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rebore tolerance

Posted: Sun Jan 10, 2010 3:14 pm
by two-stroke-brit
i am going to try and rebore the buffalo jugs at work if i can get the boss to ok it . :)
i was hopeing to just hone them and go with new pistons and rings but after the hone showed up steps and scores thats not going to happen. :roll:
how much clearance from the next oversize piston size do i want . :?
thanks mark.

Posted: Sun Jan 10, 2010 4:41 pm
by tz375
Do you have access to a proper boring fixture, or are you contemplating doing it on a milling m/c?

If you're thinking a mill will work, I would advise against it. I have seen barrels machined on a lathe and on a mill and they are never as straight as with a boring bar and thick torque plate.

more please

Posted: Sun Jan 10, 2010 6:19 pm
by two-stroke-brit
pardon my ignorance what is a torque plate and i was intending to bore it on a mill. :?
i was going to lift it from the table on 1/2 inch parralells and use a boreing head. either on a bridgeport or a cnc if available,
thanks mark

Posted: Sun Jan 10, 2010 7:51 pm
by Frank
I can tell you how my local guy does it (he uses a small cylinder boring machine but I can't see why a mill with boring bar wouldn't work.) He makes sure he has the piston before starting then bores until the piston is a tight slip fit. He then hones and ends up with .0015-.002" clearance.

The required clearance and method-of-measurement is generally given by the manufacturer. You have to be careful where you measure. I like the method described above (have had him do several for me like this) and it seems to work okay.

A torque plate replicates how the cylinder bolts load/distorts the cylinder. Theoretically it allows you to machine the cylinder in the loaded condition even though it may be oblong when unloaded. I'm not sure how big a difference it makes with air-cooled engines as they're inherently sloppy anyway.

Posted: Sun Jan 10, 2010 8:05 pm
by durgam
"loading" makes really good sense. A good way to replicate what you want would be to clamp the head down to a plate using the actual mounting holes. this will "pull" the material in the proper directions. It may only be a couple ten-thousandths of an inch, but it will be better than even a light squeeze in a kurt vice. My 2 cents.

Posted: Sun Jan 10, 2010 8:10 pm
by durgam
But to answer your initial question, I believe the bore tolerance is .01mm, total tolerance, or .0039 in. Thats on my 550, your water cooler, im sure is a bit tighter. probly -nothing to +.002. than hone,and fit your ring gap and you should be good.

queery on tol

Posted: Sun Jan 10, 2010 10:01 pm
by two-stroke-brit
i am confused you say 0.01mm or 0.0039 but 0.01mm is 0.00039 ie 4 tenths , :?
i get it about loading the jugs to bore them in the state they are in on the bike.
i think i will use the puller plate and just bore thro into the plate enough to clear the rad on the b/bar. :idea:
mark.

Posted: Sun Jan 10, 2010 11:31 pm
by tz375
A mill is indeed a fine tool and for a street motor it could well be good enough. A typical Boring machine has a really short tool ( like someone... oops I didn't mean that..) and a large diameter tool holder. That makes it incredibly stiff.

A boring tool in a mill is typically less stiff and may wander or chatter.

The torque plate is to simulate the stresses and out of round forces that it experiences in the motor. Admittedly a GT750 block is heavy and although it seems stiff, the upper parts of the bores are not very well supported.

Again, for a street motor, maybe it's not much of an issue. And yes, the hone should be used to take it out to finished clearances of .0018" - .0022" (1.8 to 2.2 thou)

WISECO pistons expand more and need a lot more clearance, so if you use them, follow their instructions.

And don't forget to chamfer all the ports.

Posted: Mon Jan 11, 2010 5:01 am
by Suzukidave
This page and the next several talk about clearance http://www.3cyl.com/mraxl/gt/manuals/gt750man/sr26.htm

Posted: Fri Jan 15, 2010 12:12 am
by H2RICK
i was intending to bore it on a mill.
Call me old-fashioned if you will but you're playing with fire trying to bore stroker cylinders that way. "Smarter" guys have already tried this method in an effort to save a few bucks. Very few, if any, have succeeded.
You will be FARRRRR better off and get a more accurate job with a real boring machine, IMO. Short term (financial) pain but long term (mechanical) happiness.....as long as the guy running the boring machine knows his job.
There is a good reason why they make boring machines.....and that is because they do a proper job.
My .02 worth.....

thanks

Posted: Fri Jan 15, 2010 1:16 pm
by two-stroke-brit
many thanks guys,
i am going to try and find a recomended shop in my area (houston).
i went to one place and saw his setup ,basically a pillar drill with auto feed and a cutting tool that he would "tap" out "abit" and try it until the piston fell in.
any recomendations are welcome.
thanks mark

Posted: Sun Jan 17, 2010 10:08 am
by bikegeezer
Box up the cylinder, and send it to Bill Bune. http://www.billbune.com/

Stu

Posted: Mon Jan 18, 2010 5:50 pm
by Suzukidave
bikegeezer wrote:Box up the cylinder, and send it to Bill Bune. http://www.billbune.com/

Stu
and the pistons so he can match them to the bores :D