Squish measuring method?

I am getting confusing figures with my squish measuring method due to piston rock....clearly these pistons will rock when cold...they are JSM pistons with correct skirt clearance...machined .040 on periphery as Jims drawing.....

So, using plasticine appled to the periphery of the piston, moving piston back before TDC....fitting head with standard .040 copper gasket...and rotating engine through TDC...remove head and measure compressed plasticine...

I see around .070 on the rear side and .040 on the front side....

....should I expect more stable readings by setting the pistons at TDC, checking the pistons are not rocked, and putting the plasticine in the head side and lowering head on tightening down then remove and measure.....

Or do you use a completely different method?

Steve

Steve, I was taught to apply the measuring material on the piston crown, but only on the sides, ie above the gudgeon pin, because the rock detected otherwise makes meaningful measurements impossible.

I was also taught to use soft solder, but of course you can only do this if you already know the squish gap is within a certain ball park.

I have tried using other softer stuff like plasticine, but find it way to difficult to get good measurements, maybe due to my hamfisted approach!!

I have used soft grade of solder to reach rim on under plug and some what to either side then with draw and mic the flattened end. Squish don't seem to matter power wise to Nortons as long as don't clash when maxed out. Might could try two lengths of solder spread apart to see which contacted first for rocked sense. If don't mash the size solder on hand can twist 2 together.

This is an old two stroke method; the only difference is most 2Ts have a central plug head- wheras most Nortons have a spark plug halfway down the side of the head.
You need to take two readings, using soft core solder twisted around itself to get the required thickness, take one reading inserting the right hand angle bent solder below the spark plug hole, and using another fresh twist of solder wire take another above the s/plug. Unless the piston/and or cyl. head has been specially machined to obtain a matching angle (like a 2T)you will have a dome head & whatever machined piston crown.

I have used plasticene with an OK result, however this method looks good:

worntorn said:

I have used plasticene with an OK result, however this method looks good:

Click to expand...

I think plasticine works OK but the issue is the rock....I have measured enough to be sure I wo'nt have impact as it is....but I want to get as much compression and squish as I safely can.....looking for that .045", and I suspect .040" would work.....

As it is I think I am going to end up with a .030" head gasket....just want a good view of what I have before I order from JSM.

Eddies comment about concentrating on the measurement directly above the piston pin is the way to go.

But I am also going to try with solder. I am happy to fit and remove the head more times at this stage of the build It is currently bare so easy to do. Will have to do it again later to confirm valve clearances to valve pockets anyway as I do valve timing. That is definately a plasticine task to get a full imprint of the valve head in the pocket.

Since these are flat top pistons in a standard offset hemispereI don't expect to measure much 'above the plug'....but I do see how all that works on 'due tempi' set ups...including feeding the solder in through the plug hole in that case...longer 2 stroke pistons don't have the same cold 'rock' issue....

If you insist on doing it through the spark plug hole it seems to me you will have difficulty being sure you are above piston pin! and in making sure you right angle section reaches right to the edge of the bore to get properly trapped and 'squished'

My current idea is to place lengths of solder around the periphery above the pin, with the ends looped in and held in place in the valve pockets with the plasticine.....sounds like it will work

I measure at the sides of the piston to get a consistent measurement that I can compare to a standard minimum clearance. For most Commando engines I use .040" as the minimum for this method of measurement. I've gone as low as .035" for 750 engines that don't get revved above 7200 rpm, and higher numbers, up to .050" for high rpm and/or large bore engines, and much larger values for low compression street engines. The minimum clearance really does depend on how high you plan to rev the engine. I used the clay method for a number of years, and it does require some care to get an accurate measurement. It's pretty easy to squish the clay a bit with calipers when measuring. I tried the solder method a few times, but finally settled on just measuring everything before installing the head. I measure the depth of the counterbore in the head, and the height of the piston above the deck, with a depth micrometer. I measure the gasket thickness with calipers, and calculate the clearance from those numbers.

Ken

lcrken said:

I measure at the sides of the piston to get a consistent measurement that I can compare to a standard minimum clearance. For most Commando engines I use .040" as the minimum for this method of measurement. I've gone as low as .035" for 750 engines that don't get revved above 7200 rpm, and higher numbers, up to .050" for high rpm and/or large bore engines, and much larger values for low compression street engines. The minimum clearance really does depend on how high you plan to rev the engine. I used the clay method for a number of years, and it does require some care to get an accurate measurement. It's pretty easy to squish the clay a bit with calipers when measuring. I tried the solder method a few times, but finally settled on just measuring everything before installing the head. I measure the depth of the counterbore in the head, and the height of the piston above the deck, with a depth micrometer. I measure the gasket thickness with calipers, and calculate the clearance from those numbers.

