checking squish height

[Nitro]

to all that run low clearances

Do you push up on the piston from underneath when you check?

Nitro Nub

 

[Ray Sametz]

to all that run low clearancesDo you push up on the piston from underneath when you check?

Nitro Nub

If your checking head clearance put pressure down on piston.

 

[Tom w]

,

 

[moparbarn]

yeah, it has happened to me, k&b 3.5 ob. to measure squish, i will measure with calipers, 2-3 times, & average readings. then i will re check with clay on top of the piston. adds down pressure and takes into account head being torqued down. sometimes the clay will read .001-.002 tighter. usually though, it's the same or less than .001' difference. if you're running them really tight, you can use 'plastigage', a clearance measuring product available at auto parts stores. directions on pack, simple and accurate.

 

[moparbarn]

just checked plastigage's website, i was wrong. there are 4 ranges available:

HPG1 - .001-.003 green

HPR1 - .002-.006 red

HPB1 - .004-.009 blue

HPY1 - .009 .020 yellow

looks like waxed string, actually a plastic. deforms to a controlled width that equates to clearance, measured with the supplied scale. no dissassembly required, can work through the glow plug hole, similar to the solder method. i've used it for YEARS, assembling full size motors, to check bearing clearance. VERY accurate.

 

[Norm Doerr]

to all that run low clearancesDo you push up on the piston from underneath when you check?

Nitro Nub

Nub;

Please come and hang out with us at this weekends race. There is a VERY easy way to do this.

Norm Doerr

 

[Joe Warren]

to all that run low clearancesDo you push up on the piston from underneath when you check?

Nitro Nub

Accucheck Is Most Accurate to check squish with.

 

[Marty Davis]

Nub:

Depends on how accurate you want to get. If you want to get REALLY accurate, look at: http://engine-analysis-software.com/measure.htm

Another benefit of this system is that you can also get super accurate timing documentation with it. These depth gauges are not expensive and should be in every serious model boaters shop for MANY uses.

The problem with measuring the clearance is that the liner has taper in it and the piston gets really tight at the top of the stroke. Unless you use a method similar to that shown on the link, and allow the liner to be raised, you won't get an accurate reading. It will be within a couple thousandths, but that is not really close enough, especially for the small engines.

Marty Davis

 

[moparbarn]

marty, in your opinion, how accurate is the measurement obtained, by measuring with the engine assembled? it seems that it would take into account any variations in liner position and head also, by having them torqued down. not doubting anything, just trying to learn the best, affordable way to get accurate results, for the average boater. your setup in the link is very nice, but out of the reach of a lot us us.

 

[Tom w]

,

 

[moparbarn]

piston up. you could measure with piston down, and figure in stroke, but that introduces too many variables and chances for error in math. plus, you would have trouble accounting for any rod and crank play difference at bdc as opposed to tdc.

 

[Tom w]

,

 

[Marty Davis]

marty, in your opinion, how accurate is the measurement obtained, by measuring with the engine assembled? it seems that it would take into account any variations in liner position and head also, by having them torqued down. not doubting anything, just trying to learn the best, affordable way to get accurate results, for the average boater. your setup in the link is very nice, but out of the reach of a lot us us.

The measurement using the device that Bill McGraw sells will get within .002" (not sure how much he gets for it currently). The Plastiguage will get within about .002-.003". Most of the measurements that you will get will show the clearance MORE than it actually is because the piston is being squeezed a ton at top dead center because of the taper in the liner. This makes the piston NOT get all the way to TDC. The gages that Terry Keeley sells will be about the same accuracy as the above range.

If you buy a good set of calipers, they will cost you $50 (you can get to .002" accuracy with the calipers).

The height gage will cost you about $75 plus a dial indicator (very inexpensive at McMaster Carr). (this will get you to .0005" or less accuracy)

The accurate measurement of head clearance is SUPER IMPORTANT and as an example the squish velocity of my best .21 engine is 55 m/s at .007" clearance and if I move it to .008" the squish velocity falls to 50 m/s and if to .006" clearance it raises to 61.5 m/s. You can see that there is a huge swing. The .001" from .007 to .006 is enough to get you into engine detonation. ONLY .001". With the larger engines it is not so critical. The .45 engines should be within .002" and the .67's about .003". You can get that close with the other measurement devices. Squish velocity does not change quite as much as you get to the larger engines with a variation in accuracy.

Hope this gives you the information that you need to make a good decision.

Marty Davis

 

[moparbarn]

thanx, marty. i was surprised at how reasonable the height gauge/stand was. i have the dial indicators. now, a GOOD surface plate........

 

[Jerry Wyss]

Plastiguage will get within about .002-.003". Most of the measurements that you will get will show the clearance MORE than it actually is because the piston is being squeezed a ton at top dead center because of the taper in the liner. This makes the piston NOT get all the way to TDC.

