Piston Skirt to Cylinder Interference?

[kstylianos]

Yesterday I received my new JE pistons to go with my replated Nikasil C's. I don't have a bore gauge or a mic. to measure either (will try to find a machine shop over the weekend to spec these out for me). Just for kicks, I inserted the JE's into the cylinder w/o rings, just to get a visual feeling of the clearances I'm working with. I inserted the piston from the bottom of the C, and it went in fine w/o any interference just up until the piston skit. It didn't get stuck but there was definite drag between the skirt and the C wall. If I held the P/C assembly upside down, the P would not fall through, but I could push it all the way through with little force. Is this normal? Please tell me I'm paranoid. This is my second set of JE's and don't want to send these back again (the dome volume of the last set was too much - 10.3 CR).

I didn't have my old Alucil P's with me last night to compare, but will try tonight.

[William Miller]

Charlie, I have a dial bore gauge and a large micrometer if you want to measure.

[kstylianos]

William,

WooHoo......Can we get together tonight or sometime this weekend?

[William Miller]

PM Me on your site or here and we'll trade numbers.

[kstylianos]

Before I get them measured this evening, I'm still curious if this skirt to C wall interference is normal.

[rcwaldo]

typically the piston skirt is the largest OD of the piston, and where this measurement is taken from. You should have clearance and should slide easily into the cylinder. JE will provide what they recommend for a clearance (cold clearance that is).

You can use a feeler guage set for establishing P to C clearances. This will be somewhat of a rough measurement though.

[john walker's workshop]

i would also be a bit concerned until the measurements were done. generally, a piston without rings should be a nice slip fit, but still be able to pretty much slip out with it's own weight. the skirt is the widest part, of course. it's probably how the cylinder surface was reconditioned.

[kstylianos]

JW, I agree with you. I think the replating built up the tolerences a bit.

If the C's need a little more honing, how much Nikasil meat (plating) can be safely removed? Think I'll call EBS on this one.

BTW: Last night I measured 94.97mm cylinder bore at the top and bottom lip of the C (at least they are consistant) and the very bottom of the P skirt was 95.05mm. These measurements were taken with digital calipers, but not at the specified location in the C or on the P.

JE recommends to measure the P diameter 13mm right above the skirt, right below the wrist pin hole. We'll see what its diameter and C bore measurements are tonight.

[Doug Zielke]

In the m/c biz, we send the pistons we intend to use, along with the cylinders to the machinist/remanufactuer. They attend to the proper clearances and specs.

[kstylianos]

The JE pistons and the replating services were both ordered through EBS. Doug, I thought the same.....they would take care of the clearances. Currently waiting to hear back from EBS.

[john walker's workshop]

that would be about .003 clearance. you would think it would slip out with that. kind of large even. what does J+E specify for clearance?

[kstylianos]

JW....the skirt was larger (~95.05) than the C bore at the lip (94.97)

Dont have the JE clearances at hand.

[john walker's workshop]

oh, i got mixed. well that's not good. the fact that you can actually push the piston thru, makes me think you measured wrong. it would not go thru at all at .003 tight.

[kstylianos]

The digital calipers didnt give me a warm fuzzy around the p-skirt.....the C lip reading was consistantly repeatable. We'll see what the actual #'s are tonight......

[Wayne 962]

Originally posted by kstylianos
The digital calipers didnt give me a warm fuzzy around the p-skirt.....the C lip reading was consistantly repeatable. We'll see what the actual #'s are tonight......

These won't get you an accurate reading - you need a dial bore gauge, no question about that. So toss your readings out the window. Also, for measuring your pistons, you will need a micrometer - the digital ones are good, but they won't be precise enough for these fine measurements.

-Wayne

[kstylianos]

Update:

I borrowed Williams micrometer and bore gauge Friday evening and spent a good 3 hours Sat. afternoon getting familiar with the tools, calibrating and measuring the P/C's.

They both (P&C's) measured out right in between the Nikasil group 0 specs....clearances were spot on also. Got them all installed Sat evening. Just need to go around one last time to measure all the deck heights and then its sealing and installing the heads and cam towers. It's nice to see it coming together....finally.

I prematurely freaked out Thursday evening when the I noticed the hang-up. Apparently there was still honing residue in the C bore from the replating/honing process. After a thorough cleaning and oiling the cylinders, the pistons slipped right through......live and learn. I know, Wayne....'Read the Book'.

[Wayne 962]

Bad, or inaccurate tools will cause a lot of worry. The digital calipers that you referred to originally are only good for spot measurements, and then, only within a certain range. Most people don't own these tools - that is why I recommend sending the parts to the machine shop. Not only do they have the proper tools, they are also typically more experienced in their usage.

-Wayne

[William Miller]

Glad I could help someone here and glad it worked out!