Author Topic: Bottom end problem or ???  (Read 2743 times)

Offline coater87

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Re: Bottom end problem or ???
« Reply #15 on: 27.05. 2009 00:41 »
 OKaaay (I guess), I may have found my kicker problem.

 Just popped the head and barrels off this motor. Bore is for sure standard, which is great. Cylinders are do-able as is, which is also super.

 Pistons- look good except for one minor thing. The compression ratio of these has got to be out of this world- I had no idea what road rocket or any other a10 piston should look like until I met you guys, I no longer think these are stock at all.

 But before I go off thinking too much, does anyone have a pic of stock G/F pistons? None of that high test stuff, just cooking pistons.

 These have the normal relief for the valves, problem comes with the dome- and I mean full dome- no flat area in the middle at all. When my buddy raced, these are what we would have called "head fillers", or "valve benders". Also, underneith the oil control rings, are 8 splash oiling holes- really reminds me of the alcohol cars.

 If these are stock or sound like it, someone tell me before I chuck these up in the lathe and make flats out of them! *smile*

 Lee
Central Wisconsin in the U.S.

Online RichardL

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Re: Bottom end problem or ???
« Reply #16 on: 27.05. 2009 05:06 »
Lee,

I need to be careful quoting years, but if I am readng Roy Bacon's book correctly and if he is, indeed, correct, the full domed pistons will be 9:1 a la '62-'63 Super Rocket, and the same pistons I am running. When you started your description, I thought you were heading to "look like mountain peaks," which would indicate 10:1. My 9:1s are running quite nicely right now, but I don't have enough miles on them or me (really, plenty of miles on me, just not enough of them riding a motorcycle) to judge the ultmate benefit or drawbacks of this somewhat high compression ratio. I've been wanting to post a YouTube video so the sound of these pistons through stock mufflers and with an undertuned cam (356) could be heard/judged by others. I can tell you that the bike has a good deal of pep.

Richard L.
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Offline coater87

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Re: Bottom end problem or ???
« Reply #17 on: 27.05. 2009 11:26 »
 Richard,

 Thanks for this information, I have a Bacon on the way as we speak. I was a little thrown off by the pics in the Haynes manual showing very flat pistons.

 So the splash oil holes are normal? I also have to check these over for the skirt splits, I dont remember seeing any and its too dark in the shed right now.

 Well, so much for my theory that these may be as high as 12/1 and that is what was causing my kicker problems. If I can get a picture of these, I will post it up for you to look at and see if they look about like yours. They still seam awfully full for 9/1, but then again I didnt even know what stock looked like!

 Lee
Central Wisconsin in the U.S.

Offline A10Boy

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Re: Bottom end problem or ???
« Reply #18 on: 27.05. 2009 18:36 »
Its all personal choice of course, but I've heard guys say that with a Gold Flash [with a cast iron head] you don't want HC pistons as the higher compression will produce high temps and can cause pre-ignition. That's why BSA brought in the alloy head to help with heat dissipation on the sports models.

The flat tops produce a 7.25 compression ratio with a cast head and make a very sweet motor.
Regards

Andy

1960 A10 - Black Golden Flash
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Yam XJR 1300

Offline snowbeard

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Re: Bottom end problem or ???
« Reply #19 on: 27.05. 2009 20:44 »
I think the oil splash holes are pretty common, at least they are there in my 60 super rocket with +060 pistons, my brand new hepolite +060 pistons, and the old beater +040 pistons I have. 

what came original stock may or may not have them, but it seems in my n=3 that they are supposed to be there ;)
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Offline coater87

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Re: Bottom end problem or ???
« Reply #20 on: 27.05. 2009 21:50 »
 Thanks guys,

 Andy, I agree about the higher compression, it is certainly nothing I want at this point. I may ask around a bit for a few measured figures on a stock piston- then honestly lathe these off. The "hump" is not hollow, but I do see a few balancing marks, so someone had an idea what they were doing.

 The more I think on it, these pistons may have been the saving grace for this bike. The brothers told me that after the (chopper) "rebuild", the bike never did run correctly, and during trying to figure out what was wrong they gave up on the project.

 Of coarse, this was after they broke every other fin off the head, and drilled 5/16 holes in the rest and threw all the hard to find bits away.

 I figure this is what happend: they added HC pistons to a good running motor, swapped out the smaller carburator for a larger bore/jetting, and added straight pipes. The Mag may have been a little weak so really poor spark now, coupled with flooding and no exhaust back-pressure and that was the end. Lucky me! *smile*

 Lee
Central Wisconsin in the U.S.