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Message started by Dennisgb on 05/19/20 at 14:24:00

Title: Piston to barrel.
Post by Dennisgb on 05/19/20 at 14:24:00

i need to get batteries for my caliper. It was working a couple of days ago.

Stupid question. If I slide the piston without rings into the barrel and can slide an .008” feeler gauge in the gap between piston and barrel is this a rough estimate of .004” circumference clearance? For some reason my brain isn’t working.

Title: Re: Piston to barrel.
Post by Armen on 05/19/20 at 16:05:46

Nope, that would be .016" clearance.
Make sure you are measuring on the thrust surface (front to back, not sides).
Very crude/inaccurate way to measure.

Title: Re: Piston to barrel.
Post by Dennisgb on 05/19/20 at 20:25:17

Thanks, yeah I know pretty crude. I measured last week with proper tools and checked out ok. Was trying to double check after cleaning piston and digital caliber was dead. Not sure how it would be .016 when piston is to opposite side against the wall and I slipped feeler in the gap.

Title: Re: Piston to barrel.
Post by norm92de on 05/19/20 at 22:06:48

You got it Dennis.

Title: Re: Piston to barrel.
Post by Armen on 05/20/20 at 07:21:55

Yup. brain fart here. You were reading .008" Shouldn't write late at night.

Title: Re: Piston to barrel.
Post by DragBikeMike on 05/20/20 at 12:26:46

I don’t want to step on any toes here, and I mean no disrespect. I just want to clear up some obvious confusion.

First, checking piston to cylinder clearance with a feeler gage is a perfectly fine alternative to a dial bore gage and micrometer. It’s not as accurate, and I wouldn’t use the feeler gage to resize a cylinder, but its just fine for verifying running clearance and also a great cross check of the more precise measuring instruments. I always cross check with feeler gages after measuring with precision instruments. Actually, there are spring scales available to verify correct drag on feelers when used for fitting pistons.

Second, Dennisqb, if you can fit an .008” feeler between the skirt of the piston and the cylinder (assuming the drag is proper and the piston is positioned correctly) your piston to cylinder running clearance is .008”. That works out to .004” radial clearance but you don’t use radial clearance to evaluate piston to cylinder.

As Armen said, you always do this check in the fore/aft direction, perpendicular to the wrist pin. That’s because pistons are not round, they are egg shaped. Pistons are also tapered top to bottom, the top being way smaller than the bottom. So, the tightest clearance is found close to the bottom of the piston in the fore/aft direction.

This picture shows a cylinder/piston combination that has right about .003” running clearance. Note that the drag on the feeler is just enough to hold the piston suspended in the cylinder, just enough to prevent gravity from pulling the piston down. The piston will continue down with just a light finger push.

Title: Re: Piston to barrel.
Post by DragBikeMike on 05/20/20 at 12:27:32

If I try to insert a .004” feeler, the piston won’t go into the cylinder without applying a whole bunch of force. If I pushed it through with this .004” feeler in there, I would probably score up the piston a bit.

Title: Re: Piston to barrel.
Post by DragBikeMike on 05/20/20 at 12:28:24

Feeler gages are an excellent alternative to expensive measuring instruments. Certainly, good enough for the shade tree mechanic trying to do a ring job.

Dennis, if your .008” feeler fit between the cylinder and piston as shown in the first picture, if the piston could be pushed through past the feeler with light force, if the feeler was positioned as shown in the first picture, you have too much running clearance. If you are very lucky, the majority of the wear is confined to the piston and you will be able to tighten things up by simply replacing the piston. Otherwise, its time for a rebore.

Title: Re: Piston to barrel.
Post by LANCER on 05/22/20 at 09:53:15

Good info on this DBM.

Title: Re: Piston to barrel.
Post by Dennisgb on 05/22/20 at 12:46:23

63656A16141317270 wrote:

Thank you for the clarification. When I stuck the feeler in there I was at the piston ring position and not properly on the skirt. I did proper measurements at multiple places and got .0025” at the top and .003” at the bottom. That coordinates with my expandable tool and micrometer. I think it’s good to go. I just need to clean off my bench so I have room to work

Title: Re: Piston to barrel.
Post by Dave on 05/23/20 at 10:13:23

5D5C5757504A5E5B390 wrote:

[quote author=63656A16141317270 link=1589923440/0#5 date=1590002806]I just need to clean off my bench so I have room to work [/color]

Sometimes I have found it necessary to use the floor! ::)

Title: Re: Piston to barrel.
Post by Dennisgb on 05/24/20 at 18:06:45

5C676A7D6C607B7D666E637C0F0 wrote:

[quote author=5D5C5757504A5E5B390 link=1589923440/0#9 date=1590176783]

Sometimes I have found it necessary to use the floor! ::)

It’s been really hard for me since my shop burned down last year. I had an awesome workspace then.

Now I’m in a dirty barn with varmits and not much bench space.

Title: Re: Piston to barrel.
Post by Dave on 05/25/20 at 04:34:25

5D5C5757504A5E5B390 wrote:

It’s been really hard for me since my shop burned down last year. I had an awesome workspace then.

Now I’m in a dirty barn with varmits and not much bench space.

I am soooooooo sorry! That is about the worst thing that could happen to a fellow. I worked in marginal conditions on and off for a very big part of my life, and finally when I turned 50 we built a house and I was able to get a wonderful garage......it was so nice to have a garage with good light and heat!

Maybe if the government sent you a stimulus check - you can buy one of those little yard barns and build a nice bike workshop!