Topic: Stupid question?? Re-using used 750 piston rings in a 650 overbore.

[Soos]

I have finally got ahold of a set of pistons that doesn't scare the h*ll outta me for a 650 overbore....

The block is bored out, and I am wondering if it is just plain stupid to re-use the 750 piston rings.
I am REALLY tempted to, since the engine machine shop i took my head (surfaced) and block to gouged me.
They quoted 125.00 for the whole job, that is until they found out their boring bar will not even fit into the bore.
They started honing to size without confirming a higher bill is ok.

Ended up paying 75.00 more for the job(after arguing them down by 100.00 for not contacting me and aproving the extra time/money involved).
So I now have to wait to buy the new gaskets I was going to get, and a set of rings just sinks my heart that much more...

From they guy I got the 750 F pistons from, they are out of a 8000 mile motor.


Anyone ever re-use used piston rings?


BTW - clearance is 0.0006 (total) over piston size.


l8r

[tbone]

The folks selling new parts will say to never re-use rings.
Ultimately it's you and your wallet that has to decide what's best.
Many a "backyard rebuild" has been done with used parts and
a lot of them are still running fine.

That said check ebay. Sometimes there are some real deals on rings.

[Terry in Australia]

So the machine shop honed your cylinders out by a few millimeters mate? I hope they're still concentric? They may well be, my cousin honed his vintage car engine out to the next oversize, but he used a special concentric 4 stone hone, not the normal spring loaded 3 stone job.

Anyway, I've often used used pistons and rings in different cylinders with no major issues, as long as you measure the ring gaps and they're within specs for the given engine application, they should be fine. Some might say that the rings will have worn "out of round" due to the additional pressures on the thrust faces, but it's been my experience that the rings move around on the pistons anyway, so you should have no major issues.

If you do chicken out though, I bought new rings on Ebay for less than 40 bucks for a set of four pistons. Cheers, Terry.  

[Soos]

Cool....

They took the bore out 1.2mm, from 59.8mm to 61mm.
And no they are not out of round.
The machine used was a Sunnen CK-10, uses 2 stones, and 2 guides.
I have used a similar Sunnen hone machine in the past, and unless you have the guides set wrong, it is nearly impossible to cause a out of round hole.

I have taken the block to work, and checked with the dial bore indicator we have, and there is about 0.0002 taper if that much on the worst cylinder.

So i'm happy as to the finish, and the taper.

If no-one has had any bad experiences with re-using rings, i don't see why I need them, considering the low miles on these pistons.
I did use a ring scraper to remove the carbon from the ring grooves, and cleaned the rings really well.

Just paranoid I guess....


l8r

[754]

First off, you dont have anything to lose..
except re & re the top end if they dont work but , if it comes off in the frame and you are capable..then go for it.

You are probably itching to try it and however it works, you know it will be even better with new rings when you can get them.

Give ER !!

[KRONUS0100]

Advice from an olds smallblock specialist....as long as they are not scored, pitted, burned or otherwise damaged..yes.  You still have to check end gap clearance before installing on the piston.  And be real careful with the "oil" rings, as these especialy are fragile.  And last...be sure to stagger the end gaps 120 degrees..Good luck.

[bryanj]

with 8,000 should be no problem, i would and i been in the "trade2 30+years

[sbc1320]

I have rebuilt a many of small block chevys, some small and big block fords, straight and v-6's. I rememeber when my machinist's boring bar was broken so he bored my block with the Sunnen CK-10 machine. Started with the rougher stones and finished with a finer one. No problems. True that many parts have been used with success, but personally I would spring for new rings of a known or descent manufacture. Might have to wait to save a little more money, but shouldn't have to redo anything. That's one thing I don't and can't stand is to have to redo my work. I had to file fit my last two motors (sbc) and had to asssemble them twice to notch for longer rods. Fixing to build a 383 sbc stroker next. I have all the top end all ready. On my sbc bottom end I grind and clearance the bottom of the cylinders and oil pan rails, then take the vacuum cleaner and vacuum as much shavings out as possible. Then I tear it apart and clean all the bearings with lacquer thinner  and blow out the block. Then reasssemble the short block. I know this latter info doesn't pertain to what you are doing, but I am just including it for future reference if anyone out there is doing any stroker motors (car engines).

[Alan F.]

Advice from an olds smallblock specialist....as long as they are not scored, pitted, burned or otherwise damaged..yes.  You still have to check end gap clearance before installing on the piston.  And be real careful with the "oil" rings, as these especially are fragile.  And last...be sure to stagger the end gaps 120 degrees..Good luck.

I'm also being tempted to reuse my original piston rings, I'm doing a top end clean and reseal on my 73 750 which has 20,000 miles, the rings are nice and clean, no pits, cracks or any damage at all.  I've honed the cylinders and they measure nicely.  I'm really tempted to reuse them, but a buddy who's a Cad/Hummer tech also urges me to replace them, his first bike smoked quite a bit, reusing old rings was why (gs500)  I'm looking for that $40 set on eBay now, thanks for the confirmation of quality, they were bought from rusty riders, right?
Thanks Gents.

