• Husqvarna Motorcycles Made In Sweden - About 1988 and older

  • Hi everyone,

    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.

    When he passed, I worked with his kids to gather the necessary credentials to keep this site running. Since then (and for however long they worked with Coffee), Woodschick and Dirtdame have been maintaining the site and covering the costs. Without their hard work and financial support, CafeHusky would have been lost.

    Over the past couple of weeks, I’ve been working to migrate the site to a free cloud compute instance so that Woodschick and Dirtdame no longer have to fund it. At the same time, I’ve updated the site to a current version of XenForo (the discussion software it runs on). The previous version was outdated and no longer supported.

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    Coffee didn’t document the work he did on the site, so I’ve been digging through the old setup to understand how everything was running. There may still be things I’ve missed. One known issue is that email functionality is not yet working on the new site, but I hope to resolve this over time.

    Thanks for your patience and support!

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?
 
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