MEASURING HEAD CLEARANCE WITH PLASTIGAGE

[joeb33050]

I just spent an hour and a half with two guns trying to measure head clearance with plastigage-actually with "SEALED POWER" strip from NAPA.
Using a bunch of fired FL sized cases, the plastigage smeared when the bolt was closed. The bolt turned relative to the case head, smearing the [plastigage. Widths varied with the smear, but there was no reading I'd believe.
I've read that some guys measure head clearance with plastigage. What am I doing wrong?
joe b.

[hotbrass]

Are you talking about head space?

Why dont you use gauges or the other techniques that are described in many threads on the topic of head spacing?

[82boy]

You put it on the back of the case, the plasiti gague is not smearing it is compressing, measuring the compression with the package, and it will give you a reading.

[stangfish]

I hope your sizing die is set correctly. Otherwise you may be stuck using reloads.

[BobT]

What am I doing wrong?
joe b.


You are using an automotive tool on a firearm, not that there aren't some instances where that will work but this isn't one of them. Plastigage works best without all the rotation that can occur when a bolt is closed on a case. The proper tools for the job are headspace gauges in the correct caliber.

Bob

[joeb33050]

Are you talking about head space?

Why dont you use gauges or the other techniques that are described in many threads on the topic of head spacing?



There are several recommendations re plasigage for checking/measuring head clearence here. See

http://savageshooters.com/SavageForum/index.php/topic,28466.0.html

for one. If I'm doing it wrong, I'd like to know how to do it right. Again, put it on the bolt face or the case head and it smears, making reading impossible-at least for me.
joe b.

[Nor Cal Mikie]

On the case head "bolt face". it will smear from the rotation. Try laying it on the face of the case shoulder area. If the case doesn't turn with the bolt head, it might work? ???

[DarnYankee]

Ever push a chain? there are easier ways without all the hassle. use some scotch tape on one case and nothing on another for a go and no go gauge.

[82boy]

Ever push a chain? there are easier ways without all the hassle. use some scotch tape on one case and nothing on another for a go and no go gauge.


Scotch tape is not the answer.

[jonbearman]

Let me give you some sound advice,Break the piggy bank and buy the go,nogo gages. Any other way could work ,but my money says gages period.

[sharpshooter]

You probably have the wrong plasti-gage. You need the .002-.006" range.
Cut a piece that will go across the entire bolt face. Put a small dab of Vaseline on the bolt face to make the plasti-gage stay in place.
If it smears to the point you can't get a reading, it is probably less than .002".

[bigedp51]

Head clearance is the "air space" between the bolt face and rear of the cartridge case.

Headspace is the distance from the bolt face to the datum point in the chamber.

The last batch of .243 cases I bought had cases with cartridge headspace .009 shorter than GO or minimum headspace. The rifles actual headspace setting is .002 longer than minimum headspace and this makes .011 HEAD CLEARANCE.

How to check and measure head clearance.

1. Measure the overall length of your cartridge case with a vernier caliper, base to case mouth and write it down. Use a new or full length resized case "BUT" remember a new case can be the shortest of the two. And remember that the SAAMI allows approximately .007 variation in case headspace length during manufacture. ;)

2. Take a spent fired primer and just start the primer into the primer pocket.

3. Now chamber the test cartridge closing the bolt fully and using the bolt face to "seat" the primer in the primer pocket.

4. Now measure the cartridge case overall length again and write it down.

5. Subtract the first case measurement from the second case measurement and this is your "head clearance". The amount the primer is protruding is the "air space" between the bolt face and the rear of the case with the bolt closed.

Below a rimmed cartridge case is being used as an example, BUT this test will work on any type of rifle cartridge case.

http://i122.photobucket.com/albums/o254/bigedp51/hspace-b.gif

NOTE: Engine bearings do not have Remchester pressed into the bearing surface to throw off the plasticgage reading when compressed. :o

http://i122.photobucket.com/albums/o254/bigedp51/Plastigagehelp.jpg

http://i122.photobucket.com/albums/o254/bigedp51/Headspace_2_lg.jpg
http://i122.photobucket.com/albums/o254/bigedp51/Headspace_1_lg.jpg

Know thy rifle

http://i122.photobucket.com/albums/o254/bigedp51/IMGP7241.jpg

[devildogandboy]

i would put my money on BigEd, he knows what he's talking about. i read and tried his methods because they seem most reasonable to ME. read and see what you believe.
http://savageshooters.com/SavageForum/index.php/topic,50137.msg344071.html#msg344071

[bigedp51]

At low chamber pressures like the 30-30 Winchester, the primer will back out of the primer pocket every time the rifle is fired, and the primer can only back out by the amount of head clearance you have.