[Terry in Australia]

Yeah mate, I bought them from Rusty riders, they look fine, but I haven't used them yet. Cheers, Terry.

[Soos]

I did end up buting a new set due to a broken ring, from rusty riders as well.

They are performing well, and looked great.
I did however opt to not use the 1 piece oil ring that came with the new ring kits.
I re-used the 3 piece that was stock to those pistons.
The ring sets I got were all gapped perfectly as well. They had between 0.007 and 0.008 gap in the rings when inserted into the bore.
Thought it was a good idea to check that just in case...

Going good after 110 miles so far.


l8r

[Terry in Australia]

So what's the engine capacity with the 750 pistons mate? Around 700? Cheers, Terry.

[Soos]

With the 61mm pistons the CC's go from a bone stock of 627cc's to a 652cc engine.

Not too bad.
Next motor i'm going to bore out will end up in the neighborhood of 740cc's though...
But thats going to take time...
Gotta bore out the upper half of a motor case to accept the 750 sleeves, bore the block for the larger sleeves, modify the length of the sleeves, press them in, bore the sleeves out, have them honed, and a COMPLETE motor re-assembly....Thank god the motor only had 10,000 miles, so i'll feel safe re-using all the plain bearings and such.

The 61mm cb750 pistons only involved modifying the pistons, a hone job, and re-assembly after getting the new rings and gasket kit.

With the head job, it DEFINATELY has more OOMPH!
Not quite enough to scare me(yet), but it definately goes better.
But some of that may be from the dyna2000 unit on it as well.
I think it gives more advance than the stock advancer unit ever could without modification.

l8r

[StrongPerf]

I'm glad to hear this is working for you!

I'm really @#$@% fed up with trying to find rings for these old things. I've got a set of 62mm KZ650 pistons for my super modded 650 motor and no rings to be found for less than $200. I was thinking of taking the stock 650 pistons and put them in my 550 motor. I can't find standard rings. If Honda has them they will be around $150.

I do have a set of 1mm over rings for the KZ pistons I was thinking of just stuffing them in and regapping them (top 2 rings only and re-use the old oil rings). I'm not such a butcher. My local Kawi dealer said he's done .5mm over like that with success but never tried 1.0mm.

It looks like the 750 piston is the only one that has rings out there for fairly cheap. I think I'll go with the 750 pistons in my 650. I don't know what to do about my 550...

Damn rings...

[hymodyne]

I've gone through two sets of 605 overbore pistons and rings and have decided to call it quits. I'm switching out my overbored jugs and pistons for a set of .25 over 550 pistons and rings.

hym

[StrongPerf]

I've gone through two sets of 605 overbore pistons and rings and have decided to call it quits. I'm switching out my overbored jugs and pistons for a set of .25 over 550 pistons and rings.

hym

Just where do you find those and the rings for them without paying a small fortune?

[Soos]

I'm glad to hear this is working for you!

I'm really @#$@% fed up with trying to find rings for these old things. I've got a set of 62mm KZ650 pistons for my super modded 650 motor and no rings to be found for less than $200. I was thinking of taking the stock 650 pistons and put them in my 550 motor. I can't find standard rings. If Honda has them they will be around $150.

Have you checked DSS?
http://www.davidsilverspares.co.uk/parts/part_6088/
However, at L18 ***PER SET***, and needing 4 sets, thats about $90 US currency.
ouch. but obtainable.
Heck they even have oversized (0.5mm) pistons AND rings!
Gotta luv DSS!

And yes, the 71mm cb750 F pistons are doing quite well... now if that head gasket would quit leaking (mind you it's been installed 3 times, don't ask) ... 
 

I do have a set of 1mm over rings for the KZ pistons I was thinking of just stuffing them in and regapping them (top 2 rings only and re-use the old oil rings). I'm not such a butcher. My local Kawi dealer said he's done .5mm over like that with success but never tried 1.0mm.

It looks like the 750 piston is the only one that has rings out there for fairly cheap. I think I'll go with the 750 pistons in my 650. I don't know what to do about my 550...

Damn rings...

Those KZ pistons sound interesting.... I thought the 2 valve KZ650 pistons had too high a compression, or dome height...mabey it was they had a 16mm wrist pin?

Do you have the stock rings? you might be able to measure them and compare to other 62mm piston ring sets and find some that will work possibly.



l8r

[hymodyne]

I've gone through two sets of 605 overbore pistons and rings and have decided to call it quits. I'm switching out my overbored jugs and pistons for a set of .25 over 550 pistons and rings.

hym

Just where do you find those and the rings for them without paying a small fortune?

which, the .25 over pistons and rings or the 605  60mm pistons and rings?

I scored the .25 over pistons on this site and  the rings on feebay.

[StrongPerf]

I've gone through two sets of 605 overbore pistons and rings and have decided to call it quits. I'm switching out my overbored jugs and pistons for a set of .25 over 550 pistons and rings.

hym

Just where do you find those and the rings for them without paying a small fortune?

which, the .25 over pistons and rings or the 605  60mm pistons and rings?

