Topic: Name That Piston

[savageman]

Hey guys, been absorbing a ton of info from you all since I got my 78 CB750 last feb.  First lesson learned was buy a runner... but the price was right and had a title.  I slowly got her sorted.  In November she was ready to fired up, a few kicks and she came to life.  Then I noticed the oil light wouldn't go out.  Tried a few things, bought a gauge for the oil galley, dropped the oil pump out three times to reprime, replace o-rings, etc.  Tried every method I could find here to get oil moving and the needle on the gauge to budge but to no avail.  For whatever reason, I decided to check compression while I was waiting for the oil to drain the 4th time.  Well, cylinder 1 is 30lbs on average off from the other 3.  Time to pull the motor.

I got the motor out with little fan fair and started into the breaking it down.  The big surprise I got was when I pulled the head off and did a quick measure of the cylinder bores, I found them to be 65mm.  The PO didn't mention a big bore kit, likely didn't know.  The cylinders look ok, no scratches or gauges, but cross hatching gone.  Ring gap in the cylinder is right at 0.7mm not horrible, and now the questions:

1) What make are these pistons and are the rings just available or .25 over rings available?  There is a "C" stamped at the top of the dome and a very faint EXH ink off center



2) How much of the cylinder sleeve is left to possibly go another .25 over?

3) Best (cost effective) option if cylinder sleeve can't be bored?  New sleeves?  New cylinder jug, original or go big bore and matching pistons?

[madmtnmotors]

Probably going to need more pictures to identify the pistons. The only oversize from 65mm that I am aware of with stock sleeves would be CycleX 65.5mm (850cc). You are beyond ".25" increments and anything over 65.5mm will require new sleeves. You could also have stock sleeves installed and do one of several things:

>Stock pistons (ewwww...)
>Overbore in any of the .25 increments up to 1mm oversize
>Overbore to match your current pistons if the skirts look good and you can find new rings
>Overbore for new pistons up to 65.5mm

or just have oversize sleeves installed and overbore up to 1025cc or greater! Of course, you need to get out your checkbook for that...

Whatever you decide you will need to check the cam. Those are some deep valve pockets in the top of that piston, so you will need to check valve to piston clearance if you use different pistons. A hotter cam is often used with 836 kits.

[BPellerine]

kind of look like old venoila pistons,high compression,1 pc oil ring?bill

[754]

Show us the side and bottom of piston.
 He forgot one option, if you can get rings, rebore another cyl to fit your pistons.
 They look like they might be 12:1 , did it ping ?

[madmtnmotors]

He forgot one option, if you can get rings, rebore another cyl to fit your pistons.
 

I thought I had it covered, just a different method of replacing the sleeves. Either have new sleeves pressed into current cylinder block or replace the whole cylinder block with sleeves that have a smaller bore. Seemed like a potato/potatoe to me. You might get a better deal on a replacement cylinder block than buying new sleeves as long as you are confident with the sellers measurements if you are not able to inspect in person prior to purchase.

>Overbore to match your current pistons if the skirts look good and you can find new rings

 

[PeWe]

78 CB750 need higher piston domes, right?  F2 head with deeper domes and larger valves?
Measure bores and pistons to get the clearance.

My old 836 cyl with NOS RC 65mm has 0.1mm in clearance and still running good without taking oil.
Edit: clearance around 0.004"which is rather much. 0.0025"  (0.065mm) is probably what it should be
 
I have new rings for them. Same ring size as for my old Acton Fours 65mm and older RC forged pistons that I burned hole in. The newer RC are of older version though. Rings known to be of same size as Honda CB350 twin +1mm

The seller has still new 65mm rings for sale for a very low price. Rings cost usually double. Might fit yours too.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=361834612842#ht_500wt_1288

[Tim2005]

A few options here of what it could be

http://www.satanicmechanic.org/bigbore.shtml

[754]

Sorry Wilbur, i read thu too fast..
 I can vouch for Dream 305, i have met him  few times,bought a set of Std rings from him , Iwill use them on the hi comp 750 TRW pistons I got for Christmas..
Gonna have a small Nasty sounding motor with. 315 and Webers..
,

[savageman]

Thanks for the input guys, here's a couple more pics.  3 piece oil ring.  @754, didn't run it long due to suspected lack of oil flow, any pinging wasn't very noticeable.

[754]

I am guessing Wiseco and probably street compression ratio like 10.5-1.

[madmtnmotors]

Any markings inside the piston?

[scottly]

I pulled the head off and did a quick measure of the cylinder bores, I found them to be 65mm.  The PO didn't mention a big bore kit, likely didn't know.  The cylinders look ok, no scratches or gauges, but cross hatching gone.  Ring gap in the cylinder is right at 0.7mm not horrible, and now the questions:

Ring gap is not a direct indication of cylinder bore wear. If the bore is within specs, a light hone and new 65mm rings may be all that's needed.
Regarding the compression test, the low cylinder may have been due to other factors than ring sealing, like valves. Too late now, but the drill is to add some oil through the spark-plug hole; if the compression goes up, suspect rings, and if it doesn't suspect valves.

[754]

If you can get a .004 feeler guage between skirt and cylinder, you wont/shouldnt be reusing them ,.....those cylinders and pistons together I mean.

[MRieck]

You should do a leak down test along with the compression check to get a real idea of what is going on.