How to use plastigauge for measuring head clearance

[XtremeRcBoats]

I bought some plastigauge to measure the head clearance. Have not used this stuff before. What is the best way to use it, can you do the same as with a tin wire and stick it through the plug opening?

 

[FloridaScaleBoater]

Cannot use the same as with tin wire/solder.
Plastigauge is not bendable.
How I use, remove head and cooling jacket, cut a piece of plastigauge to fit side to side on piston top,
Reassemble head and cooling jacket, tighten to specs, rotate engine over by hand.
Remove head, measure the squashed area of gauge using the colored measuring squares on package.
Adjust with shims to desired clearance.

 

[Daniel's Racing]

You never want to rely on anything that takes compressive force to take measurement.

Especially when building engines in around .010" squish

 

[Dave Gilles]

You never want to rely on anything that takes compressive force to take measurement.

Especially when building engines in around .010" squish

i'd personally trust plastigauge being that it's for crank bearing clearances. Solder on the other hand I have never been able to get super accurate. Lots of tolerance stacking there.

 

[Daniel's Racing]

Problem is the compressive force required to compress the material also compresses the rotating assembly and stretches the top end assembly and skews the reading by some amount. You want a totally relaxed assembly with no forces being applied when measurement is being taken.

 

[Wasted wages]

Problem is the compressive force required to compress the material also compresses the rotating assembly and stretches the top end assembly and skews the reading by some amount. You want a totally relaxed assembly with no forces being applied when measurement is being taken.

We could argue these points all day long,,,
I’ve done them both ways… but Danny’s gauge is the only one that gives repeatable results every time. Worth every penny!

 

[Danny King]

Throw it in the trash and physically measure it.

 

[timothy johnson]

How hard can it be!

 

[Kris Flynn]

when an engine is running it is under compressive load?

 

[FloridaScaleBoater]

when an engine is running it is under compressive load?

That's kinda why I like the plastigauge.
You are actually getting a measurement that is showing any slop in rod, wrist pin & crank pin.
Just my thoughts, but what do I know??

 

[Buckshot]

I’ve never used plastigauge or solder. I’ve always wondered if the location of the compressive material would affect the accuracy of the measurement as it’s placed in different locations around the circumference of the cylinder. i.e. it would measure a tighter squish directly above/parallel to the piston pin and less squish 180*/perpendicular to the piston pin due to the piston rocking in the sleeve. This might not be the case as I’ve never tried it.

 

[Daniel's Racing]

You can never get repeatable results is what I have found is main reason.

One time around a tdc crush produces one measurement and a couple more times back and forth across tdc and the thing measures .005" difference. Then compare either to a proper squish gauge and it is not the same either. Which is it ? It can relax while running like when you throttle chop the engine and the flywheel is driving the crank not the piston. You need to know what is the closest it can get. They can be tighter at the edge where you didn't get solder/plastic gauge in between.

I can't see setting engines in the .010"-.012" range getting variances like that. I set them to .001" tolerance so how would that even work.

MOST IMPORTANT OF ALL.
If your not bottoming the piston in the head you have no idea what area or where it is going to make contact first and setting the start point to zero based on that.

It might be .005" tighter right at the edge but plastigauge or solder didn't quite catch that detail, eh ?

You start dealing with dishes and domes and pistons that don't have same angle as squish band you are gambling on where the piston actually makes contact with the head.

If you had a dead flat piston and a removable button with dead flat squish band plastigauge may be a more viable measurement.

 

[Kris Flynn]

Hi Daniel,

I have not seen a squish gauge in use or even photos of how they actually measure. But how do they account for dished/domed pistons and tapered head button squish?

I have just been using a digital vernier and also a digital depth gauge to get my numbers...it does not really account for dish and tapers...but in all honesty, it is how I have seen some REALLY FAST guys do it, so it must be ok

 

[Daniel's Racing]

Hi Daniel,

I have not seen a squish gauge in use or even photos of how they actually measure. But how do they account for dished/domed pistons and tapered head button squish?

I have just been using a digital vernier and also a digital depth gauge to get my numbers...it does not really account for dish and tapers...but in all honesty, it is how I have seen some REALLY FAST guys do it, so it must be ok

None of that matters as it is all taken into account while zeroing the gauge.
You have to mate the piston tight to the button during assembly and set the gauge to zero. Once you have done this there is a way to repeat that start position later down the road. But anyhow you mate piston to head until they are touching and set to zero. Now assemble the engine and roll the piston slowly around tdc and the lowest number you see it read as tdc goes by is your actual squish clearance.

>>>>>>>>>

Hmmm,

Clevite 77 Plastigage

Clevite 77 Plastigage was designed to be a final check of total vertical oil clearance during reassembly. It is not meant to be a replacement for properly measuring crankshaft journals, housing bores, or bearing dimensions before engine reassembly with accurate micrometers and gauges. Plastigage is available in four different sizes to check total vertical oil clearance on connecting rods and main bearings. Each package has a measuring scale printed in inches and millimeters. The strips are color-coded for easy size identification and are soluble in oil.

Warranty

I like this one, reasonable idea.

Plastiguage is a method used to measure bearing clearance. While it is not as accurate as a micrometer and bore gauge, it can still provide a reasonable idea of the oil clearance at the point being measured.

 

[Barrie Lever]

when an engine is running it is under compressive load?

You would have thought so, but what these measurements of any kind don't take into account is system stretch.

This means that the piston actually goes past the compressed measured position, think about the inertia of the piston and then bringing it to a stop.

I have empirically gained figures on 40 pylon race engines and the stretch is quite surprising.

Having said the above you need some hard and fast figures to measure and move to, we measure actual head volume as that is what we are all really interested in.

 

[Daniel's Racing]

Some methods are going to work better for some engines than others. I build mostly gas engines. The pistons are domed and squish bands are angled. We cut the squish bands and combustion chambers. The only accurate way is a depth gauge fixtured in a adapter through the plug hole. You start with loose piston buried metal to metal up in the one piece cast jug and set the gauge to zero. Asemble and roll around tdc and lowest number is your squish. It is the only way to accurately measure the clearance. Some stock untouched jugs the edge of combustion chamber has a ridge that plastic gauge and solder would not catch. We usually cut this away on the stroker engines and make it uniform to the cylinder wall.