Hard to turnover after new rings

Status
Not open for further replies.
rx7171

rx7171

Joined
Oct 8, 2007
Messages
656
I've done an overhaul of my 72 Combat Commando, including opening up the crankcase to confirm bearings ok (Superblend) Garaged since 1977!
Prior to putting on the head and after the clutch was installed I thought I should turn it over to make sure everything was ok.
That is my problem. Even standing on the kicker either during the upstroke or down stroke it will not swing through.
With the clutch pulled in everything swings freely except no piston travel of course.
I put a torque wrench on the mainshaft nut at the alternator and with about 30-40 foot pounds the pistons will travel up and down, and at the top and bottom of the stroke it rotates much more freely (since less piston movement).
I've used the torque wrench to turn it through probably 100 revolutions and I can see the fine honing lines starting to polish out and there is a dark slurry showing up in the 3 in 1 light oil I'm lubricating the walls with. I'm thinking this is the face of the rings and the cylinder walls polishing each other.
The turning over has gotten slightly easier but I still can't kick it over with the kicker and once I've got the head on it will get even more difficult against compression.
I can't believe this is a typical situation after installing new rings.
The problem appears to be during piston travel they are causing excessive drag.
The bores don't indicate any odd pattern of vertical scratches although there is slight difference of polishing.
They are the same Standard ones that were in the bike when I bought it. I was able to start it ok then.
The pistons have stayed on the rods. When I was putting on the Barrel it did seem to take more effort than I expected. Some tapping with rubber mallet.
My guess is when I installed the new rings I messed up. Like the pistons are Standard size.
I put one ring in each bore and checked for end gap which although on the tight end of the exceptable range seemed OK.
I didn't check all four of the compression rings which I should have. The first two were the same so I figured the manufacturing standards were consistant.
Now I wish I had. Can undergapped rings cause this. I'm quessing they would have to be severely undergapped to do that. Are Standard rings that oversize?
Is this hard a turnover normal? It sure doesn't seem so.
If normal how can I get it started if I can't even turn it over.
Thanks in advance for any advice offered. :|
Bob
 
Hi Bob,
You might want to pull the cylinder, if for no other reason than your own peace of mind, and check the endgap on all the rings. I really doubt that binding rings would cause what you describe because they would have simply broken when you fitted the cylinders if they had no endgap at all. They are very brittle. Who's rings are you using? How was the honing done? Hepolites seem to come undersized usually. You can never rely on quality control and what's in the bag may or may not always be what it says, so as long as you haven't put the head on you might want to check. It would be a good chance to see just how much an endgap is increased when you knock off the high spots on the rings and cylinder. I've done it and I got five thou doing just what you did.
 
Thanks for replies.
I'll pull the barrel to take a look. No point in flogging a dead horse and causing real damage.
I figured rings but I thought I'd check with the experts in case I'm misreading it, just in case this much stiffness was common.
When I see what's happening I'll post a follow up.

Thanks everyone

Bob
 
There are two types of oil ring available. One to fit in a piston with a deep oil ring groove and one for a shallow groove. If you have installed the wider oil ring in a shallow groove it will cause lots of drag and will cause damage if started. Compare the radial thickness of your old rings with the new ones to find out. Jim
 
Just remembered the 'slotted piston' issue. There were certain years in the 750cc engine where pistons had slots under the oil control rings. They have had a reputation for cracking and would be a reason to replace them even if they seem to be in good shape. Google Commando slotted pistons. Muy malo.
 
check the rings fit all the way into the groove, more than likely you still have carbon in the bottom of the grooves causing the rings to bind
 
bpatton said:
Just remembered the 'slotted piston' issue. There were certain years in the 750cc engine where pistons had slots under the oil control rings. They have had a reputation for cracking and would be a reason to replace them even if they seem to be in good shape. Google Commando slotted pistons. Muy malo.
Click to expand...

He lists his as a 72, I think they updated the pistons on the Combats. If they didn't we would have never heard of bearing problems because the pistons would have gone first. :mrgreen:
 
Here's what happened to mine. Early Hepolite.

Hard to turnover after new rings
 
Don't know for sure. Ebay purchase. Ran them for four-five thousand miles. Soon after a drag race with a Harley one piston flipped it's lid.
 
Apparently, the solid skirt 'Combat' pistons were introduced on all 750 models from engine number 204166.
 
There are only four things I have seen do that. Oil rings improperly installed, carbon in groove, wrong oversize ring, and as Jim mentions wrong ring entirely. Look at this as a learning experience and good practice. At least you did it yourself, I was usually trying to find somebody else's problem.
 
L.A.B. said:
Apparently, the solid skirt 'Combat' pistons were introduced on all 750 models from engine number 204166.
Click to expand...

Close! I'm 204183

I'll look close when I have it appart just in case.

A lot of good advice.

Thanks
Bob
 
[quote="Cookie" and as Jim mentions wrong ring entirely. .[/quote]

I'd be surprised if the rings are wrong type since I got them from a small shop that specializes in restoration of old British bikes from a fellow with years of experience.
I'll check them out very carefully though just in case.

Thanks
Bob
 
Do make sure when you have the barrel off that the crank and con-rods turn freely with minimal resistance.
 
I've got the engine turning over the way it should now.
Pulled the barrel and removed rings.
No obvious build up of carbon in ring grooves.
Compression rings are gapped just fine. 0.010, so not the problem. Oil scrpers even more, which is normal.
Now the oil rings.
I noticed when I first disassembled the engine that one piston was obviously pretty fresh compared to the other.
There was evidence of possibly a broken piston that had been replaced. Shortly after the unwinding of the rocker arm adjuster (broken locking nut) which dived down onto the cam follower probably caused the bike to be garaged since 1977. I bought the bike in startable condition with the rattle of the adjuster on the follower.
When I took off the old rings I noticed the oil scrapers looked pretty shallow (horizontal, not thickness)
When I put the new rings on the oil scrapers appeared noticibly deeper?
Didn't give it much thought at the time but since the compression rings aren't guilty I'm only left with oil rings.
When I looked carefully at the newer piston it appears the oil ring groove is not as deep as on the old piston, and the new deeper oil scrapers are bottoming out and arn't able to compress away from the cylinder wall.
When pistons started to loose their tops did they attempt to solve the problem by reducing the depth of the oil ring groove. A possiblity.
So my old piston with deeper groove can handle the deeper oil scrapers but my newer piston needs the shallower scraper.
Since the oil scrapers are not as critical as compression rings and the newer piston did not appear to have much time on it I elected to put the shallower old scraper rings and oil control ring back in the newer piston.
Problem has now gone away. Turns over like it should.
Weird problem but thats how its looks to me.
Now for figuring out the Mikuni VM conversion and installing the Pazon ignition.
Thanks for averyones suggestions.
Bob
 
Status
Not open for further replies.
Back
Top