Cb250 Pistons for CB72

[DrMowl]

Hey there,
Since my CB72 cylinders were already too worn out I had to change the bore to 56mm and use CB250 pistons. A good friend of mine who works a lot on CB72 engines ensured be that it's possible to use them.
Now the Pistons fit in the cylinder but not in the cylinder head since they have a wider diameter and a higher roof (? the topside of the piston has a different form).
I was wondering if anybody here did this kind of conversion and knows how much I can mill off the cb250 piston. Another possibility would be to change the cylinder heads but that would be much more effort I guess.

Thanks for your help!

[OldScrambler]

Somewhat 'dicey' advice..............especially when oversize CB72 pistons & rings are available.

You should maintain no less than 0.75mm clearance between the piston and the head/valves. DO NOT mill the pistons........that's very risky..........but the sharp edges should be smoothed..........The skull in the head should not be machined but you may get by with applying an emery-cloth to the edges of the chamber. You will have to check the clearance of the piston to head-gasket openings.............and may need to use a 305 unit.

I would consider installing a 1mm thick plate made of mild steel or a good aluminum alloy at the bottom of the cylinder. You could go a little thicker if needed..........but too much will affect the cam-chain. Install the base-gasket on top of the plate.......and another below it if you want more clearance up top.

[G-Man]

How about asking your friend?

You might just go to +3 mm and use a set of Hap Jones Pistons made for this engine.

http://www.ebay.com/itm/Honda-CB72-CL72 ... 5c&vxp=mtr

G

Hey there,
Since my CB72 cylinders were already too worn out I had to change the bore to 56mm and use CB250 pistons. A good friend of mine who works a lot on CB72 engines ensured be that it's possible to use them.
Now the Pistons fit in the cylinder but not in the cylinder head since they have a wider diameter and a higher roof (? the topside of the piston has a different form).
I was wondering if anybody here did this kind of conversion and knows how much I can mill off the cb250 piston. Another possibility would be to change the cylinder heads but that would be much more effort I guess.

Thanks for your help!

[DrMowl]

How about asking your friend?

You might just go to +3 mm and use a set of Hap Jones Pistons made for this engine.

http://www.ebay.com/itm/Honda-CB72-CL72 ... 5c&vxp=mtr

G

Hey there,
Since my CB72 cylinders were already too worn out I had to change the bore to 56mm and use CB250 pistons. A good friend of mine who works a lot on CB72 engines ensured be that it's possible to use them.
Now the Pistons fit in the cylinder but not in the cylinder head since they have a wider diameter and a higher roof (? the topside of the piston has a different form).
I was wondering if anybody here did this kind of conversion and knows how much I can mill off the cb250 piston. Another possibility would be to change the cylinder heads but that would be much more effort I guess.

Thanks for your help!

Thanks for that advice! That might just be the cheapest option considering that I can sell the cb250 pistons.
Does this set work without any work on the heads?
Edit: And do you have any experience with these pistons?
Just talked to the machine shop. they want 260€ to get 1mm off the cylinder..

[DrMowl]

Somewhat 'dicey' advice..............especially when oversize CB72 pistons & rings are available.

You should maintain no less than 0.75mm clearance between the piston and the head/valves. DO NOT mill the pistons........that's very risky..........but the sharp edges should be smoothed..........The skull in the head should not be machined but you may get by with applying an emery-cloth to the edges of the chamber. You will have to check the clearance of the piston to head-gasket openings.............and may need to use a 305 unit.

I would consider installing a 1mm thick plate made of mild steel or a good aluminum alloy at the bottom of the cylinder. You could go a little thicker if needed..........but too much will affect the cam-chain. Install the base-gasket on top of the plate.......and another below it if you want more clearance up top.

Thanks for your input! I also though about a thick plate at the bottom but i don't think I have the right tools to build one that will fit perfectly. I will probably go with G-Mans idea!

[G-Man]

Alternatively you could get a fresh set of barrels with more meat in them.

G

[honda brian]

Hi Doc, have you thought about making a new cylinder base gasket from 1MM thick material. I used a 2MM aluminium spacer for the base of a set of barrels which had been heavily modified for racing. I was putting the engine back to road use. The camchain was hard to fit but not impossible if the barrels are pulled down using spacers on the cylinder studs. You should be OK using a 1MM gasket/spacer. I have used the Hap Jones 3MM oversize pistons and found no problem with them. Can't remember if I needed to rework the head, but it's not a big deal. Put the head on without a gasket, and if you can turn it over, then you should have enough clearance when you assemble it with the gasket fitted.
Regards Brian.