• Husqvarna Motorcycles Made In Sweden - About 1988 and older

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    As you all know, Coffee (Dean) passed away a couple of years ago. I am Dean's ex-wife's husband and happen to have spent my career in tech. Over the years, I occasionally helped Dean with various tech issues.

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1971 250 MX - Rebuilt Engine - Piston Rings seized

N

NMc

Husqvarna
A Class
Hi Everyone. I need a little expert help for my 1971 250 Cross. Any advice or guidance gratefully recieved.

The engine was rebuilt a while ago by someone who knows these engines ( ex MXer who ran Huskys in the UK ). Got new piston from Vintco and a few other parts I needed. I'm new to two strokes, and when I threw it back into the back and buttoned it up, I pick up the wrong can of fuel and ran it for about 5 minutes. Result was a badly scratched cylinder and a torn up piston with seized rings. My error. Not doing that again.

However, a year or so later ( my projects are slow ) I have the engine back in the bike. I had the cyclinder bored out and the next size up piston from Vintco. I fitted the new piston myself, and while it was the first time I've done that I was very careful. Everything checked out. Compression was great. Made sure I had the 2 stroke fuel made up ( temporary tank ) and fired it up. Within a few kicks, it fired right up. It ran for a few seconds and I hit the kill switch. Exhaust was not fitted correctly ( no springs ) so there was a little smoke but all looked fine. Happy boy.

A week later, I'm back in the garage and have buttoned up a few more things. Rechecked torque on the head. Exhaust springs on. Quick compression check, and it's around 140. I have fuel ( again, very careful to check it's the correct mix 2 stroke ). Check for spark ( it's a new Vape ignition ) and I kick it over. It takes alot of kicking, but eventually fires up. Runs on its own for 5 seconds and then dies. Smoke coming out of the front of the engine and I'm wondering whether I've blown a gasket, but then remember the cylinder and the head have no gasket. Kicking it over now feels like there's no compression, so I do a check and it's below 50. Not good. Check the manual and it talks to lapping in the head and cylinder spigot as an initial run can find leaks in that mating, so I bought some lapping paste and take the head and cylinder off.

And there are scratches on the cylinder. The ring is seized in the piston. The piston above the ring feels like I've been hitting it with a screwdriver. It feels jagged when I run my hands over it. It's not as bad as the previous piston that I ran in the bike with no oil, but it doesn't feel like I want to put it back in the cylinder again and the ring is now seized in place, so is not sealing which is why I have no compression.

So I'm stumped as to what has happened and I've missed something fundamental. I've added some photos here and a video of the first run.

Video of first run here. Nothing untoward.

IMG_6726.mov

drive.google.com drive.google.com

This was the first piston that I messed up and ran without two stroke fuel. See that pitting above the ring gap ? That looks like it's happening again.

IMG_6812.JPG

drive.google.com drive.google.com
 

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That ring gap is no good also it looks like you don't have a pin in the piston to index the ring take the ring off put it in your cylinder and measure the gap when you have the ring off check the piston groove and see if there's a pin in it there should be I'm pretty sure otherwise the rink could rotate around and get caught on one of the ports.
 
By any chance did you use assembly lube or oil up the ring and piston before assembly? How tight was the ring on the piston when first assembled?

Might want to touch base with Vintco and explain what has happened - they will want to see the pics.
 
Thanks for the replies.

I did check the gap before putting the ring on. The ring from Vintco has a recess to sit around a pin in the piston that seats it and stops rotation. I'll take the ring off and do some fundamental checks again.

There was plenty of assembly lube when I fitted the piston and slotted the cylinder over the top. That took some effort to co-ordinate compressing the ring with dropping the cylinder over the top.

It's the damage on the piston above the ring that I'm concerned about. That is what is scoring the cylinder. The ring gap shouldn't impact that.

I'll get onto Vintco as well.
 
Looks like you have done everything 'by the book'. Let us know what your conversation with Vintco reveals.

BTW - what is the cylinder squish - with the head off, where does the top of the piston sit at TDC? Any marks on the cylinder head combustion chamber it self?
 
I'm wondering if there may have been debris from the bore job left inside one of the transfer ports or in the crank case from the previous seize when the 2T oil was absent.
 
