Proper way to measure piston-cylinder w/feeler gau


Formula

Member
Sep 21, 2000
59
0
I took my cylinder (1990 RMX 250) to 2 dealers to get measurements for a new piston. They both used a slide/dial guage and said "should be able to use a standard piston". Based on that I got one of them to order me a Wiseco kit. When it arrives I would like to be able to measure the clearence for myself at the shop so to know if what they are saying was accurate, (then we can talk about it in person if it is not the right one!) . I have one of the Sears flat feeler guages with the width of the blade about 1/2 inch.

At what points do I measure and how do I get a very accurate measurement with this type of guage.

If it is not the best way to measure I do have a wire guage but the prongs are all bent at right angles coming off the ends and I do not feel as comfortable bending them straight to get them to reach.

On a side note I have a question regarding float height for my (stock) Mikuni TM38. The Clymer for my bike says measure from base of main jet to the prongs that the floats rest on. I have done that and want to double check with you guys.

All the advice I have received here has been great, and if I had inquired earlier as to what the noise I heard when accelerating was I may have saved myself some trouble. Instead "wait till winter- then look inside" said my brain, cost me.

Thanks again.
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
To measure the bore with a feeler guage, insert the piston, and measure the clearance at the skirt, at several points in the cylinder at the top, middle, and bottom of the jug.

As for the float, that is not the way I do it, but if the manual says to do it that way, you will be fine. I hold my carb upside down, and measure from the carb float-bowl sealing surface to the top of the float. I set this distance at 16mm for my bike, but it will certainly be different for your bike.
 

Formula

Member
Sep 21, 2000
59
0
What kind of numbers am I looking at for the clearance - max. and min. for new and then when I check again in x amount of hours.

Thanks.
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
You measure one spot at a time. If you had a feeler gauge stuck in two spots across from each other, you would have .006" clearance.
 

jski

Sponsoring Member
Apr 5, 2001
178
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Spanky,

Can you measure the piston clearance through the exhaust port by just removing the pipe or does it require a top end tear down to do so?

Thanks,
Jim
 

Formula

Member
Sep 21, 2000
59
0
Since I am also new with using feelers acrosss the width of the blade is it a go - no go measurement or do I expect a slight or large amount of drag when I have the right feeler in there. I'm saying that because when I am doing ring end gaps it's more of a go - no go feeling that I am familiar with.
Using the width of the blade may lend itself to more error for someone like myself who has not used it in this manner.
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
You guys are scaring the hell out of me:scream:
Using a "feelermeter" is about as accurate as using the "eyecrometer".
Pistons have unique shapes based on the geometry of the ports. Normally 2-stroke pistons are tapered from top to bottom and egg-shaped with the blunt side on the exhaust skirt.
The piston and bore need to be measured independently, thats why every manufacturer lists specs for the cylinder and the piston. Generally speaking, if your plated cylinder is brown in color not gray, its ok, thats using the eyecrometer. The best way to measure the piston to cylinder wall clearance is to start by measuring the maximum piston diameter by adjusting the micrometer for the largest point near the bottom of the piston measured from the center of the intake and exhaust skirts. Then use the micrometer to set the dial bore gage at zero. When you insert the bore gage in the cylinder it will read the difference (actual clearance dimension) and the key areas of the cylinder are at 90 degrees near the top of the cylinder where the rings peak and the bottom of the cylinder where the piston bottoms. Look for taper and out of round dimensions and compare it to the factory service manual. Plated cylinders always wear the most at the top of the cylinder where the rings peak.
Sorry to be touchy about this particular subject of metrology (the science of measurement) its just that on a weekly basis I get screamed at by customers who get their brand new plated cylinders returned then have their knucklehead friend stick the piston in the top of the cylinder upsidedown and wiggle it or shove a feeler gage in it and find that it accepts a 10 thousanths shim. They always start off the same way "My buddy says you messed up my cylinder". Then I have to explain this all again, and of course his buddy doesn't own $2,000 worth of gaging or taken a semester of metrology at college so he thinks I'm a back-pedaling idiot trying to baffle him with BS.
Feeler gages are for checking spark plug gap and valve clearance, not for cylinder bores.
If you're really concerned about the bore clearance, take the cylinder and piston to a performance machine shop and have them measure it with the proper gaging.
Good luck:)
 

SFO

LIFETIME SPONSOR
Feb 16, 2001
2,001
1
Thanks for cleaning that one up, Eric

I was starting to think I got lost in the Twilight zone.
I never heard of using feeler gauges to measure clearance either and was feeling confused.
I know it is difficult to use a telescoping guage, let alone to tell someone to buy gauging equipment costing a couple hundred bucks.
Measuring after it is blown up would be a little late, what about verifying the #'s before assembly?
There has got to be some science here and not just replacing parts.
Your machine shop should be happy to gauge all work in your presence, if not, don't use them.
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
Thank you for that info, Eric. No one has ever explained that to me in such detail before. I do own and use a micrometer and bore guage, but I did not realize you could not at least determine if a cylinder was at least serviceable with a feeler gauge and careful measuring. From your description, I haven't been measuring it correctly with the micrometer, either. :(

