Hi All,

 

I wonder if the more experienced eyes on NSR World would mind taking a look at an MC18 cylinder that we’ve just purchased for our MC21/MC18 hybrid race bike:-

 

https://www.flickr.com/photos/124222392@N04/

 

It’s hard to see from the pictures but there’s a hair line crack in the exhaust port bridge as highlighted in red. Having taken some time to gather our thoughts and consider whether it’s worth the risk to use it as a short term fix while my other cylinder is off to the platers, we’ve decided against it as broken collarbones, numerous dents in the bank balance and years of repairing fibreglass bodywork have taught us that it’s not worth taking risks when putting together a 2 stroke motor especially for race purposes.

 

We’re thinking that eventually the nickasil plating will flake off the bridge at the position of the crack resulting in a seizure. It could be first time out, or in 2 years time but we’re sure that it’s certain to happen particularly when on the exhaust port side of the cylinder which runs hotter than the intake side and it being the biggest port so there’s less route for heat transfer between the piston/cylinder hence this is where most seizures happen.

 

We would be keen to hear your thoughts and if you concur with our line of thinking? Would you be prepared to take a risk on it? Your honest opinion on this matter is welcome and if you think we’re being over cautious then please let us know!

 

Many thanks for your time!

I had an MC18 barrel welded and replated. It didn't last very long.
Like all repairs I guess it depends on who does it
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Why do things simply when you can complicate them

We've seen a lot of barrels over the years, with cracked exhaust bridges and/or front transfer ports. Like Rob, we've had them repaired, and some crack again, but the majority seem to be OK.

The niggling question is always, how long was it cracked before it was discovered?! Naturally, we would never re-use a barrel with a crack in one of our builds, but it does make you wonder if some of them have been running like it for thousands of miles already.

It's very important to allow the motor to warm up gradually. This is especially true for the MC16/18s (hexagonal barrels), which have much smaller water jackets around the barrels, and consequently heat up much quicker.
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Andy.
NSR-WORLD.COM

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Thanks for the feedback guys, would we be best having it repaired then? How many would be prepared to take a risk on it given that it's being used on a race bike?

Thanks in advance for any advice as my head says "repair" but my bank balance says "take a risk!"

As with anything, you're going to have to calculate that risk. At best, a seizure could simply result in £130 worth of dead piston and rings, and a coast to the edge of the track, but at worst it could destroy an £1100 crank, render the barrel (and head) unrepairable, and put you in hospital!

Here's the best way I can put it... We're happy to sell you piston and rings, if you want to try it, but don't ask us to build it!
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Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

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Andy,
 
We really appreciate your input. You have confirmed what we were already thinking and will just bite the bullet and get it repaired. It’s not worth the risk.
 
This will also assist us in our discussions with the person who sold us the cylinder. We bought the cylinder a few weeks back to keep us up and running as our front cylinder had a light seizure.
 
We didn’t have the opportunity to inspect it other than a few pictures being sent of the bore which looked okay and when asked, the seller advised that the bore looked to be in pretty good condition. (pretty damn good was his exact description)
 
We only spotted the crack when it arrived in the post and from my dealings with the person I genuinely don’t think they knew there was a problem with it. We’ve since went back to him with a very reasonable response to advise him of the crack and that we had reservations about using the cylinder. We suggested that we were prepared to send it back to him for a refund or alternatively, if he was willing to contribute to 50% of the repair costs this would also be an option. (50% would equate to around £100)
 
Although he was reasonably sympathetic in his response, he stated that we never asked if there were any cracks in the exhaust port bridge,  he thinks we’re being over cautious, he’s not willing to offer a refund and his suggested contribution to the repair is £20. He said that if we have concerns about using it then we should sell it on to someone else but we would never consider doing that.
 
Anyway, enough of our tales of woe, we’ll just chalk it down to experience!
 
Onwards and upwards…I’ll PM you regards piston and rings as we’ll need to sort this out before sending it off for repair.hi
 
Thanks again!
 
Jamie