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Post by jacoblong09 on Mar 18, 2019 19:26:25 GMT -5
i ordered a new big bore kit for my 139qmb 4t engine. it's 100cc, and i originally wasn't worried about the effects the extra power would have on the crank, but now i'm worried that the power may cause damage to the stock crankshaft and crank arm. I want to know if i should be worried if the stock crank will support the increase, or if i should get a better crank to avoid damage.
thanks
- jacob
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Post by 2stroked on Mar 18, 2019 22:10:54 GMT -5
How many miles on the bike? How hard has it been ridden? Is there play in the bearings or ConRod?
There's a lot of "talk" about the "100" cc kits beating the hell out of cranks, but I personally have never seen a or even heard of a first hand account of it. Its always "my neighbor's grandson's best buddie's step dad had it happen".
Not saying it won't happen. Not saying g it will. There have been plenty of people here run those with stock cranks. Maybe one or two will chime in.
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Post by jacoblong09 on Mar 20, 2019 6:49:14 GMT -5
cool, and i'll guess that the low mileage i have (at least what the odo says, idk when the engine was swapped), that applies even more. i'll use the stock crank. actually right now i'm waiting for my kit tomorrow it is arriving, and i still need to order an exhaust, so i cant even start it till i get that installed as well  . - jacob |
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Post by 90GTVert on Mar 22, 2019 7:22:31 GMT -5
I put in a new crank and killed it in 2,003 miles with a 50mm kit. To be fair though, I was revving it out to 9-10,000RPM with a cam, BVH head, etc... and pushing it as hard as it would go to get 50MPH out of it with stock gears. You can venture over to the thread linked to see some actual experiences with 50mm kits. 49ccscoot.proboards.com/thread/16593/50mm-bbk-thread-installed?page=1 Unless you have reason to be suspicious of the crank, I'd install the kit on the current crank. Most of the folks not tearing up engines in short order seem to be tuning more mildly. Keeping RPM down. |
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Post by fugaziiv on Mar 22, 2019 8:06:20 GMT -5
A lot depends on how many miles are on the stock crank, and how hard those miles have been.
Also, since 100cc kits don't actually exist, it depends on the bore of the kit you bought and the manufacture as well. BBKs for the QMB139 are normally found in 47mm, 50mm and 52mm. So, some more details will help.
Matt
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