1. 2 Stroke Husqvarna Motorcycles Made In Austria - About 2014 & Newer
    TE = 2st Enduro & TC = 2st Cross

TE/TC All About Setting the Squish Clearance

Discussion in '2st' started by KXcam22, Dec 19, 2017.

  1. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    Always lots of head miss-information around. Milling the head on a 2 stroke is an easy way to get more snap in the power band and improve runability, and doesn't increase compression by much. I have done this on almost every 2 stroke I have owned. Contrary to popular opinion, it is not done to increase compression ratio but to improve combustion efficiency by tightening up the squish band. The purpose of the squish band is to force the combustible mixture into the center of the head chamber. The mixture that remains in the squish band gap is cooled by the proximity of the head and piston and does not ignite. Depending on the gap, this “lost” mixture can be as much as 15% of the overall intake charge but usually ranges around 10%. In my case it was 11.3% (calculated) lost charge that did not burn. This represents lost HP. Reduced squish also contributes to improved tolerance to jetting. It is likely that the increased combustion efficiency allows the engine to happily burn less ideal A/F mixtures.
    squsih band.png


    Ideally the squish band should be almost zero, but with the wide variance in tolerances in mass produced motors they can't get there. Why? The piston, rod and cylinder all have production tolerances in length, say +/-0.010" each. If you got a Monday bike the piston could be as much as +0.030" closer to the head than expected. If the factory squish was set at 0.029" then boom. Plus the rod stretches a bit at high RPM so they make the clearance far too big from the factory to be safe. An additional factor is the rod big end bearing clearance. For example, my KX500 had a std 0.010” bearing clearance, meaning the rod could be wiggled up and down about 0.010”. This added an additional 0.005” to the rod stretch value. Not something you would expect and hard to measure. Husky/KTM is also one of the few manufactures that offer different base gasket thickness, which means that once the squish is set that it is importance to always use the same thickness base gasket. Ideally this is set before hand with the proper setup having the piston align with the bottom of the exhaust port at BDC and the top of the cylinder at TDC. My new 300 was closed enough from the factory to not mess with.
    If the squish clearance is too large it can sometimes burn and then cause something similar to detonation. It also contributes a bit to pipe bang as the squish mixture goes out the exhaust port and likely contributes somewhat to spooge as well. Most guys with a modified head report a noticeable decrease in pipe bang and spooge. In an ideal world, or if I was still racing, I would set the squish clearance to the bare minimum and then, if required, do some bowl work to increase the volume slightly to keep the compression high but within an acceptable range to run your desired fuel. It takes a lot of compression before top end rev is effected.
    An aftermarket head may be better for some, depending on you desire and willingness to mod your bike, but something like 75% of the gain comes from setting the squish band to a more optimum number. The remainder comes from reshaping the bowl or other cool things like the RK Tek head has. In my case I am happy spending $50 to get the 75%. I am suspicious of aftermarket heads or head modifying companies that don’t ask for a squish measurement. It is a necessary requirement to prepare a proper head. The advantage of doing it yourself is that it then becomes a recognized clearance when rebuilding, like setting the X and Z dimensions.
    Typical squish clearances are:
    50-80cc 0.6-0.8mm
    100-125 0.7-0.8mm
    175-250 1.0-1.4mm
    300-500 1.1-1.5mm
    (note: values are per Bell 2 stroke Performance handbook)
    These values are from 1973 but I have used them for years and found them to be very good.

    Measuring Squish
    Squish can be measured in 2 common ways, using a micrometer and the engine dimensions, or by assembling the head with a compressible media on the piston crown and turning the engine over. Common media is modelling clay but resin-core solder it typically used as it is the easiest to measure, and easier for the engine to compress. Solder glued to the piston crown with grease to keep it in place with a small loop in the solder to keep it from rotating. It is important to cut the solder ends with a knife so that the ends are sharp and blunt. Both ends must touch the cylinder wall from L to R. Front to back doesn’t work since the piston will rock on the piston pin and give you odd results. Due to the taper of the squish band it is these ends that give us the measurement, so we want them to be exact and accurate. I use 2 diameters of solder, a before and after, to reduce the amount of stress on the engine. It sucks to buy solder just for measuring squish but I was able to beg some for free from an auto repair shop, since you need less than 6” of each size. The engine must be turned over using the nut on the flywheel and not the kickstarter. When milling the head the final surface must be Oring finish level. A milling machine is best but it can also be done on a lathe with some final lapping of the Oring surface on a piece of glass with 400gr. Not necessary for this application but the ultimate is a final lapping the head to the cylinder with valve grinding paste, but that is messy and only done with the engine apart.
    Squish Solder.JPG



