OLd School

Jimbo

You can fool some of the people some of the time
I remember years ago i was taught in one of my automotive engines class that a way to check piston to cylinder clearance was to put a long feeler gauge in the cylinder and slide the piston in. If it would slide in with out too much force with a .005" feeler gauge it meant you that .005" clearance.
If it wouldn't go in with a .005" you would then try a .004" and so on until you found the size that fit to your liking.
There is some judgement involved here.
It won't tell you if you have tapered or out of round cylinders
By today's standards this is a very crude method but i still use it if i question the readings i get with my measuring tools.
I use plastic feeler gauges that very easily conform to the curvature of the piston and the bore but i'm wondering if any one else does this and what is the width of the feeler gauge you use??
 

flattop1

Dawg 89
don't have the long feeler so I have to move piston down the cylinder. standard 1/2 inch.
double checks my Chinese bore gauge. plus a simple check to see what clearance you have before choosing to hone and re ring or go up 1 size . on my diy stuff , though you pretty well know the answer if you been running the engine a while.
 
Could you use plastigage like we use on rod bearings?

I never use(d) feelers gauges to check piston to cylinder wall clearance...seems pretty crude like you said, especially when we have access to much more accurate methods of measuring the cylinder wall clearance (ie outside mics and a dial bore gauge.) I do tend to check pistons with both mics and dial calipers just as a standard though (just out of habit.)
One thing that is a good idea is to have your dial bore gauge calibrated (or at least check it against another known correct bore gauge.) The points do wear! I purchased my Sunnen dial bore guage new probably 20+ years ago and I've had it recalibrated/certified each year since. I recently broke down and purchased the Sunnen bore gauge setting fixture. It is so much more convenient than trying to set the gauge in a pair of mics. (and much more accurate I think.) I use different bore gauges for the different engine types we work on here -- ie one for flatheads, one for the animals, and one for the big block stuff (390's etc.) That way I can have them set to the correct bore size and use them without having to reset them each time I use them. I've got everything from the old Sunnen that I use every day in the shop, to a Mititoyo and cheaper Fowlers and they are all very accurate and hold their measurement well as long as you are careful in handling them.

How do you accommodate for some of the piston "taper" or "barrel" shape when you use the feelers gauge, Jim? I'm assuming you're going down past the bottom of the skirt on each side? How do you keep the piston square in the cylinder?

Why do you ask, Jim? Just curious. :)

I do use long feelers gauges to set coil air gap. I think they are stainless and about 3/4" wide and about 10" long. I picked up a ton of them at an aircraft tool cage auction.

Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cuts
www.CarlsonMotorsports.com
765-339-4407
bcarlson@CarlsonMotorsports.com
 

Jimbo

You can fool some of the people some of the time
Brian
Like i said this isn't the preferred method.
Actually i use a tri mic to measure the cylinder bore and a conventional digital mic for the piston.
I use a dial bore gauge mainly to measure for cylinder taper and out of round. I also use a snap gauge sometimes
Like i said i only do this when i question my readings. You'd be surprised how well it works.
Why did i make this post?
The main reason was to start a discussion.
 

Outrider

Member
I haven't done it that way in a long time (that sort of thing happens when you start using engine builders :) ), but I learned it way many years ago. Somewhere wrapped up for protection in one of my old tool boxes is a bundle of long feeler gauges from 0.0015 to 0.006 dedicated to that method. Done carefully, it's way ahead of whatever is in third place with regard to accuracy, so it certainly is a good double check on what now are the more traditional methods when in doubt.
 

barluc99

Member
I agree with Jimbo on the use of tri mics. Although they do not have the accuracy of a bore gage the tri mics have 3 points of contact and will give a better representation of your roundness because your bore gage only has 2 points of contact.
 

Jimbo

You can fool some of the people some of the time
I think if you want to get picky a snap gauge or a bore gauge will be more likely to tell you if a cylinder is out of round more so than a tri mic.
 
