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The limit? Squish


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#1 GoinBoardin

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Posted 10 December 2017 - 12:51 AM

Little wordy but bear with me. 2013 Summit 800 Etec. I put some cast pistons in it last fall, around 3500 miles; same compression height as the stockers (I measured) but with a 0.6mm base gasket (0.7mm was stock). Never checked squish, just ran it. Ran great (better than ever). Put close to 2000 miles on last year. 

 

This fall, checked compression. 90psi PTO, 75 MAG. Cheap gauge, but 15psi difference is not going to fly. Crank runout was 0.0045" PTO, never checked MAG. I picked up a trued crank, and set of RKTek Wossner's. I put the bottom end together earlier today, top end this afternoon with another 0.6mm gasket.

 

Start checking things. Crank, nice & true (0.001" or less both ends), turns nicely. Good so far. Checked piston projection (as per BRP manual): 0.084" measured. Doo specs 0.072-0.076". Hm, that's going to be a little tight. Squish is at 0.025". I knew that was tight. Jump on here, people are saying 0.040 is safe, 0.030 is tight. No one talked about anything tighter. Why is it so tight? Wossner's too tall? No. Different crank, yes, but an OEM reman, seems unlikely the rods are longer. Same cylinder, same case. Just how tight was it last year? When I pulled last years pistons, I noticed a small shiny spot on the exhaust side, right at the piston edge. Figured it was a touch lean. Not quite; there's a matching spot in the head. Piston was kissing head!

8037.jpeg

20171209_215247.jpg

Yikes.. 

8038.jpeg

 

Check your squish when you're changing things, even if it's just 0.1mm on the base gasket. Doo may not have built your's as loose as the internet will lead you to believe. 

 

BTW, no evidence of detonation and this thing really ran well last year. So tight is good, but not this tight!

 



#2 jdrmx

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Posted 10 December 2017 - 11:04 AM

Id run the oem base gasket, that will gain you .005". Has your head been planed?


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#3 GoinBoardin

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Posted 10 December 2017 - 01:07 PM

Head has not been planed. Yeah, the 0.7mm will open things up a little; enough to prevent witness marks. I'll run that or an 0.8mm, I have both sitting on the bench. Hard to use OEM piston projection specs with aftermarket pistons..more measuring is in order. Thanks for the input.

 

I might put the OEM piston back in with 0.6mm gasket, measure squish & piston projection just because I've never seen someone measure both & I'm curious how tight this was straight from Doo. 

 

The wildcard is port timing. While I might have gotten a tight squish straight from Doo, does that then mean I have poor port timing for top end power? Don't know..not an engine builder.  



#4 Ski-B

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Posted 10 December 2017 - 04:28 PM

No  .010" diff in vertical height won't make an diff on port timing.

 

If you raised the cylinder .5mm to 1mm then you would see a diff in port timing.

 

My guess is stock pistons with .7mm gskt won't project above the cylinder like the aftermarket pistons. Factory pistons with .7 mm gskt should be in the .040"-.045" squish. With stock head and cylinder. Have been pretty consistent from what i have seen and measured.


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#5 GoinBoardin

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Posted 10 December 2017 - 04:56 PM

After careful measurement and a little parts swapping I have some findings. All measurements were done in triplicate. I may not have Mitutoyo instruments but I did use a dial indicator where applicable. Cylinder bore checked with snap gauge & vernier caliper. 

 

I used 1mm rosin core solder for squish checks. The squish measurements with solder match measuring piston deck height & cylinder head relief measurements (measure how far above/below the deck of the piston edge is, and add/subtract from combustion chamber depth at outer most edge). This solder is very soft; I do not think there is any spring back. Solder was placed atop the piston, spanning the pistons full width, directly over the wrist pin. The head was torqued down, and piston was used to crush solder by turning it over by hand 4x. 

 

BRP, Sno-X, and Wossners all have the same amount of crown, or height difference, from the pistons edge to the crown top. The piston edge is difficult to measure. I came up with 0.080" for that measurement, however, this is the most error prone measurement in my opinion.

 

BRP & Sno-X measure out the same in terms of total piston height & piston compression height. 