This is why this site is so valuable.:)
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
Hi Spanky,
Glad you picked up some tips, remember I'm always a little tongue and cheek with my posts, mostly poking fun at the guys who shake the wigglalipers and feelermeters in my face at work.
Bill has a good point, if a guy won't weild a dial bore gauge in your presence, he's got something to hide. The old plating shop I worked at, Max Power, uses the dial bore gauge to get more work. One time my friend Todd brought his Arctic Cat snowmobile cylinders in to be checked and they said "The problem is your cylinders are shaped like ice cream cones". I checked them out and they were straight and round. I wonder if those guys were reffering to standard cones or hand-rolled waffle cones, those cones are my favorite especially when they put a malted milk ball in the bottom of the cone to keep the ice cream from leaking out the bottom. Oh well gotta go, gettin hungry for ice cream:silly:
 

David Trustrum

~SPONSOR~
Jan 25, 2001
1,396
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I remember this mechanic telling some dumb 15yr old (me) that my A model XR was stuffed when he managed to squeeze a 12 thou feeler right down the side of the piston & it was still loose. He was right because it smoked enough to make an Arab smile. Sadly I had to put it back together (complete with old gskts :eek: ) as it was also my road bike (allowed to ride here at 15) & I of course had no money. Makes me cringe or laugh to think back now.

If you don’t own a dial gauge, then slightly more accurate than eyecrometer (& if the wrath of Eric doesn’t befall me) is to put your new ring at several positions in the bore & measure differences in ring end gap. This will tell you more about taper, more likely in sleeved cylinders,

but arguably -after you scotchbrite the carbon off the top of the cylinder above the rings high point, if you feel a lip with your fingernail it’s time to get it measured properly then re-plated

. . . unless of course you are a dumb broke 15 yr old.:silly: :silly:
 

smarttoys

Sponsoring Member
Apr 29, 2001
199
0
What scares me is that there are alot of "mechanics" that use a feeler gauge to check piston to cylinder clearance (along with some other questionable practices). 10 years ago when I first started working at a dealership I was the only mechanic there that used a micrometer and dial bore guage for that. The other mechanics at the time came over and watched me use it and said "what's that?" "I'll stick to my feeler guages.":eek: :scream:

Oh well, to each there own.
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
I've got a funny story for you guys about the proper use of micrometers. When I went to tech college for metal machining we had this idiotic teacher who would open the doors to the shop then go to the cafeteria to smoke cigarettes and work college chicks. Scarey idea having a bunch of newbies loose in a metal shop. The funniest story was a guy who used 4 digital micrometers to clamp down 2 pieces of metal in order to saw them. I asked him what he thought those things were, he said "they're torque clamps! I use a vise-grips to tighten them up to the same torque and read the torque rating on the little digital display". I said oh thats a novel use of micrometers, I'll go get the teacher to show him, I'm sure you'll get a gold star!;)
 

MikeT

~SPONSOR~
Jan 17, 2001
4,111
11
Originally posted by EricGorr
I wonder if those guys were reffering to standard cones or hand-rolled waffle cones, those cones are my favorite especially when they put a malted milk ball in the bottom of the cone to keep the ice cream from leaking out the bottom.

Got to try that. Thanks for the idea Eric! You are really a down to earth type of guy.
 

WoodsRider

Sponsoring Member<BR>Club Moderator
Damn Yankees
Oct 13, 1999
2,812
0
Eric - That has to be the most detailed response on the correct method of measuring piston to bore clearance that I have ever read on DRN. As a former inspection supervisor I've been through countless hours of metrology training. A couple of the most difficult tasks I've ever done is teaching mechanics and machinists how to use and read a micrometer and how to set-up and read a dial bore gage.

Another funny story. One of our metal butchers... er, machinists was tired of wringing gage blocks together for setting his bore gage. He decided to shorten a $500 12" gage block by .750" on a grinder. Needless to say we ended up with seven 6-cyl. marine engine blocks where press-in sleeves would fall right through the hole. :scream:
 

Formula

Member
Sep 21, 2000
59
0
Thanks for the help, now I can tell what piston I need. Since the Breyers ice cream bar stick wont fit, nor will the Revel stick fit. I've narrowed it down between a semi-gloss business card and 2 pieces of paper from the Far Side mini calendar but only the months with 30 days.

When I go to the dealers I get generalities, no quantitative data, no numbers, just an empirica,l "yah it's standard". I guess I have 2 options - go to a reputable machine shop, or do it myself and invest in some precision tools.

Now not to deny all that I have just learned but somone mentioned the area of the bore at the head gasket that the rings don't quite make it up to. Can, I say can a person expect this measurement to be indicitive of the original size of the bore. Or can the difference between that and lower in the bore be an idication of wear. Can a rings end gap be used as evidence at these 2 points for the amount of wear you have?

Thanks again.
 

David Trustrum

~SPONSOR~
Jan 25, 2001
1,396
0
One supposition doesn’t totally equal the other. On a really shagged engine (& I mean a sleeved engine because a plated barrel will be through to ally) there will be a nasty lip there. This is where the rings are really working, remember they are changing direction while being pressed heavily against the bore by the gas they are compressing.

Needless to say if there is a decent lip it’s ‘brown bread’. However there may be an unusual amount of carbon build up or piston residue from detonation so take that into account.

Really this is a method that will tell you is it is totally kuzed, or acceptable for a playbike. If it’s your pride & joy racebike, take the time to find someone clued up to measure it.

If they boast they were to shop classmates with Eric Gorr, ask to check their posterior for boot imprints.

Thanks! that story brightened up my morning no end.
 

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