New JE piston to cylinder wall clearance

[Guest]

Hi, During my build something didn't seem right with the piston clearance.

My JE pistons were used to bore the cylinders to the right size and I asked for 35 thou clearance and gave them the instructions that said to measure up .5 inch from the bottom of the skirt and use that as the measurement reference. ( there is 0.0030 difference from the top of the piston to the reference point. So they are not square.

I got the block back and didn't think anything of it, pistons seemed to fit fine.

However when I measured the clearance between the skirt reference and the cylinder bore I got 0.0005" of clearance

I measured the very top of the piston and its 0.0030 smaller than the reference point on the skirt like the instructions said.

I think the tech just thought they were square or didn't read the information. I don't know I'm not a machinist.


So my question is. do you measure 35 thou at the top of the piston or
DO as the instructions say and measure from the skirt reference?


I have a feeling I have to take the block back to them and have it done to the instructions but I thought I would ask the question.


Thanks

 

[49er]

Hi, During my build something didn't seem right with the piston clearance.

My JE pistons were used to bore the cylinders to the right size and I asked for 35 thou clearance and gave them the instructions that said to measure up .5 inch from the bottom of the skirt and use that as the measurement reference. ( there is 0.0030 difference from the top of the piston to the reference point. So they are not square.

I got the block back and didn't think anything of it, pistons seemed to fit fine.

However when I measured the clearance between the skirt reference and the cylinder bore I got 0.0005" of clearance

I measured the very top of the piston and its 0.0030 smaller than the reference point on the skirt like the instructions said.

I think the tech just thought they were square or didn't read the information. I don't know I'm not a machinist.


So my question is. do you measure 35 thou at the top of the piston or
DO as the instructions say and measure from the skirt reference?


I have a feeling I have to take the block back to them and have it done to the instructions but I thought I would ask the question.


Thanks

The clearance for my 71 mm JE pistons in my 850 is 0.0025" (two and one half thou). That's measured 0.5" (13 mm) up from the bottom of the skirt.

I think you may need 0.0035" on yours, not 35 thou. The clearance increases as the piston diameter increases. What size pistons are you running?

Never measure clearance at the crown. These vary significantly due to piston design. A piston with a thick crown requires greater clearance there, to allow for greater expansion during hard running.

 

[Guest]

Thanks 49er.

I think I jumped the gun while measuring, its been a long day

The instructions say 0.0025-0.0035 for these pistons which are JE 78mm 4 valve 1100 pistons. (going into a 400 )

I was measuring and and setting 0.035 on my caliper thinking it was 35 thou that I needed when what i needed was 3.5 thou

The tech did it right and I misinterpreted

The clearance increases as the piston diameter increases.

What do you mean by that? I thought it would be the opposite.

Is it that the sleeve expands and creates more clearance while the piston doesn't expand as much in that area ?

Never measure clearance at the crown. These vary significantly due to piston design. A piston with a thick crown requires greater clearance there, to allow for greater expansion during hard running.

I measured the crown and got 0.030 and thought the tech could have measured there instead.

I was just trying to justify what I didn't understand. I wouldn't measure there but thought that's what could have happened.


I just had a brain fart

 

[1980GS1000E]

The piston clearance at the skirt reference point stays pretty consistent throughout the temperature range. The piston will expand at the crown when heated up, since that's where most of the metal is, and as a result, the piston clearance will diminish somewhat, ideally to the same clearance that is at the skirt when cold. The piston is actually machined in an oval shape at the wrist pin and is of a slightly smaller diameter. These measurements are determined by the designer so that when the piston is at operating temperature, it assumes a nice round shape and the clearances are consistent top to bottom. One of the reasons why it's never good to run an engine hard until it has fully warmed up.

 

[49er]

What I meant was that as the piston sizes increase (example 60 mm to 100 mm), there is more metal to expand in the larger one, so the skirt clearance will be greater for that piston.

In racing applications, air cooled HC pistons will get slightly more clearance than liquid cooled ones.

 

[Guest]

.0035 is a little loose for that piston. He should have stayed closer to the .0025.

 

[Guest]

Thanks Big Jay,49we and 1980GS1000E.

I understand now.

.0035 is a little loose for that piston. He should have stayed closer to the .0025.

Why is that ? I'm still learning and I can barely even measure the clearance.

I have two sets of instructions.

1 is a blueprint like sheet that says .0025 and looks like it was specifically for that piston when it was manufactured. However just below that it says this is only a guideline.

2 is The specific JE piston instructions with specs for different applications.
That sheet says to start with 0.0030-0.0036 and go bigger by 0.0001 to0.001 depending on the application. It gives specific's and leaves no room for interpretation.

So I asked the question on here and 0.0035 was suggested as what I should go for. I didn't know what was best at the time.

 

[GregT]

Given the applicaion and the number of unknowns here I'd be quite happy with .0035in clearance.
Oversize liners in a block with less material around them - and it's a small twin which means it's going to be run harder than those pistons would be in a road four. An extra thou or so isn't silly.

 

[rapidray]

The newer pistons have a higher silicon content in them, therefore, causing them to expand LESS. You should have gone .0025 on the clearance. You will see more oil use & a little less power that you would have gotten with a one thou tighter clearance. I used to work at CP Pistons so what I'm telling you isn't something I heard from my cousin's friend's girlfriend's dad whose uncle used to race. Ray.

 

[Guest]

Thanks for your input Ray, I value your opinion and experience, there's no doubt there. But I'm a little confused by what you said since my pistons are non-silicon.

This is what confuses me:

4032 high silicon aluminum require 0.0022"-0.0028" for my size piston.

This agrees with what your saying and I understand that high silicon would require less clearance because of its lower coefficient of thermal expansion than the 2618 alloy.

Here are the instructions from JE for those.
http://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc4032.pdf



In the instructions that came with the pistons I have, it says:

Your pistons are manufactured from 2618 non-silicon aluminum alloy and requires 0.0030"-0.0036" for bore sizes 2.5"-3.65" mine are (3.070")
piston part # 128324
suzuki m/c gs1100 4 valve
Bore 3.070"

Here are the instructions
http://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc2618.pdf



What can you tell me?

I mean this is based on what I'm reading and no previous experience with these pistons.
If I'm wrong. So be it. I'd rather know than think I do


Additional reading.
http://www.importtuner.com/tech/impp_1110_piston_anatomy_tech_knowledge/viewall.html




P.S How much piston slap should I expect with these?

 

[platinum2]

piston , instructions are just like wisecos
if your piston has a smaller crown it will not be the same as the skirt( of course), so they use the widest part of the piston to measure for the bore size, usually at the pin area, the skirt should be the same .002 is tight i think, but what ever, - a race motor is tore down all the time, a street motor is built upon every day all day use applications
a good machinist would have double checked with the manufacturer, but they say it comes down to the builder so they will not be liable lol

 

[Guest]

lol
I gave the shop both instructions when I had it bored and suggested 0.0035''
I left it with them to decide what is best after telling them exactly what I was building.