Topic: cb900f pistons in cb750

[jus750]

Hey im rebuilding a 750f2 1978, going along well need help with piston installation.
 It had 900 pistons with 4 groves on the top for the valves(i think out of a  dohc model)
I have new rings and have honed the bore but have forgotten which way they came out. there is 3 grooves on one side and 1 on the other. Does the 3 grooves face the exhaust or does the one groove face exhaust. any help would be awesome. Im so close to getting this thing on the road.

[Terry in Australia]

G'Day mate, can you post a pic of those pistons? I'm presuming the "grooves" are valve reliefs, but a DOHC engine with 4 valves per cylinder would have 2 valve reliefs at the front, and 2 at the rear. Post a pic mate, it's worth a thousand words. Cheers, Terry.

[mick7504]

Can you post some photos?
No worries trying to help but not quite sure what you mean.
Is it piston problems or ring problems or both?

[jus750]

ITs piston probs . I am sure I have 900 dohc pistons i just dont know which way they go back into the bore. 750 pistons have an arrow or dot which faces the exhaust. as you know . these pistons dont have a dot or arrow they have 4 cutouts for the valves of the 900 dohc 3 on one side and 1 on the other. just need to know which side should face the exhaust the 1 side or the 3 side. any ideas. I am having a little prob with my camera which is a little inconvenient.

[mick7504]

Still a bit in the dark as what you have mate.
As Terry said the DOHC pistons have 4 valve reliefs.
2 at the front and 2 at the back.
In the picture below you can see how the valve pockets are offset.
Pistons 1&3 are offset one way and pistons 2&4 the other way.
Are yours something like that?
What diameter are the gudgeon pins?
It's all a bit of a guess without a photo.
Mick

[jus750]

That motor is pretty clean wish mine was like that my pistons are different im off to the shed now to try photo and gudgen pin dia

[jus750]

Cant figure how to put up pictures now gudgeon is 14.98 mm

[mick7504]

No worries. The pin is 15mm so that's a good start.
That indicates that they are Honda pistons.
There is a couple of ways that you can post pictures.
Either sign up for a free account at Photobucket or have a look here. http://forums.sohc4.net/index.php?topic=14605.0
If you post photos from the "Additional Options" remember to keep the file size under 256kb.
It doesn't matter with photobucket because you are posting a direct link from a web hosting site.
Ask if you're not sure and we'll sort it out.
Mick

[jus750]

alright resized the picks hope they are ok included a pick of my motor in homemade frame
fingers crossed

[mick7504]

Don't know what those pistons are.
Someone else here may recognise them.
I think the GPZ750 Kawasaki had a 66mm piston and could go 1mm o/size to 67mm and also had a 15mm pin.
Just a wild guess on these that you have here.
What bore size are they?
The valve relief with the other 2 small ones looks slightly larger in the picture than the one by itself.
If it is, I would be inclined to put that to the intake side.

[Terry in Australia]

Ahhhhhh................... Now I understand mate, what someone has done is "fly cut" new valve reliefs in those DOHC pistons to allow for two big valves, hence the 3 on 1 side, and 1 on the other. The two small fly cuts on one side are for the DOHC's inlet valves, (normally bigger, with slightly higher lift) and the "1 slot" side is for the exhaust. Cheers, Terry.

[mick7504]

Hey Terry
Did you see the posting times for yours and mine?
They were right on to the second but I don't know how mine got there first.  
Mick

[jus750]

legend. now i can start building. On one side of the piston is stamped honda which i forget to mention.After reading all the threads oil flow or lack of it seams to be the biggest problem when rebuilding on of these motors and starting them up. I will not be using any gasket sealer and all oil gallerys are clear. is there any way i can make sure flow is ok once i start this sucker. ? I know you should pump it with the kick starter for a while. Thanks for all the input on the pistons

[Terry in Australia]

Hey Terry
Did you see the posting times for yours and mine?
They were right on to the second but I don't know how mine got there first.  
Mick

There you go mate, great minds think alike, at exactly the same time!

legend. now i can start building. On one side of the piston is stamped honda which i forget to mention.After reading all the threads oil flow or lack of it seams to be the biggest problem when rebuilding on of these motors and starting them up. I will not be using any gasket sealer and all oil gallerys are clear. is there any way i can make sure flow is ok once i start this sucker. ? I know you should pump it with the kick starter for a while. Thanks for all the input on the pistons

No worries mate, if the engine hasn't been started for a long time make sure you pull the oil pump out (three screws) and give it a clean out, and prime it with fresh oil. Tip the bike over on it's left (alternator) side, open the oil gallery plug adjacent the points cover, and pour plenty of fresh oil in there. Replace the plug. Open the valve covers and squirt as much oil as you can in there too. Kick the engine over with the ignition off 20 or so times to endsure that the opil is circulating, (if you have an oil pressure gauge, more power to you, they're great to have, all my CB750's have them mounted permanently) then turn the engine on and fire that sucker up. Have fun! Cheers, Terry. 

[jus750]

sounds good. i have split the case and installed a new cam chain , had hard done and bores honed so motor should be pretty fresh, will post pics when i got her running. have forgetten how cam chain tensioner goes for now. should have it sussed tommorrow. problem was bikes were apart when i got them .previous owner had given up i got a k7 f1 and two f2's
i making 2 good bikes the two f2's will be selling my other stuff soon