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Author Topic: Piston to cylinder clearance  (Read 2325 times)

indian55-123

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Piston to cylinder clearance
« on: December 15, 2009, 03:03:00 AM »
I was looking through my manual for piston to cylinder clearance but nothing is given.
Does anybody know what the clearance is for a 55 500 Twin? I need to let the machinist know so he can finish the bore job. Also, what kind of finish is best on the crosshatch in the cylinder?  I've had the crank turned and balanced so I hope to start reassembly soon and I'm starting to get excited. Any help with these two new questions will be much appreciated.
                  Thanks, Duane

ScooterBob

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Re: Piston to cylinder clearance
« Reply #1 on: December 15, 2009, 04:00:12 PM »
I've been a Certified Master Engine Machinist for more years than I can count - The "rule of thumb" is .0015" per bore inch if you have no specs at all. The material that the piston is made from is a BIG determining factor in piston clearance. Old-style aluminum pistons with no silicone in the allow expand a LOT! Heating a 3" piston on the bench with a heat gun has shown me up to .006" EXPANSION - this would NOT be a good deal in a bore where you have .002" clearance! The theory is that you let it warm up SLOWLY and the piston won't seize, hopefully. High silicone alloy pistons (Keith Black Hypereutectic for instance) expand very little - and the expansion is quite carefully controlled with the casting of the piston. It is not uncommon to clearance these types of pistons at .002" in a four-inch bore. Forged pistons require a bit more claerance as well as they expand a bit - in a three-inch bore, I'd not be afraid of up to .006" depending on the piston type.

All-in-all, the thing to keep in mind is that the bore is STRAIGHT and ROUND. These two factors allow the bore to be "loose enough to run" without risking piston slap or a tendency to smoke like a Southern Barbecue grill on start up. If you are a civilsed rider - go tight - if you plan to run the eyes out of it - loosen it up just a little and enjoy! I hope this helps a little!
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rotorwrench

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Re: Piston to cylinder clearance
« Reply #2 on: December 16, 2009, 10:13:42 PM »
That falls right in with the "rule of thumb" that scooter bob mentioned. I'm sure that if boring and honing ended at that spec or maybe slightly more clearance, then you shouldn't have any problems if bedded in carefully.

indian55-123

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Re: Piston to cylinder clearance
« Reply #3 on: December 17, 2009, 01:07:32 AM »
Yes , scooter bob had it right on. I forgot to ask the machinist why he needed to know how smooth I wanted the cylinder honed but I will ask tomorrow when I pick the parts.
Everything I've read about these engines says to bed in very carefully for the first 1000 mi. Thats probably why the pistons and cylinders were so trashed.
Thanks again for the feedback, I certainly appreciate it. Maybe if I keep at it I'll be able to pass something along to the next bewildered one.
 Thanks again, Duane

Rammo

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Re: Piston to cylinder clearance
« Reply #4 on: November 15, 2012, 10:38:30 AM »
Hi

What would be the clearence if the cylinder barrel is pumping out oil via the piston gap and also heavy smoke. Can i say the cylinder barrel to piston is having more than a clearence of .002.

Thanks
Ram

ERC

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Re: Piston to cylinder clearance
« Reply #5 on: November 15, 2012, 10:41:54 PM »
The factory spec for original pistons is .005" the ring gap is .012- .015. If you have some aftermarket ones you should get the specs they recommend. Scooter Bob covered it pretty well.  ERC
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Arizoni

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Re: Piston to cylinder clearance
« Reply #6 on: November 15, 2012, 10:59:46 PM »
Scooter Bob's recommendation of .0015" per inch would result in a aluminum piston in a cast iron cylinder clearance of around .005" (.126mm).

Normally, oil burning and smoking is a sign of worn out piston rings, not of excessive piston to bore clearance.

Excessive piston to bore clearance is usually noted by the knocking sound of piston slap.
Jim
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SRL790

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Re: Piston to cylinder clearance
« Reply #7 on: November 17, 2012, 05:16:46 PM »
I'm not sure about the 500 twin pistons but the bullet pistons are oval and tapered.  Measure piston front to back at the bottom of the skirt (widest point) and use this to figure out the clearance.

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ERC

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Re: Piston to cylinder clearance
« Reply #8 on: November 17, 2012, 11:12:39 PM »
The 500s are the same.  ERC
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Arizoni

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Re: Piston to cylinder clearance
« Reply #9 on: November 18, 2012, 01:45:14 AM »
Using the .0015"/inch value with the 64mm (2.5197")  bore shown for the 500 twin in the book "Royal Enfield -The Postwar Models", the piston to cylinder clearance should be 0.0038 inches.
Jim
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barenekd

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Re: Piston to cylinder clearance
« Reply #10 on: November 18, 2012, 02:38:55 AM »
The book on solid tapered pistons is.004 at the bottom and .006 at the top. If the pistons are split skirt .0035 at the bottom, .0045 at the top. Current thinking on honing is to keep the walls smooth.
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