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July 29, 2014, 02:49:10 AM

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Author Topic: Piston-Cyl tolerences  (Read 417 times)

TejK

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Piston-Cyl tolerences
« on: July 22, 2013, 06:56:05 AM »
Hi Guys,
I was trying to figure out the correct piston to cylinder clearance( without the piston rings) but couldn't find any related topics. My friend got his head removed on account of a leaky valve seal. His bike has seen minimum use since he bought it in 2009 and has genuine 12,000kms on the bike. Blue smoke on starting the bike led him to have the bike checked and resulted in detection of a worn valve seal!! The mechanic suggested new rings to be added as his old rings also seemed worn. I saw that the piston w/out the rings has considerable movement space within the cylinder - seemed too loose to me somehow and I could pretty much move it around with a light touch!! Is this the case with all AVL's ??? I always thought the tolerances were much tighter with the engine being of better materials than the standard cast iron barrel one. Anyway - with the new rings and valve seal, the bike is back running fine and has good compression.

In my bike I had a new liner and the piston and cyl clearance was much tighter. Am wondering whether its made too tight by the lathe shop by mistake?? It would help if someone can shed light on the cylinder piston tolerance in the AVL.
Thanks ,
T

ace.cafe

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Re: Piston-Cyl tolerences
« Reply #1 on: July 22, 2013, 12:49:09 PM »
The actual piston-to-bore clearance as honed is specific to the piston being used.
Different types of pistons have different materials and expansion characteristics, and have diffrerent needs.

As for the top of the piston being able to be moved around by hand, that's because the top of the piston is much smaller than the biggest piston dimension. The biggest piston dimension is measured at the skirt, probably around the same height as the wrist pin, and measured from front to rear. That's where the piston-to-bore measurement is taken.
The top of the piston must be considerably smaller to allow for the rocking motion of the piston in the bore.

Proper running clearances at the skirt measurement are very critical, and must be precision measured with very accurate measuring tools.
Depending on the piston in question, it might need as little as .002" clearance, or as much as .006" clearance, or anything in between. And you have to get it right, or it will seize if too tight, or slap if too loose.
There is no "guesstimation" allowed in this area. You have to know exactly what to do.
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KB8ANY

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Re: Piston-Cyl tolerences
« Reply #2 on: July 23, 2013, 08:31:22 PM »
The service manual calls out piston to bore clearance as:

Cylinder: new min: 84.025 mm; new max: 84.035; service limit: 84.155.

Cylinder-to=bore clearance (measuring the piston at the skirts): new min: .08; new max: .1 (i.e., one tenth of a millimeter); service limit: .3 (three tenths of a millimeter).

In other words, measure the bore at several locations, take the average, measure the piston at the skirts, and calculate the difference between the two circles.

I've had the barrel off, and can tell you it feels really loose over the piston.

Good luck.

Paul 

TejK

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Re: Piston-Cyl tolerences
« Reply #3 on: July 24, 2013, 07:20:27 AM »
Thanks Ace,
I was basically referring to the stock AVL piston and the clearance thereof. So the piston is definitely smaller at the combustion chamber end and is measured at the skirt. Its difficult to get the kind of close numbers which are possible with good measuring devices abroad. In India it's mostly a vernier or some other simpler device for measuring. Additionally, the higher ambient temperatures here somehow prompt the machine shop guys to give more clearance "to avoid seizing" as they put it, but this results in the piston slap after a few thousand miles.

Thanks Paul,
Those number should help as a reference. Is it possible you can share the complete service manual for the AVL if you have it in a pdf or a word document. It would be of much help.

Cheers,
Tej

dampking

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Re: Piston-Cyl tolerences
« Reply #4 on: July 24, 2013, 08:03:58 AM »
Tej - Are you using the 500s piston or the 535s?

Regards,
Sanket

TejK

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Re: Piston-Cyl tolerences
« Reply #5 on: July 24, 2013, 08:39:04 AM »
Sanket :
Its the stock part for the AVL - 500 cc piston on my friend's bike.

D the D

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Re: Piston-Cyl tolerences
« Reply #6 on: July 24, 2013, 02:02:04 PM »
Is there some law that prevents people in India from ordering an inexpensive dial caliper from overseas?  We can get adequate dial caliper for under $25 + S&H by mail order in the US.  And before everyone goes off about the cheap calipers, I have one that cost $19 that is every bit as accurate as my $250 Starrett I got 30 years ago.
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dampking

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Re: Piston-Cyl tolerences
« Reply #7 on: July 24, 2013, 02:21:53 PM »
There are zillion sources from where you can get one (in India) but our darn mechs don't like those. At the same time I don't like them people ;) but yea it takes some good amount of time to hunt down real good bullet docs and Garages. I think we have only a couple of such people who get the right job done using the right tools here in Bangalore. To be honest Chumma's *India plan* is our/my last hope! *Fingers Crossed*



SRL790

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Re: Piston-Cyl tolerences
« Reply #8 on: July 24, 2013, 05:13:03 PM »
I recently rebored my Triumph with a clearance of 0.0045" as measured with a micrometer.  Since there had been a discussion previously on this forum about measuring with feeler gauges when I was finished I did just that.  A 0.004" feeler slid right in but a 0.005" was tight.  So from my experiment reasonable accuracy, with care, can be achieved with a set of feelers.  My ten cents.

Andy
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Kevin Mahoney

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Re: Piston-Cyl tolerences
« Reply #9 on: July 24, 2013, 05:51:27 PM »
I have always subscribed to feeler gauges for piston/skirt clearance although you should make sure your machine shop has good instructions when they fit the piston. I do not think that anything less than .0045-.005" is a good idea especially on iron barrels. Lastly a very cautious break-in is needed, especially for the iron barrel and 535. I think the right number is at least 1,000 miles and not the 500 that is published in the owners manual. An iron barrel will generate more heat than it can dissipate when new if pushed. Less so for the AVL and a non issue for the UCE which cools extremely well.