Ken

Click to expand...

Thanks Ken....

I am still going to try the solder as an option, but I favour your suggestion to measure each component and caculate.....now to find a useful depth micrometer I can afford ;-)

It does complicate matters a little using JSM pistons with a machined periphery, but that is just another measurement to add into the calulation.

Based on what I have right now I am safe to assemble as is! 77mm bore, 80.4 crank, I currently think my clearance over the pin is .055" with a .040" gasket....just want to be really sure of that.

I don't want to machine any more off anything anyway, but taking account of your comments and JSM recommendations of .045" I could probably go to a .030" gasket when I am happy with the way it is running....

I intend to aim for peak power at 7000, occasional 7500, very occasional 8000 when saving a gearchange between corners (well thats the plan)

Steve

Steve
static squish is one thing, but what is the dynamic clearance ?
years ago when building competition engines where squish was an important consideration, I would first measure components and calculate the clearance.
This would then be checked during engine build up using the soft core solder method. What then bothered me was the possibility of the piston coming fractionally higher at TDC on the overlap when no compression was present to stop components flinging upwards due to inertia - in other words, the dynamic squish.
I eventually decided to mark the piston crown under the squish land using a spring loaded centre pop. The resulting mark when viewed with a magnifying glass would resemble a small volcano - the centre punch mark being surrounded by a raised rim a few thou higher than the surface of the piston. After a full throttle max. rpm run, the head was removed and the punch marks examined. A slight flattening of the top of the rim would indicate the piston and head had been that close, how close could be measured by a vernier.
If you are intent on having the tightest squish possible, this method will let you know exactly how close, otherwise defer to the old workshop maxim 'close enough is near enough'.
Incidentally I found out over the years that not all engines respond the same way to squish, and found some that ran very much better with no squish at all, which would seem to indicate that burn efficiency is the all important issue.

re; I am still going to try the solder as an option, but I favour your suggestion to measure each component and caculate.....now to find a useful depth micrometer I can afford ;-)


Why bother , a Digital LCD Vernier Caliper Gauge could do the job just as well and be useful for other jobs

http://www.ebay.co.uk/sch/i.html?_sacat ... per&_frs=1

Works BSA 750 riden bu John Cooper had 30 thou of squish..he reved the engine to 10,000 to beat Ago on his home track..the pistons kissed the head..makes you wonder where thet 30 thou went to? Proberly a heat issue, carn't see the crank flexing that much in a three. Rod stretch?

Bernhard said:

re; I am still going to try the solder as an option, but I favour your suggestion to measure each component and caculate.....now to find a useful depth micrometer I can afford ;-)


Why bother , a Digital LCD Vernier Caliper Gauge could do the job just as well and be useful for other jobs

http://www.ebay.co.uk/sch/i.html?_sacat ... per&_frs=1

Click to expand...

Well I bought a used one from ebay for about that money....result....

Following Kens methods I calculate .054" to .056" was fairly happy with that...for now....then I remeasured and calculated and got .060".....

Using solder measured at 058" it remained unsquished.

2 Twisted length of squished solder and a digital caliper I already had, I meausred the solder at ~ .065" why .005 difference who knows?

You could question the accuracy of either instrument. This is first build and I accept Snotzo's input that you can cetainly over worry the whole thing.

So, either way it is getting assembled like that. It should be safe.

Thanks for your input all of you.

I will purchase a couple of gaskets from JSM for experimenting over the winter after it has been run.

Some things to help settle your mind - higher compression pays back quite a bit less after upper 9:1 CR reached and mainly helps to allow a hot cam profile to start and idle as going from 9 to 10 CR only adds 2-3 percent power increase and most pilots can't detect under 10% change in power according to the years of reading feedback. We really covered the scope of squish in Norton and other hemi's to see squish don't seem to matter at all in hot Norton engines. With the lighter JSM kit and steel rods 7000 rpm should not be causing much crank bowing or rod stretch at TDC so likely can fudge a bit more CR safely if limiting rpm to normal factory level for nicer deeper bark-pop sound at idle and blip ups. Once stack height clearance settled can then dig in some more to re-do the push rod geometry rocker contact sweep.