Cracks under the cylinder, can it be repaired?

James912

Member
Apr 12, 2011
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Florida
I finally quit procrastinating, took a week off from work, started working on my bike. Replaced a shot bike tire valve. Swapped out a nt carb that had the flanges breaking like they were plastic and got it running. Drove around the neighborhood to give it some time to warm up. I felt air hitting my right leg constantly from the engine. So I drove back under my pole barn and found this when looking for where the air was coming from....
https://photos.app.goo.gl/MDkGZBjqUpzHZ1V72
 

ZipTie

Active Member
Jan 8, 2016
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Mpls Mn
How many miles on this engine, where and when was it purchased?
what cc? That looks pretty fatal in my opinion. I have not noticed this exact type of fracturing on any other post as of yet.

This air filled cast aluminum probably would be pretty weak if welded up in that location. I presume it overheated very badly, probably from the leaky intake carb manifold and running so lean it got super hot. I was surprised it even ran after it cracked and or when it was so lean.

I would just put a new engine on it. I believe good 40mm engines, with balance cranks and better bearings plus bigger 8mm studs with allen head cap screws can be had for about 100.00 now. What is ones time worth after disassembly, welding filing, re assembly etc, and then it might just fail again right away? Who knows? Others will hopefully weigh in here...
 

James912

Member
Apr 12, 2011
586
0
16
28
Florida
How many miles on this engine, where and when was it purchased?
what cc? That looks pretty fatal in my opinion. I have not noticed this exact type of fracturing on any other post as of yet.

This air filled cast aluminum probably would be pretty weak if welded up in that location. I presume it overheated very badly, probably from the leaky intake carb manifold and running so lean it got super hot. I was surprised it even ran after it cracked and or when it was so lean.

I would just put a new engine on it. I believe good 40mm engines, with balance cranks and better bearings plus bigger 8mm studs with allen head cap screws can be had for about 100.00 now. What is ones time worth after disassembly, welding filing, re assembly etc, and then it might just fail again right away? Who knows? Others will hopefully weigh in here...
It's a thatsdax f50 49cc. It's been sitting in my shed for several years. Before I quit riding it last time back in like 2012 I redid the cylinder studs, and base gasket. It's has only like 150 miles on it.
 

crassius

Well-Known Member
Sep 30, 2012
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USA
without a picture, it is hard to tell, but if the area isn't under a working load, it may be fixable by drilling to keep crack from spreading and then filling with jbweld to seal the leak
 

Greg58

Well-Known Member
May 1, 2011
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Newnan,Georgia U.S.A.
James I hope that helps you find a repair. At this point if it were mine I would try a two part epoxy, I know a lot of people are against putting anything inside the engine but I see no other choice. The epoxy products have advanced greatly, some can penetrate slightly and hold real well. It needs to be on the inside and clean it well first.
 
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crassius

Well-Known Member
Sep 30, 2012
4,032
154
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USA
except for the threaded hole for the head stud, there seems no real problem there - if there are enough threads to hold the head, then it is just a matter of stopping any air leaks - I'd think about how it looks and feels, then maybe get epoxy or jbweld onto the stud and glue the stud right into the case, then seal a bit more from outside