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Post by humanshield on Jul 30, 2016 12:59:51 GMT -5
I'm just wondering what the current consensus is on larger bore BBK's FROM THOSE WHO HAVE ACTUALLY USED THEM PLEASE.
Those here that had early failures may have not properly cleaned the cases after clearing the case for the larger cylinder or maybe they ran their motors at high rpm.
Do you use a 50mm or larger BBK?
If so, how did it work out for you?
Did yours fail early?
Do you have any suspicion of the reason for the failure?
How hard do you run your scooter?
Do you take it easy or gun it every chance you get?
I'm at 3,500 miles with my 50mm BBK from 02starrider with no problems and expect to get to 5,000 or more.
It is not uncommon for me to hit 8,200rpm while riding but I rarely go over that.
Please chime in if you've installed a 50mm or larger BBK and what happened.
I'm wondering if it's the high rpm, racing style of riding that destroys 50mm motors....or if it's simply the weight of the piston that hammers the bearings out.
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Post by arkie on Jul 30, 2016 15:54:40 GMT -5
Ok so you got my vote and by now I hope you know the story behind it. I would also like to add that there was no case cutting involved for my install and never revved more than 8300. and that only while I was testing slider weights.
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Post by humanshield on Jul 30, 2016 17:34:53 GMT -5
Ok so you got my vote and by now I hope you know the story behind it. I would also like to add that there was no case cutting involved for my install and never revved more than 8300. and that only while I was testing slider weights. Have you detailed what happened somewhere on the forum? I could probably start a thread asking how amny 47mm BBK's failed after installation and I'm positive there would be some. Hopefully over time we'll get a lot more responses. Maybe there are 50mm BBK's out there with pistons that are WAY too heavy? |
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Post by arkie on Jul 30, 2016 17:41:33 GMT -5
Big end failure. I have since replaced the CS and upgraded to the airsal piston. Which IIRC is lighter than the standard 50mm, also domed a big plus.
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Post by Deleted on Jul 30, 2016 17:57:05 GMT -5
I am posting here, not to vote, but only so I can follow the results. I think those large pistons are too heavy (And create too much pressure) for the stock crank but, we will see what the evidence shows here.
Bill
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Post by spaz12 on Jul 30, 2016 18:10:15 GMT -5
I keep meaning to bring this up... Read up about the Honda Metropolitan before 2006. Because of only one breather line (or something like that) they had a lot of bearing failures. The cases were building up a lot of moisture inside and that's what was causing the issue. Anyway, might be relevant to the qmb crank bearing failures...
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Post by humanshield on Jul 30, 2016 18:26:11 GMT -5
Big end failure. I have since replaced the CS and upgraded to the airsal piston. Which IIRC is lighter than the standard 50mm, also domed a big plus. I wonder if it would be possible to lighten the 50mm piston a bit? |
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Post by Deleted on Jul 30, 2016 18:37:37 GMT -5
Big end failure. I have since replaced the CS and upgraded to the airsal piston. Which IIRC is lighter than the standard 50mm, also domed a big plus. I wonder if it would be possible to lighten the 50mm piston a bit? You could drill a series of holes below the piston rings on the skirt. It would not make it much lighter and would make it a bit weaker but, you could do it. Bill |
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Post by cwazywazy on Jul 30, 2016 22:04:26 GMT -5
I went all out with a 52mm domed piston and big valve head. (Ported and "polished" by me.) I installed that with an oil cooler months ago and it's been running strong since. I have no doubt that it will fail someday, but it's not like it blew up immediately. I'm pretty much always WOT unless I'm behind traffic or at a stop, so it gets run fairly hard. Tuned for about 7k. I have no idea how many miles I've put on it, probably somewhere under 150-200 because I do not ride it regularly or for long distances ever. I don't think I've even gone 10 miles in one ride.
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Post by 90GTVert on Jul 31, 2016 6:55:08 GMT -5
Failure at 2,003 miles with a brand new stock crank and no grinding on the cases (while they were together) with a 50mm cylinder kit.
NCY "performance" cam, ScarppyDog big valve head, port matched intake/spacer, 18-20mm carb, UNI pod filter, P&P exhaust, timing advanced 2-3 degrees, lots of CVT work, and stock gears (taller gears not available back then). I wedged a 120/90-10 rear tire onto the short case to try and help some with the drive ratio.
Shell Rotella T 15W40 changed multiple times in the first 500 miles and then every 500 miles or less.
I managed to finally squeeze 50MPH out of it on a long stretch tucked down the best I could on the same night that it failed. It had to be 9,000RPM+ to reach that speed. Normally, it cruised around 40MPH; prob in the 8000RPM range, but I can't recall for certain.
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Post by humanshield on Jul 31, 2016 7:56:22 GMT -5
In addition to Arkies suggestion that build quality from the manufacturers could vary, I'll throw this out....
Low oil pressure will cause failure quicker with a larger BBK. It's a possibility that some of the failures were simply due to low oil pressure. One of the 4 stroke failures voted on may have been one with known oil leak problems.
Low oil pressure could be a result of a worn oil pump, poor manufacturing tolerances, leaking crank seals or improper installation of sleeves and gaskets during the build. Heck, even improper torquing of the head bolts might cause oil problems the way these motors are designed.
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Post by humanshield on Jul 31, 2016 7:58:19 GMT -5
Big end failure. I have since replaced the CS and upgraded to the airsal piston. Which IIRC is lighter than the standard 50mm, also domed a big plus. I have both on hand. As soon as I get some time I'll weigh them both and post the comparison. |
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Post by arkie on Jul 31, 2016 9:35:07 GMT -5
There is (somewhere) here a comparison of lots of different pistons by weight. I can't seem to find it now.
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Post by tortoise2 on Jul 31, 2016 10:23:36 GMT -5
There is (somewhere) here a comparison of lots of different pistons by weight. I can't seem to find it now. Link
Then there is the sealer-obstructed oil passageway scenario.
NOT selecting at least a 125cc scooter sill remains the most common regret . . as evidenced by the numerous repeated BBK queries by the "game the system" crowd. Doesn't take all that much effort to get a motorcycle endorsement in most states . . study the manual, pass the written, and drive around a few cones for the practical. Get insurance, register the scooter, and concentrate on avoiding collisions . . instead of eluding statute consequences.
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Post by Deleted on Jul 31, 2016 10:59:16 GMT -5
There is (somewhere) here a comparison of lots of different pistons by weight. I can't seem to find it now.  Bill |
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