Squish!!

[Mekanix]

Still troubleshooting my GSX running rough in the other thread but this needed its own thread.

Using a WSM top end kit, not oem, Should I be using the OEM squish measurements?

I think so but the manual says I need a 1.2-1.6mm squish.

Well I'm at 2.00mm that's using the 6 hole base gasket that came in the kit. (0.6mm thick)

To fix that means a thinner gasket.
but I can't get small enough.
5 hole gasket I get 1.9mm squish
4 hole gasket I get 1.8mm squish
3 hole gasket I get 1.7mm squish
no gasket I get 1.4mm squish

I have another head that measured a bit better though but that only saves me 0.1mm which makes the 3 hole gasket give me 1.6 which is in spec.

Like to be at 1.45mm because I'm picky but not sure how to get there. Maybe plane the head is the easyer way to go or should I bother?
AS IN, will the WSM piston hold up to the increase in power?


Another thing, The head domes are polished, Does that matter for any reason ?

Thanks guys

 

[68ragtop]

Any chance your checking without the case halves or top end parts torqued down to specs? I don't think polishing a dome with do anything but maybe help cut back carbon build up. If anything it could make your chamber slightly bigger & lower your compression a couple lbs if too much is removed.

 

[Mekanix]

Case, cylinders, and head torqued to exact spec.
First time I checked was with the engine all together before changing heads.
Tried using 1.67mm solder and it didnt squish

 

[Strizzo]

Uh, if a 3 hole is .3mm and you get 1.7mm then shouldn't no gasket give you 1.4mm?

Seems like you could machine the bottom of the jugs to reduce the distance between the case and head.

 

[Mekanix]

Your right, I missed that.

Machining the bottom would be one, or plane the head or just the top of the cylinders.

Do you think this is done on purpose by WSM so their piston's will last longer ?

 

[mikidymac]

I have never had an issue with WSM components and use them myself.
If you are using "your" OEM cases, cylinders and head you should theoretically be able to use the same base gasket as came from the factory and be fine. However squish is very important especially on the 787 so it is good that you are checking it.

What were all 4 of your measurements? You should be checking both sides of both pistons directly over the wrist pin.

 

[racerxxx]

Before I would even THINK of machining I would measure the wrist pin to crown on an OEM and a WSM piston, actual gasket thickness etc.......

You might get a good read here http://www.groupk.com/sd785updt.htm

Read from the bottom up Remember this is more about race engines but could shed some light on your situation.

Check out : Precision Top End Assembly of the Laydown Rave Motors

Also check out the Piston Crown Variation. Now, they are talking about crown height etc...... but if your pin to lowest point of crown is off this is where your squish could get out of whack.

The also mention that with an OEM base gasket there is variation on that too, but on a stock ski it might not matter a few .00X But once you start stacking the tolerances up you can get out of line pretty quick.

What amazes me the most is I remembered reading all this back in the day when I built my first 787 and it's still on the interwebs!

 

[Mekanix]

While I'm inside for the moment here are my measurements without any base gasket...

MAG 1.20//1.28------1.24//1.36 PTO After a few tries I've gotten around 1.2-1.36mm. It's inconsistent since I'm not always exactly where I was the previous measurement and sometimes I catch the ring.

I have a 3, 4, 6 hole gasket available but that makes it just squeaking into the limit of 1.6 in certain places with the 3 hole...

3 hole theoretical.
MAG 1.50//1.58------1.54//1.66 PTO

I'm testing with 1.7mm solder and measuring the most squished part.



Original 6 hole gasket:

MAG 1.80//1.88------1.84//1.96 PTO

Another thing. Since I'm practically splitting hair's here, I think the polisher did take a bit out of the squish band.



What should I do now?

Put the 3 hole on and calling it a day
Use my other slightly pitted head that will drop the results a bit and maybe let me use a 3 or 4 hole (testing with it when I go out again...)
Or use the head I have on there now and have it planed and then use what ever gasket will work? 3,4,or 6...


And the dreaded option, Swap in my spare brand new cylinders and top end kit to see if its better...

 

[Mekanix]

Base Gasket Thickness - "Jon's Story" - One of our early test boats was an XP that was to be raced in region one offshore. We shipped the kit to the owner, who had it assembled at a local shop. After the break-in, the test rider reported a little better top end, along with average bottom end power. We were concerned because all our other test riders had reported big increases in bottom end as well as peak rpm. After a few unsuccessful attempts to resolve the problem with carb tuning, he came to our shop with the machine. After a quick test ride, we found this XP to have mediocre acceleration and a 6900 rpm peak (that's 150 rpm short of the norm). Back at the shop we checked out every possible problem. Besides the indicated compression being just a little low, the only other inconsistency was that the squish clearance seemed to be about .010" (.25mm) too thick. At the time, we didn't think this was the problem...but it was the only thing we saw that we could "fix". We removed the .024" (6 hole) base gasket and replaced it with a .016" (4 hole) base gasket. The next morning we gave the boat a short break-in...then gassed it. The difference was unbelievable. The boat pulled viciously up to 7040 rpm...just like all our other test boats. No one was more shocked than us, that .008" of base gasket thickness could turn a weakling into rocket. After this experience, we specified for our assembly instructions that a .038" - .042" squish clearance must be maintained on all Group K modified top ends. Since then, we have not experienced this problem again. (Note : This XP won the 90 minute 1200 pro overall at the Havasu Global Offshore Finals)

Well now, They say that 0.2mm can be devastating :O

So for a stock engine I'm going for 1.2-1.6mm and I can only hit 1.5 at best with a 3 hole gasket...

