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Post by sailracer on Aug 6, 2013 12:13:53 GMT -5
I didn't think I did any damage when the engine with the bbk soft seized, but on closer examination, I did.The 2nd ring froze to the piston in the groove. The bbk came from scrappydog, but they won't sell the piston alone. I got the ring out and cleaned out the grooves, and ordered a new set of rings. I also bought a 50mm hoca piston set from partsfor scooters, that piston looks domed. Does anyone think this will be a problem? The jug and head are scrappys, and piston from pfs
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Post by scootnwinn on Aug 6, 2013 12:29:21 GMT -5
You will have to check the squish to be sure before you do final assembly. You want to use a bit of grease and insert 4 pieces of thick solder. You turn the engine with head on and check to see how deformed the solder ends up. You will want to make sure that it won't interfere with your valves. Should be fun. It may be possible to figure this just by checking how far the piston sticks out at top dead center...
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Post by brandjur on Aug 6, 2013 12:31:24 GMT -5
It shouldn't be a problem as long as the cylinder is good and there aren't wear marks. Did the old piston rings scratch the cylinder walls?
Hoca parts are usually synonymous with racing, so the dome is there to increase the compression. You might not like the extra compression as it can lead to harder starting and a tighter tolerance with the a/f mixture. To compensate, people add a 2nd base gasket to lower the compression.
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Post by sailracer on Aug 6, 2013 13:07:25 GMT -5
Yeah clearance and compression are exactly my concerns. The cylinder walls look great. Thats why I didn't think there was any damage. Something told me though to remove the piston, and there it was. 1st ring ok though go figure. wrist pin turned blue too. I don't trust it anymore.Anyway, I got a set of rings for it if the hoca don't work out
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Post by jjoshua20213 on Aug 6, 2013 18:14:36 GMT -5
[replyingto=salracer]sailracer[/replyingto]Ebay has everything!
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Post by tvnacman on Aug 6, 2013 19:49:17 GMT -5
I don't agree with an extra base gasket to compensate or hard starting . I would work with timing for start up.
John
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Post by brandjur on Aug 6, 2013 22:02:37 GMT -5
I don't agree with an extra base gasket to compensate or hard starting . I would work with timing for start up. John I do agree with you, but the piston has already been purchased. The dome of the piston has already affected the quench area of the engine and yes, Hoca pistons are designed for increased compression. Has an extra gasket been practiced in all types of engine building, YES. From turbos to stroker engines. Also, for extra clearance from the valves, there is no other choice but to add the base gasket. Personally, I wouldn't have chosen the Hoca piston for mostly normal driving. Just wondering, are you proposing a product you carry for the timing change? |
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Post by sailracer on Aug 7, 2013 8:29:29 GMT -5
I bought the hoca so I could have a choice. Here's the way i'm looking at it- The original 37 mm piston from the qmb 139 has quite a dome on it.(convex) The piston that comes with the BBK is dished.(concave) This would suggest to me that the engine was designed to run best with a domed piston, and the hoca, is the better piston for the job. I am going to try it. I'll dry fit and check clearances first, and adjust as necessary. If i don't like it, i can stow it for the future builds,and put back the 50mm dished piston with new rings and wrist pin. ;D
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Post by sailracer on Aug 7, 2013 8:40:02 GMT -5
I was just thinking, Since I have the original motor in the scooter and the build on the bench, I'm going to run a compression test on the running motor and compare the numbers to the other motor with each the hoca, and the BBK supplied piston . That should tell a lot  |
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Post by brandjur on Aug 7, 2013 11:25:57 GMT -5
I was just thinking, Since I have the original motor in the scooter and the build on the bench, I'm going to run a compression test on the running motor and compare the numbers to the other motor with each the hoca, and the BBK supplied piston . That should tell a lot  There ya go! That's how to do it. |
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Post by rockynv on Aug 8, 2013 4:09:34 GMT -5
Yeah clearance and compression are exactly my concerns. The cylinder walls look great. Thats why I didn't think there was any damage. Something told me though to remove the piston, and there it was. 1st ring ok though go figure. wrist pin turned blue too. I don't trust it anymore.Anyway, I got a set of rings for it if the hoca don't work out Sounds like you forgot to use assembly oil as you were putting it back together and the dry startup got you. Those first few minutes after you start up a rebult engine for the first time are critical and if you don't use an assemble lube can lead to a soft or hard seizure. www.autozone.com/autozone/accessories/Melling-4-oz-Mell-Lube-engine-assembly-lube-and-break-in-fluid/_/N-25qx?itemIdentifier=179584_0_0_ |
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Post by sailracer on Aug 8, 2013 10:15:03 GMT -5
Just got the Hoca piston in the mail.I gotta say, In terms of quality, there is absolutely no comparison to the hoca vs the piston supplied in the kit from scrappydog's.When I get the gaskets in tomorrow, I will do a compression test with 1 base /2 bases, and the regular BBk piston. Hoca looks top-notch,even comes in it'sown box |
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Post by sailracer on Aug 8, 2013 16:28:37 GMT -5
>:(these***$###@-- chinese motors!!!! I picked up a compression tester with nice brass metric fittings, screwed it into the plug hole, tested, and the darn fitting stayed in the head, but the hose unscrewed  Had to remove half the shroud just to take it out! why cant they put the plug in an easier place! :-Xchinese( got it out,and superglued the fitting to the hose...wont unscrew again) anyway, tested the stock 37mm piston qmb139-factory=160 PSI Ill do the scrappy and the hoca later |
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Post by sailracer on Aug 9, 2013 10:44:26 GMT -5
Ok either something is wrong, or scrappydog BBK sucks. I put new rings on the piston that came with the BBK. Qmb 139-37mm piston-all stock=160PSI=starter cranks easy QMB139-50mm BBK A9 cam SDS=  -100 PSI ???starter lugs  When the new base gaskets arrive, I will test the hoca piston with1 &2 base gaskets (SDS is scrappydog scooters) rechecked the motor in the scooter (just to make sure gauge is ok-180 psi,and it turns over easy.) |
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jet
MJ, Orange coil, Blue Performance CDI (Orange NCY did not work), NGK Iridium, 5g Sliders
-100 PSI ???starter lugs