mysterious knocking what do you think

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pan57
Posts: 32
Joined: Mon Jul 16, 2007 11:25 am
Location: Baton Rouge, la

mysterious knocking what do you think

#1

Post by pan57 » Mon Jul 07, 2008 4:21 pm

I had a mysterious knock on my brand new rebuild with about 700m on it. I could not pinpoint the knock so I started with taking the heads off. I found that the front head and piston was real heavy with carbon and the back had a lot less. This figures I had an intake leak which I will take care of when I put it back together. But I figured that was not the source of my knock so I continued to pull the cylinders off and found that I had seized my front cylinder. I figured the cause of it was during the break in period my linkert developed a leaky float and dumped a bunch of gas in the front cylinder and washed out the oil. That is the only thing I can put my finger on right now. I measured the ring gaps before I put it together and everything was within specs. I do have some wobble in my front piston so I will have to change my pistons, wrist pins and have my cylinders bored out to .0030. I have 2 questions first could having a seized piston or piston with out oil develop a knock and second if I have my linkert dialed in will I run into this problem again. it will take me two weeks to get the pistons and cylinders back in the meantime I will check the cam cover, lifters and oil pump to make sure there are no damage spots with those parts. What do you think?



VT

#2

Post by VT » Mon Jul 07, 2008 4:55 pm

Knocks are hard to isolate. The last knock I had was a pinion shaft taper not being a perfect match to the right-side flywheel taper. I was picking up metal shavings in my external oil cannister.
Having piston wobble isn't good. You should find out the reason. Rods twisted? Unless you like building the motors and tearing them apart again, I'd suggest having someone else build it, since there are so many variables that can go wrong. If you don't build the motors everyday, you might miss something, get it all back together for another 700 mile test ride and have it fail, imo.
These people are good and closer to you than Stett's. Pay them whatever they want and ride the machine for a long time, worry and knock-free.
http://www.accurate-engineering.com/

pan57
Posts: 32
Joined: Mon Jul 16, 2007 11:25 am
Location: Baton Rouge, la

#3

Post by pan57 » Mon Jul 07, 2008 7:01 pm

Thanks for the heads up about the rods being twisted. I had the bottom end completely rebuilt by the local guy who has been building bikes for about 35 yrs. He is also doing all the sizing and machine work. Basically all I do is check his measurements with what is in the book as I’m assembling it. I think the wobble in the piston may be from the wrist pin bushing. I will know more tonight after I take the old piston and wrist pin out.

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#4

Post by Panacea » Tue Jul 08, 2008 1:05 am

Why re-bore the rear cylinder? Mike

pan57
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Location: Baton Rouge, la

#5

Post by pan57 » Tue Jul 08, 2008 1:47 am

I have to re-bore the rear cylinder because I am not sure but I don’t think I can run two different size pistons on the bike. The front cylinder is scared up too much to just re-hone so I will have to go to the next size up. And because I do not want to run different size pistons I will have the back one bored also. I may be wrong but the guy who’s done all my major work said this is the way to go. Let me know if I need to question his judgment. Chris.

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#6

Post by Cotten » Tue Jul 08, 2008 4:22 am

Since both pistons are connected to the same crank throw, the motor really doesn't care much at all if the pistons match.

V-twin motors are very forgiving.

....Cotten

pan57
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#7

Post by pan57 » Tue Jul 08, 2008 4:35 am

Thanks Cotten for straightening me out but I want to be sure I understand you right that I can run the front piston .0030 and the rear pistons .0020 over and not hurt the bottom end or the heads.

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#8

Post by Cotten » Tue Jul 08, 2008 12:36 pm

Pan57!

I think you mean .030" and .020", but it is still an insignificant difference as long as fits and assembly are correct, of course.

Once upon a time, the head came off of a valve in my tired old stroker.
With most of a summer ahead of me, I honed out the damaged cylinder from .050" to .070" for a fresh piston and slapped it together. The machine ran as flawlessly and smooth as ever.
That winter I tore it down to find that the remaining .050" piston from years before had a very low compression dome (for hack duty). The .070" was standard 8 to 1, so with the added volume there may have been as much as a full point of compression difference between the two. Yet even the plug burn was identical.

With only one crank throw, a v-twin really doesn't know it has two cylinders. It is really a single that only fires half of its combustion chamber each revolution.
(That's why Harleys do not have "distributors".)

....Cotten

Guest

#9

Post by Guest » Tue Jul 08, 2008 1:56 pm

Thanks I'll talk to the guy who is doing the work and see what he thinks. When I first noticed the problem I did a compression test and came up with 95 lbs on the front and 105 on the rear should this be higher

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