 

Wossner is 0.001" shorter in total piston height, but most interesting is the 0.005" taller compression height. This is why I have 0.025" squish when using this pistons in conjunction with a 0.6mm base gasket. Kelsey mentions making his pistons slightly taller on his website, this is what he's talking about. 

 

Stock piston mounted in my engine, with a 0.6mm gasket, gives 0.030" squish and 0.078" piston projection. Using the factory selected 0.7mm gasket that was originally in this engine, squish would open up to 0.034" and piston projection would drop to 0.074". Dead center of the BRP specified 0.072-0.076" piston projection. 

 

What about piston to cylinder clearance? Somewhat unrelated to squish, but interesting nonetheless. BRP specs piston to cylinder clearance as 0.0053-0.0063" new; wear limit is 0.0079". With 3500 miles on my stock pistons, they were at 0.0075". 2000 miles on Sno-X pistons, 0.0080". Stockers weren't quite done, Sno-X not really up to the job in my opinion.. The fresh Wossners measure 3.2205" in a measured 3.226" bore for a 0.0055" clearance. I'm happy with that. These measurements are done on the skirts of the pistons, and bore checked just above exhaust port (as per BRP manual).  

 

Okay so back to the piston kissing the head & gasket selection. Based on measurements, I know the Sno-X pistons were at 0.030" squish all last season. They weigh 447g with rings. Stock this motor was at 0.034" squish with a 473g piston with no piston to head contact. One difference in this comparison is crank bearing wear--and I know the bearings were getting worn because the MAG rod end was starting to turn blue (pictured). Loose cylinder clearance certainly wasn't helping things either.

20171112_193355.jpg

 

Stock 0.7mm base gasket would put the lighter (415g with rings) RKTek Wossner in the same place as the heavier (447g with rings) Sno-X with a 0.6mm gasket in terms of squish & piston projection. I bet with the lighter piston, correct cylinder clearance, & fresh crank bearings, there would be no piston to head contact. I'm sure Kelsey intentionally wanted to close up the squish band. Considering my engine was dead center on BRP piston projection specifications, I don't think I have some fluke build that slipped by BRP with extra tight squish. More likely, I think, is that BRP & the aftermarket are wise to the advantages of tight squish in battling detonation. I do know it is more complicated than just squish.. 

 

I hope I'm not stepping on toes here: anyone can measure parts. The Wossners are very nice pistons. The take home for me is that you need to be careful. If the pistons hit the head any more than mine already did, the ring lands would likely be crushed on the exhaust side to the point of sticking the rings. This is not easy on rod bearings either (may explain blueing on my older crank?). 

 

Now do I run 0.8mm or 0.7mm gasket? A phone call to the piston maker is in order I think. 


Edited by GoinBoardin, 10 December 2017 - 05:01 PM.


#6 jdrmx

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Posted 10 December 2017 - 05:03 PM

Without measuring anything, there no reason a replacement Wossner piston wont work with a factory base gasket and a uncut head. Unless the piston is made wrong, wich is highly unlikly.


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#7 jdrmx

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Posted 10 December 2017 - 05:09 PM

Ive used every brand of piston and never measured anything, (not that its not right), with unmodified parts, and slapped them in without issue. Now if I am raising compression and porting, then its different. Piston wall clearance has alway been the critical measurement when replacemnt pistons are installed. To replate or to not replate is the criticle part. If your piston is contacting the head with stock replacement pieces somthing isnt right.


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#8 GoinBoardin

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Posted 10 December 2017 - 05:09 PM

No  .010" diff in vertical height won't make an diff on port timing.

 

If you raised the cylinder .5mm to 1mm then you would see a diff in port timing.

 

My guess is stock pistons with .7mm gskt won't project above the cylinder like the aftermarket pistons. Factory pistons with .7 mm gskt should be in the .040"-.045" squish. With stock head and cylinder. Have been pretty consistent from what i have seen and measured.

Exactly how do you measure squish? 



#9 GoinBoardin

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Posted 10 December 2017 - 05:13 PM

Ive used every brand of piston and never measured anything, (not that its not right), with unmodified parts, and slapped them in without issue. Now if I am raising compression and porting, then its different. Piston wall clearance has alway been the critical measurement when replacemnt pistons are installed. To replate or to not replate is the criticle part. If your piston is contacting the head with stock replacement pieces somthing isnt right.