I've got to ask again, with all the taper, or barrel shape, pistons that we use today, how do you keep the piston square in the bore using this feelers gauge method? Wouldn't that affect the measurement accuracy? On older very large bore pistons (ie diesels) that had square shoulders on them, maybe this would have worked -- I have to wonder if this is where the idea came from since everyone is in agreement that it's not the best method, rather something learned from our youth. I've honestly never heard of using feelers gauges to check bore sizes. Maybe I'm still on the young side. :)
I've used the snap gauges and found them extremely inconsistent. I do, however, still use snap gauges to check the i.d. of exhaust pipes and occasionally an intake runner. Even then I take several measurements and average them.
If you find yourself questioning the accuracy of your tool of choice (dial bore gauge, etc.) -- have it checked out. Our Sunnen rep would typically come through our area with a setting fixture and I'd at least have him check it against my way of setting my bore gauge against a pair of mics. Also, if you have a friend that works in a good machine shop, he can have your tools certified when they get theirs done to save a bit of money as well. The Sunnen dial bore gauge setting fixture is really the cat's meow. I wish I would have gotten one 20 years ago, but I thought I really couldn't justify the expense. Man, was I wrong!
 

Jimbo

You can fool some of the people some of the time
Yes you need to have your all of your measuring tools checked periodically but if you have an engine that needs to be done for a race tomorrow it's pretty tough to drop everything else and get them checked.
Actually, if you measure the top of the bore with a tri mic your dial bore gauge doesn't need to give you the exact bore dimension.
It doesn't matter what size it tells you the cylinder is. You can just use it to measure for taper and or out of round.
If you use a good honing system it really should not be out of round
 

flattop1

Dawg 89
not being a pro builder Iam gonna input a little any way.
The feeler gauge does not know if the piston is barrel shaped or square it only knows if its gonna fit in or not . if a .003 feeler and the piston fits and a .004 does not then at least .003 and less than
.004 clearance. may not be round may have taper but that is still what you have. as too the piston being square it will be measuring on the high spot of a barrel shape piston, and if the piston does not square up in the bore it is likely to large any way. obviously not as accurate as bore gauges tri mic's and such but pretty much fool proof in as much as giving cylinder to piston clearance. I totally agree with Brian on the fixture to set the dial gauge fumbling with a mic and seting the bore gauge gives you cause to wonder if its gonna give you the desired results, great for showing out of round though no matter what it reads.
 
I understand that the feelers gauges will "get you close"....but if the piston is not square in the cylinder, then your feelers gauge can't be measuring accurately due to the shape of the piston...Ie piston rock. Consider instead of a thou or thou and a half taper, that you had much more taper....if you rock the piston even slightly, the measurement won't be accurate at measuring straight across the piston - ie, the more taper, or barrel shape, a piston has, the more inaccurate this method of measuring. For sure, the more cylinder clearance you allow, the more piston rock you will have and less accuracy with measuring with a feelers gauge. Now, is it good enough to be within .001"? Quite possibly. I think it's safe to say that on multiple cylinder engines with much larger bore sizes, .001" may not be a big concern...I tend to think that when we are dealing with a single cylinder application, it is critical. Hey, can't knock whatever method you choose to back-up / validate your measurements though (especially if there is a concern with the accuracy of your primary tool.)
 

Jimbo

You can fool some of the people some of the time
I'LL SAY IT AGAIN. THIS IS NOT THE BEST METHOD BUT SIMPLY ANOTHER METHOD FOR CHECKING YOUR CLEARANCE.
There is a difference from one person to another what they want for cylinder clearance as well as what you need for various brands of pistons. A CV / JE requires a different clearance than a stock Briggs piston or for that matter what is recommended for a Burris piston.
To imply that you need exactly .00XX" of clearance is highly debatable. If that is the case every refresh is going to require going to the next size larger piston.
Many pistons are built with a larger diameter at the bottom of the skirt than at the top of the skirt. The ring land area is normally much smaller that the skirt are to allow for expansion. (the piston is hotter at the top than at the bottom)
Pistons skirts are also often not round but biggest 90 degrees from the skirt and somewhat smaller 45 or 60 degrees from the skirt. Usually there is no skirt directly under the wrist pin.
With the piston connected to the connecting rod and a plastic feeler gauge of the correct thickness (.00XX") and WIDTH (lets say 1") in the cylinder when you in slide it into a freshly bored, honed, straight and true cylinder there is no piston rock since the feeler gauge takes up all of the clearance.
We use feeler gauges to check ring end gap, ring clearance in the groove, rod side clearance, valve lash etc etc and it works just fine.