I scored the .25 over pistons on this site and  the rings on feebay.

Ok so I'm guessing you got lucky and the pistons aren't readily available. I haven't seen that ring deal very often. I guess I'll just go with the 750 pistons and new (good) rings. I think I hear that my 550 can take the 61mm bore without re-sleeving and surely the 650 can... Thanks

and good job SOOS. You've been the R&D department for lots of us!

[Soos]

Well, some things work, and some don't.
You cannot know how far you can go until you go to far.



And it's more my nature i'm beginning to believe.
To tinker, and mess with whatever I can get my hands on.
A side note:
If I haven't said it before, CB750F pistons (IMO) are more desirable for a cb650 overbore than cb750K pistons are.
The F pistons closely match the dome height of the cb650 more, even though you do need to trim about 1mm off the deck height to match the cb650's.

The K pistons(or the ones I have) are NOWHERE the height the cb650 pistons are.(from the center line of the wrist pin to top of piston dome)  I would say(eyeballin it here) about 2-3mm shorter.
However, as with the F pistons, the K pistons also need about 1mm trimmed off the deck height to match the cb650's.


My guess is that the stocker K pistons would keep the CR in the neighborhood of stock.
Yeah less dome height, but a larger diameter, there is SOME dome at least.
The F pistons with their higher dome gives a higher CR than stock, no question.
In my motor before the overbore my father in laws pressure guage read about 160-170 across all 4 cylinders.
Now after a head job, and the F piston overbore that same pressure guage reads about 205-210 psi across the board.


I would like to see the CB750A pistons...

If I had a CNC mill at home, and a nice manual or CNC lathe, I'd make my own friggin pistons and rings among other REALLY nice custom bits and pieces.
But as It is, I'm about $10k away from even HOPING to be able to do that(or a small buisness loan ? ). Always wanted to be the arse telling the guys they have to stay late/work the weekend.




l8r

[Terry in Australia]

If I had a CNC mill at home, and a nice manual or CNC lathe, I'd make my own friggin pistons and rings among other REALLY nice custom bits and pieces.

l8r

Geezus mate, you can get custom pistons made for a heckuva lot less than 10K, ha ha! Cheers, Terry.

[Soos]

Once past the initial machine purchase, with me doing all the work, everything I make would be at cost of material, electricity, and possibly maintenance.

But the range of other parts I could make beyond pistons....
custom tripple trees come to mind...



l8r

[StrongPerf]

If I haven't said it before, CB750F pistons (IMO) are more desirable for a cb650 overbore than cb750K pistons are.
The F pistons closely match the dome height of the cb650 more, even though you do need to trim about 1mm off the deck height to match the cb650's.

The K pistons(or the ones I have) are NOWHERE the height the cb650 pistons are.(from the center line of the wrist pin to top of piston dome)  I would say(eyeballin it here) about 2-3mm shorter.
However, as with the F pistons, the K pistons also need about 1mm trimmed off the deck height to match the cb650's.


My guess is that the stocker K pistons would keep the CR in the neighborhood of stock.
Yeah less dome height, but a larger diameter, there is SOME dome at least.
The F pistons with their higher dome gives a higher CR than stock, no question.
In my motor before the overbore my father in laws pressure guage read about 160-170 across all 4 cylinders.
Now after a head job, and the F piston overbore that same pressure guage reads about 205-210 psi across the board.

I've got a lathe in the garage and can take care of the crown mods. Do you think the K pistons would be best for the 550. The gentlemen's express article doesn't say if they are f or k pistons in the picture. I still haven't read the last page.

205-210 psi? Sounds like it's time for a bigger cam!

Did say that 750 rings got thinner around 1978?

[moham]

I've got a lathe in the garage and can take care of the crown mods. Do you think the K pistons would be best for the 550. The gentlemen's express article doesn't say if they are f or k pistons in the picture. I still haven't read the last page.

It doesn't say but seeing as that was the first year of the F it seems like they would have made the distinction (this is an assumption on my part, could be wrong). The same magazine reviewed the F in that year.

Crazypj said in one of my posts on the same subject that he had done the 750 piston mod and suggested using k0-k1 pistons as the tops were thicker. He didn't go into much detail beyond that.

[Soos]

The difference in thickness would be immense between the F and K pistons after modifying the deck height and the dome height to be used in a cb550.
The K (69-76) pistons being the preferable ones, because there would more meat left after machining.

If you wanted to see how much compression you can run, or run on methanol or CNG(anyone done that to a 550?) then a set of F pistons might work in a 550, but I would say it's iffy at best. IF the pistons are able to clear the head and valves it MIGHT work.
But I think the CR would be crazy high.
As I stated before, with the same tester unit, the pressure increased in a cb650 with F pistons, so in a 550 with the smaller dome in the head... could be a screamer or a grenade.


The F pistons I have are in my '79cb650 motor, so pictures are all I have to compare them to the K piston set I got recently, and a few drawings and measurements I did around the time I was getting ready to machine them for my 650 motor.



But anyways....


Happy modding!



l8r