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I reduced base gasket thickness by 0.1mm and used Sno-X pistons. They measure identical to BRP pistons. However, the skirts had collapsed, enabling them to rock more in the bore, and tighten up exhaust side squish just enough to barely kiss the head. 

 

I agree completely, you don't want contact. That's why I'm boring you all with long posts & excessive detail.. thanks for the input. 



#10 jdrmx

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Posted 10 December 2017 - 05:20 PM

I know its not cheap but i wonder if you should send the cylinder out for replate. When you measured the wall clearance did you measure from the area around the ports? This is the most worn area.


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#11 GoinBoardin

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Posted 10 December 2017 - 05:40 PM

Just rechecked the bores. At the ports, top, bottom, side to side, front to back, all within 0.0005" of each other. So it measures fine and the plating is in good shape. The excessive piston to cylinder clearance on the old pistons is because they are worn out (skirts collapsed). 

 

I was in MN a few weeks ago, went to Sled Head Racing to buy the trued crank, Wossner's, gasket set, and get his opinion on my cylinder. He could have sold me a cylinder but instead cleaned & inspected it and said it looks good. He ran a hone through it for me, gave me assembly & tune tips, then sent me on my way. 


Edited by GoinBoardin, 10 December 2017 - 05:40 PM.


#12 Ski-B

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Posted 10 December 2017 - 06:59 PM

Exactly how do you measure squish? 

I pull the plugs out. Cut a piece of rosin core solder. Bent it in an L shape. Insert through plug at bottom dead center, feel edge of cylinder wall inline with wrist pin, pull up on solder to touch head and roll the primary clutch over TDC once and then roll it back over TDC. To get two taps on the solder. I find the tightest squish point on All stock motors is...just back from the edge of solder. If i measure right out the the edge? There is a small burn from between cylinder wall and piston that is not crushed. So the tightest point seems to be back slightly with calipers.

 

I wonder if your solder is moving on you... pulling it over four times? 


2020 Freeride 850. My ride.

2018 XRS 850 Sons sled.

2017 Freeride 860R etec. 860BB. Daughters sled.

2015 Freeride 800R etec. All Stock. Wifes sled.

2015 BCX  800R etec. Trail ported. Spare sled.

2014 BCX 800R etec. All stock. Spare sled.

1998 Formula S 380. She is a keeper.


#13 GoinBoardin

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Posted 10 December 2017 - 07:21 PM

I pull the plugs out. Cut a piece of rosin core solder. Bent it in an L shape. Insert through plug at bottom dead center, feel edge of cylinder wall inline with wrist pin, pull up on solder to touch head and roll the primary clutch over TDC once and then roll it back over TDC. To get two taps on the solder. I find the tightest squish point on All stock motors is...just back from the edge of solder. If i measure right out the the edge? There is a small burn from between cylinder wall and piston that is not crushed. So the tightest point seems to be back slightly with calipers.

 

I wonder if your solder is moving on you... pulling it over four times? 

Having solder on only one side will inflate the squish measurement as the piston rocks in the bore (side with solder won't come up in the bore as far). I bet that explains the 0.040" squish. 0.040" squish would mean a 0.068" piston projection (on my sled), which is 0.006" off of center of Doo spec. Considering an average piston to cylinder wall clearance is likely around 0.006 or so, there can be some piston rocking. Doo doesn't spec squish in the shop manual, likely because it can be problematic to accurately measure, while piston projection is pretty easy. Piston crown shape/height is likely held to a tight tolerance during manufacturing; so measuring piston projection as a means to determine if squish is acceptable likely works well in practice. 

 

My engine is on the bench. With the head off and piston in the hole ~1/4", I cut a piece of solder to span the bore. Install & torque head. Turn the crank about 15 degrees about TDC to crush the solder. Using 1mm (0.039") solder, if it is crushed at all then the squish is tighter than 1mm.



#14 sledsrockcolorado

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Posted 10 December 2017 - 07:26 PM

.0055 with Wossners 😲

#15 GoinBoardin

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Posted 10 December 2017 - 07:43 PM

.0055 with Wossners

You saying too tight with a forged piston?






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