The piston is either under "compression stroke" or "power stroke" when its near TDC,, at what possible point

is the piston going fly "up" that additional .002 or .003". That .003" play would mean .006" rod clearance wouldn't

it? why would you have that much?

JW

 

[Marty Davis]

Plastiguage will get within about .002-.003". Most of the measurements that you will get will show the clearance MORE than it actually is because the piston is being squeezed a ton at top dead center because of the taper in the liner. This makes the piston NOT get all the way to TDC.

The piston is either under "compression stroke" or "power stroke" when its near TDC,, at what possible point

is the piston going fly "up" that additional .002 or .003". That .003" play would mean .006" rod clearance wouldn't

it? why would you have that much?

JW

Jerry:

The piston can not go all the way up to the top with the taper in the liner. The rod clearance will provide some of the lack of going to the top. Remember that you have clearance on both the bottom and the wrist pin end of the rod.

Marty Davis

 

[moparbarn]

marty, if you have a solid, non flexing rod, and a piston the same, what gives, not allowing the piston travel? the crank pin makes a full revolution, moving the crank pin through tdc. if i can turn it through a full stroke by hand..... you make it sound like a physical impossibility? the same lost motion you mention, doesn't this affect squish, while running, no matter how carefully measured on the bench? help me understand.....

 

[Marty Davis]

marty, if you have a solid, non flexing rod, and a piston the same, what gives, not allowing the piston travel? the crank pin makes a full revolution, moving the crank pin through tdc. if i can turn it through a full stroke by hand..... you make it sound like a physical impossibility? the same lost motion you mention, doesn't this affect squish, while running, no matter how carefully measured on the bench? help me understand.....

When the engine is running, the metal expands and the fit of the piston to the liner is much looser. The tighter that your engine is, the more difficult to measure accurately the head clearance cold.

Marty Davis

 

[moparbarn]

that makes more sense to me. the increased taper when cold can and will affect measurements. how much "fudge factor" is involved in the on the bench vs running parameters? static squish, compression and squish/quench velocity is all affected by these differences, isn't it? or, is more of knowing what a specific measuring style will yeild? ie- your measurment says .006 squish, and mine (assembled) says .008, as long as we measure consistently, and changes are based on our OWN measurements, does it really matter? in these very technical discussions, it is good to be on the same page, but for most of us, we are changing things based on our measurements. and, while testing at the pond, some measuring procedures can be difficult. i know this kinda flys in the face of "good shop procedures", but i'm trying to relate a lot of this info to us common folk :lol: :lol: . don't mean that in any bad way, in either direction, just that a lot of folks don't have the access to the facilities, or the skill to use them. having been an auto tech for 30 years, i understand the measurements, and how to measure them, just not the same as you, or (probably) as precisely. thanx for what you, and others have taught me thus far, and thanx, in advance, for helping me understand more.

 

[Marty Davis]

that makes more sense to me. the increased taper when cold can and will affect measurements. how much "fudge factor" is involved in the on the bench vs running parameters? static squish, compression and squish/quench velocity is all affected by these differences, isn't it? or, is more of knowing what a specific measuring style will yeild? ie- your measurment says .006 squish, and mine (assembled) says .008, as long as we measure consistently, and changes are based on our OWN measurements, does it really matter? in these very technical discussions, it is good to be on the same page, but for most of us, we are changing things based on our measurements. and, while testing at the pond, some measuring procedures can be difficult. i know this kinda flys in the face of "good shop procedures", but i'm trying to relate a lot of this info to us common folk :lol: :lol: . don't mean that in any bad way, in either direction, just that a lot of folks don't have the access to the facilities, or the skill to use them. having been an auto tech for 30 years, i understand the measurements, and how to measure them, just not the same as you, or (probably) as precisely. thanx for what you, and others have taught me thus far, and thanx, in advance, for helping me understand more.

...will affect measurements. how much "fudge factor" is involved in the on the bench vs running parameters? static squish, compression and squish/quench velocity is all affected by these differences...

Not if you use the height gauge. The liner actually raises when the piston comes up toward the top and with the dial indicator, it will still measure the TDC super accurately. You then push the liner down with the piston anyplace except at the top and make that measurement to the top of the liner (pushed all the way down and seated on the case). The difference gives you the measurement from TOL to top of piston. Then just measure the head button projection and the difference is the head clearance down to .0005".

I guess that you can "build in" some amount of difference in measurement, but I personally like to know EXACTLY what the measurement is. It took a LONG time to be able to measure all of this accurately. I started with a degree wheel and light to try to estimate timings, etc. like most people have.

Marty Davis