This is not the way i would do it but after googling the subject it seems it's quite common to do it this way.
Here's a video. http://www.youtube.com/watch?v=nb7tXgiI6Ts
 

flattop1

Dawg 89
still conflicted here on the piston being square to the cylinder.
We are working with pretty tight tolerances, so literally if you rock the piston in the direction of thrust , you will gain as much on one side as you lose on the other.
the tapered piston and the elliptical shape should be no issue.
Barrel shape I just can not visualize how you are going to get more than .0025 thousandths by rocking the piston in the bore.
In as much as increasing the clearance to exaggerate the issue then you are out of standard practices and it really does not fit the situation.
Really surprised no one else has chimed in.
bore gauge and micrometers, snap gauges, feeler gauges , indicators or tape measure ; they are all dependent on the operator to know what they tell them and how to utilize that info.
 

gregh

New member
I have not chimed in because I found this a bit entertaining. A dial bore gauge and a mic is the only accurate way of doing this job if you have those tools. I have a $1500 tri/mic as well but it sees very limited use in these motors at all. I see no benefit personally of ever taking it out of the tool box for a small displacement easy to get at bore measurement like the motors we deal with.. It is used for cam journal bores on v8's mainly in my case as mine has two finger loops and it can get at right angles down deep in a v8 engines cam journal area. This is especially true when I change a v8 to roller cam bearing type cams after I line bore the cam journal. Calibration of a dial bore gauge needs to be done every time you use it?? You just use your mic why would you need a fixture. Calibrate your mic with your cali block and transfer over to your dial bore guage. You check your size taper and oor in seconds. I would never waste my time using telescoping snap gauges if I have a dial bore gauge?? Man if you are building engines use the accurate tools that you have in your arsenal. I have heard of the feeler gauge method but that was just used for a rough reference before deciding if the bore required an oversize. Not trying to be a smart Azz but if you all have the tools why not use them for the purpose that they were intended for.
Looking really forward to Steve Bakers engine telephone conference now as we will get some spirited talk I am sure!!
Greg
 

Vmax

New member
The fellow in the Vid has NO idea what he is doing. I have a freind that builds top of the line engines for AMA dirt track and he has used feeler guages for cylinder clearance for about 40 years now. It is a subjective feel as too what the process tells you. My friend can set it up to within .0005" ever time that I have ever checked behind him.

You can check all aspect of the cylinder with a feeler gauge IF you have the feel for it. Taper out of square low spots high spots the whole gambit.

He also uses Hole guages and mikes for all OTHER holes.

(;-) TP
 

flattop1

Dawg 89
At lest the guy in the vid was honest ; he had no idea what was normal but was going with it anyway:) Didn't look like he was even sure which hole the piston went in.
 

gregh

New member
The fellow in the Vid has NO idea what he is doing. I have a freind that builds top of the line engines for AMA dirt track and he has used feeler guages for cylinder clearance for about 40 years now. It is a subjective feel as too what the process tells you. My friend can set it up to within .0005" ever time that I have ever checked behind him.

You can check all aspect of the cylinder with a feeler gauge IF you have the feel for it. Taper out of square low spots high spots the whole gambit.

He also uses Hole guages and mikes for all OTHER holes.

(;-) TP
I have a hard time believing that someone can measure to the half thow using a feeler gauge consistently--just saying not trying to start a peeing match -- just cant be done with repeatability. Everyone tries to be a nice guy and they always seem to agree with the regular guys I for one will question those claims when I feel it necessary.

Greg
 

Vmax

New member
That is kinda like saying that a human cannot hand scrape a machine way to .0001" over 12" but they do it every day.

Some of the most accurate machines in the world were hand scraped to spec (;-)

Just a thought, (;-) TP
 

Vmax

New member
I would be willing to BET you can do it as well IF you practiced it a while. It is NOT as hard as it seems and is VERY fast to do compared to draging out all the equipment then checking and calibrating it all to spec.

NEVER discount human abilities (;-)

(;-) TP
 

sandhillbandit

New member
It's probably not the best but it is a way for po boys that can't afford all the other measuring devices or pay an engine builder to atleast get their back yard motor built. Im not a top notch engine builder by any means, but after visiting a reputable engine parts manufacturer a couple of weeks ago and talking with some of their engine builders they claim to still use the.feeler gauge method. Im not going to throw their name out there but I will add they do a great job at manufacturing an awful lot of billet parts in south GA for a lot of kart racers. :)
 
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