The pictures above show a base gasket.
Whether it's in good condition or not is unknown My biggest concern is that the piston ring is are not poking out of the ring groove they're stuck in it why
And I agree with crash all those components should have been washed thoroughly then lightly oiled for assembly
Those scratches are not good and that piston is machined really badly it looks like to me from the pics
I'd be interested to see the cylinder that was bored out. Did they have the piston and ring when they did the work
 
Last edited:
Crashaholic said:
Excuse me, but the cylinder requires a base gasket.
Click to expand...
OK. The service manual I have says that's not the case, and it's an expected metal to metal contact that needs lapped in. Unless I'm translating my Swedish very badly. I will double check, but between the cylinder and the head there is no gasket in the parts manual either.

1756377094809.png
 
Last edited:
Crashaholic said:
I'm wondering if there may have been debris from the bore job left inside one of the transfer ports or in the crank case from the previous seize when the 2T oil was absent.
Click to expand...
I don't think this is a factor. The piston crown was clean, so I expect that's just me dis-assembling and removing a protective rag.
 
Crashaholic said:
I'm wondering if there may have been debris from the bore job left inside one of the transfer ports or in the crank case from the previous seize when the 2T oil was absent.
Click to expand...
This is quite possible. I'll dig a little further down to see if there's obvious debris or crap down there. I didn't think to look.
 
DeathFromAbove said:
The pictures above show a base gasket.
Whether it's in good condition or not is unknown My biggest concern is that the piston ring is are not poking out of the ring groove they're stuck in it why
And I agree with crash all those components should have been washed thoroughly then lightly oiled for assembly
Those scratches are not good and that piston is machined really badly it looks like to me from the pics
I'd be interested to see the cylinder that was bored out. Did they have the piston and ring when they did the work
Click to expand...
They had the piston and ring and they showed me the work, and the ring gapped in the cylinder to spec ( that I gave them ).

1756376361007.png


They're one of those old school machine shops that have been around for 100 years, so it's much more likely that I've dropped the ball than them.

There is a base gasket, but not one between the head and the cylinder. The cylinder and the crank cases have a gasket between them.
 
Last edited:
Cylinder to Cases Gasket Yes
Cylinder to Head No Gasket Just Lapped together with compound and re Cleaned thoroughly
Wash the cylinder and all ports before install Yes
Did you have the cases Apart?
Clean the crank well completely?
Just throwing this stuff out there as this is quite unusual with new piston and bore.
Pics of your cylinder
Great Luck
 
Thanks everyone for the replies. I'm fully open to this being an issue I caused. I've also got a chat with Vintco to see if they can help pinpoint what's going on.

I'll use it as a learning exercise if I have to split the cases again. Still not got to the cause, but you've pointed me in a direction. I'll report back.
 
DeathFromAbove said:
Cylinder to Cases Gasket Yes
Cylinder to Head No Gasket Just Lapped together with compound and re Cleaned thoroughly
Wash the cylinder and all ports before install Yes
Did you have the cases Apart?
Clean the crank well completely?
Just throwing this stuff out there as this is quite unusual with new piston and bore.
Pics of your cylinder
Great Luck
Click to expand...
Thanks DeathFrom.

Wash the cylinder and all ports before install - It was a full cases apart rebuild by an engine builder. So yes, but not after I'd run it and scored up the cylinder so that might be where to look next.

Did you have the cases Apart? - Yes for the rebuild work.
Clean the crank well completely? - Not after I messed up post rebuild. I took off the cylinder, rebored to size and dropped in the new piston. This is what I'm leaning into now as the cause. I'll know more once I've prodded around some.

Cylinder not great. Those aren't deep. You can't catch a fingernail in them and I expect they'll hone straight out, but there's just lots of them. The bike has genuinely run for about 30 seconds total so something is marking it up. The piston crown is really rough though on the edges, which I'm sure is what is causing it which would align with lots of crap in the ports that is getting picked up and pushed around. It was not like that when I put it in, so no thoughts that it's a bum piston.

IMG_6808.JPG

drive.google.com drive.google.com
 
I'd clean the s*** out of that stuff and just put it together with lube and if the piston moves up and down without binding I would run it 20 to 1 fuel oil ratio let it smoke for a while
That cylinder doesn't look too bad but I can't tell from here whether or not the ports have a chamfer on their edges should have a chamfer around all edges of all ports just to break that edge.
All those marks in the boar look like paint but again I'm not there to look at it Great luck
 
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