I have seen cartridge headspace vary as much as .009 on brand new brass and on wild exceptions like the .303 British Enfield you can have .016 head clearance and still be within headspace limits.

Below is an example of a primer backing out then being reseated as chamber pressure increases and causes the case to stretch and conform to chamber dimensions. (at low chamber pressures this does not happen) The red and yellow colors represent the areas of highest stretch and strain

http://i122.photobucket.com/albums/o254/bigedp51/deform.gif

Your primer will always tell you more than plastigage will and all you need are two eyes and a set of vernier calipers to "read" your primers.

[devildogandboy]

;D hey, is biged GREAT or what? this guy is GREAT! read his info and see if it doesn't make sense to you. made sense to me annd i tried it and it worked!

[joeb33050]

I tried the plastigage again and failed, but I think I know why.
With 1 primer and 10 fired cases I used the primer protrusion method and measured head clearance. This on a 243 Win barrel just put on a M10.
Head clearance varied from .000" to .004" with the average .0016".
It looks like the head clearance is too little to measure, at least with some cases, with plastigage with .002"-.006" bounds.
When I can I'll set headspace on a barrel a little looser, and try both plastigage and primer protrusion.
Thanks to all;
joe b.

[bigedp51]

There is a reason "WHY" headspace gauges are not made of plastic and "WHY" you should have "REAL" headspace gauges. Without stripping the bolt you "may'" get varying readings with the primer method and only be "ball park" at best depending on your cases and bolt closing method.

With a brass cartridge case you can dry fire a fired case and move the shoulder forward .001 every time you pull the trigger. This is why the reloading manuals tell you to not use cartridge cases used for reduced loads again for full power loads..........the cases have too much excess headspace or are too short for the chamber after being used for reduced loads.

I have real headspace gauges and use the primer method to tell me what is in between the GO and NO-GO gauges. Some days you win and some days you loose..................

[82boy]

Why would the primer have anything to do with this? You are measuring with a fired, and SIZED case. primer knocked out.

[bigedp51]

Why would the primer have anything to do with this? You are measuring with a fired, and SIZED case.
primer knocked out.


Because anyone who reloads will have a spent primer and hopefully a set of vernier calipers to measure head clearance.

Because plastigage is meant to only be compressed and how many engine bearings rotate 90 degrees when tightened smearing the plastigage and giving a false reading .

http://i122.photobucket.com/albums/o254/bigedp51/IMGP0704.jpg

Now chamber this test round and then measure your head clearance without buying plastigage. ;)

http://i122.photobucket.com/albums/o254/bigedp51/IMGP0706.jpg

http://i122.photobucket.com/albums/o254/bigedp51/IMGP0710.jpg

British .303 Enfield Head clearance

http://i122.photobucket.com/albums/o254/bigedp51/9-3.jpg

http://i122.photobucket.com/albums/o254/bigedp51/headspacestretch-1.gif

After collecting the Enfield rifle for over 20 years I know a little about the word headspace and head clearance. I nearly went blind playing with my bolt head. ::)

http://i122.photobucket.com/albums/o254/bigedp51/IMGP1355-1.jpg

[joeb33050]

Several of you guys have STRONGLY recommended the use of "GO" and "NO GO" gauges, and I strongly agree that that is the best and safest method of gaging during barrel changes. For almost all situations. I have read the "Headspace" sticky at the top of this about five times, end to end. I have only two actions, a Striker and a M10, and I have changed barrels at least 30 times, setting headspace as carefully as possible and measuring head clearance as a check. For reasons clearly explained by others on this forum, I prefer the fired FL sized case method. Using one and two strips of Scotch tape is one method of checking headspace. Using plastigage , if I can get the method to work for me is another. And using the primer protrusion method is a third and easier method. The more methods the better, I think.
So, I'm not ignoring you GO/NO GO gauge guys, but I've made my choice and I think it's an informed choice.
Thanks to all, this has been very helpful.
joe b.