Going back out to swap heads and test again... brb

 

[racerxxx]

Ok, they don't say it's devastating. Remember, this is not a stock engine they're discussing. If it was running offshore they used their 92 Octane Sleeper kit HERE Scroll down, there is port work, carb work and head work..... None of which your stock engine has done so that small change in base gasket thickness in that application cold make a big difference.

I think the point I was trying to make was that if the center of wrist pin to edge of piston crown is different(shorter) on your WSM pistons from stock there is one tolerance, if the base gasket measures over what it "should" measure there is another over tolerance. If you can get back to "spec" even if it's at the top end of the spec, you're still in spec. When I set my 95 engine up I went to just under the middle of the range (1.35mm or 0.053"), true middle is 1.397mm or 0.055". The engine originally had a 4 holer on it, and I dropped to a 3. Now, was the factory wide on mine? Probably not, I changed the crank and pistons so technically the 4 holer from the factory could be too thick now and squish had to be recalculated especially with the crank change.

One thing I didn't like that you mentioned is "It's inconsistent since I'm not always exactly where I was the previous measurement and sometimes I catch the ring." Not sure how you catch the ring. The way I do the check is to rotate the PTO so the piston is at TDC, then go back just a hair and put the solder in so it slides across the piston parallel to the pin. Then while holding the solder in one hand I slowly pinch the solder while rotating the PTO with my other hand. Then I will turn the PTO over by hand and only crush the solder once. Then just in from the little blip at the end of the solder is where I measure with just the very thin tips of the verniers. I will do multiple bumps on a new chunk of solder and measure those, while the solder is out I will cycle the crank one revolution and check teh other side and do this about 3 times on each side and each jug. I never run the piston all the way down and then back up with solder in the jug. The solder I use is way thicker than the stuff you have. To convert to metric my solder measures 2.93mm. To make sure your parallel what I do is cut off a strip of solder and make a [ by laying it on the table but I'll keep the bent legs close so when I put it in the cylinder what is out of the cylinder looks just like what is in the cylinder....I can get a great visual if I'm twisted off center or not if that makes sense.

 

[Mekanix]

Got it, thanks.

That little lip on the end of the solder is what I meant by catching the ring.

I now have the 3 hole gasket installed and everything torqued down with the other head.
Results:
MAG 1.44//1.44------1.44//1.58 PTO

 

[racerxxx]

Your in spec, that 1.58 is crazy shit for that piston to be that different.

Sent from my SGH-T889 using Tapatalk

 

[racerxxx]

That lip is what rolls over the edge. I thought you were nipping the solder, it was freaking me out to think that the droppings were just falling down into the bottom end.

Sent from my SGH-T889 using Tapatalk

 

[Mekanix]

lol yeah I would be freaking too ! I did it pretty much the same as you, Just rocking the crank before and after crush point. Sometimes I would get different results because the solder wasn't paralel with the wrist pin but after a few tries I was getting more consistent results.

That one side did the same with the other head too. Always 0.12-14mm higher than the other 3...

Tried shifting the head around. Ended up just lining up the edges.

 

[TheMachin0]

which way are you pointing the solder you need to point it at the mag or pto the piston will tip if your checking it side to side may help get better readings. I run 175lbs at .65 to.7 roughly

 

[Mekanix]

which way are you pointing the solder you need to point it at the mag or pto the piston will tip if your checking it side to side may help get better readings. I run 175lbs at .65 to.7 roughly

Parallel with the crank and wrist pins.

At first the solder was getting pushed to one side or the other because I couldn't see what was happening.

This engine was also at pretty close to 170 for some reason and that was with the 2.0mm Squish!
I'll get a good measurement when the engine goes back in.

How well does it run like that ?
Is the throttle smooth from idle to 5000.
At .65-70 wouldn't that cause detonation without high octane?

 

[TheMachin0]

it runs great 27 hours I got on it this year I built it last winter. took some carb adjustments for the f/a. I got squish advise from dan some kind of old jetski guru he used to race I purchased my head from his ebay site and it came with lots of advise. from what he told me the closer to .5 gets more performance .5 and under is boom time. I have never ran less than 91 octane ethanol free in any 2strok I own.

how confident are you in the measuring device maybe borrow a friends to compare

 

[Mekanix]

Can't say I'd trust the gauge so i'll check callibration at work tomorrow.


I was just playing around shifting the head side to side and managed to get the pto side down a bit and matched the two on that cylinder.
That threw the other side way out of wack though
Id say my cylinders are too close together!

Going to try re-aligning.

 

[Mekanix]

Measured the pistons vs stock vs pto to mag.. Same deck height from edge to wrist pin bore within 0.02mm.

I cheated and assembled the head on the the cylinders to see if its lined up inside and then bolted on the exhaust manifold to keep the spacing right. It was a bit hard to get the whole thing back on the pistons.
The head fit's cylinders contour a bit better. I marked the head in a few places to match up after torquing the cylinders down. It would be nice if there was dowel pin's ! or an alignment tool for this!

 

[Mekanix]

Update:

Noticed an increase in snappiness on the low end and that's about it.

Also this still has not fixed the original problem of running rough on low end.