Henry Abe's, Action Fours and Yoshimura directions for their piston kits and could'nt find any of them. I supposed you could'nt neither