    The stock squish clearance on my 2017 300 was 0.0785", measured with 0.110” solder. I milled off a safe 0.028" to leave me a 0.050" squish clearance which is 1.3mm. My actual squish ended up as 1.27mm, measured with 0.060” solder. My compression ratio increased from 13.2 to 13.9, which is not much of an increase. On my 200 EXC the squish is set at .89mm.

    I used to do the same head mod with my twin cylinder 2-stroke road racers. On one bike, I had to mill 0.120" off the head and toss the head gasket. That is huge, but the final engine ran nice at 11,500 RPM. In that case the huge squish was an anti-pollution thing as a tight squish speeds up combustion which creates NoX. It might even be a reason why the Euro inspired KTM motors come with such a large squish clearance. More than anyone wanted to know I am sure. Hopefully this info helps somebody. Cam.
  2. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    So at 1.27mm squish and 1100ft asl i only have 180 psi. Engine has 2.4 hours. I was expecting more but hopefully it increase a bit when it is fully broken in. My 200 has a solid 200 psi with the same guage. Cam
  3. Motosportz CH Sponsor

    Location:
    Vancouver WA
    Husqvarna Motorcycle:
    2021 TE250i, 570 Berg, 500 KTM, 790R
    Other Motorcycles:
    many
    Very interesting, thanks for taking the time.
  4. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    Turns out it was 180 with the muffler plug in. Doh! Will have to retest. Cam.
    Cosmokenney and silverstreakNZ like this.
  5. demi Husqvarna
    AA Class

    Location:
    Cheyenne, WYO
    Husqvarna Motorcycle:
    12 WR 300
    Other Motorcycles:
    '14 SH 500, 81 Gilera 125 C1, 17 KTM
    no TMs now???
  6. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    Sorry, whats a TM? Cam.
  7. demi Husqvarna
    AA Class

    Location:
    Cheyenne, WYO
    Husqvarna Motorcycle:
    12 WR 300
    Other Motorcycles:
    '14 SH 500, 81 Gilera 125 C1, 17 KTM
    um, maybe you are being funny. Guess you have moved on from them? Not to many Cam's from Kamloops, BC on moto boards with nice bikes.
  8. John Bunker Husqvarna
    A Class

    Location:
    Mid Michigan
    Husqvarna Motorcycle:
    TE150 2017
    Other Motorcycles:
    XR250 Honda 1985
    Hi Cam, good thread you started.

    2 stroke head design has too many variables and I think should be considered a black art, your case in point, “Squish Thickness.”

    From my understanding the squish band design’s main roll is charge velocity but its design can change greatly from road racings to off road enduro riding. Band width is narrower when road racing compared to the wider band width of off road bikes. The other variables to consider are parallel band surface to the piston or up to 3 degree difference, the minimum distant (squish thickness) at the piston OD, the radius size at the blend to the dome, the cc of the head, and of course the never ending different dome sizes and designs.

    I’m attempting to try my hand at machining some different head designs for my TE150. I’ll turn them from 7075 but noticed that the SX insert looks to be made out of brass or aluminum-bronze (Ampco) so might make a few out of that material as well.

    Also, on the newer KTM-Husky the combustion chamber insert inlay should be about .002 proud of the outer cylinder head.




    Input from other members on this subject would be greatly appreciated.
  9. Picklito Husqvarna
    Pro Class

    Location:
    Washington
    Husqvarna Motorcycle:
    430, 430, 430, ,400, 175
    Other Motorcycles:
    KTM250xc, KTM500mx, KTM440exc
    KXcam22,
    Very nice information. Thanks for taking the time to put it on the forum. I'm also a squish band guy, and I get frustrated by all the misinformation out there.

    John Bunker,
    Not sure if we've discussed this (I think we have) but I flat sanded my 150xcw head for a squish of .040" and then .036". When I had the insert and shell in my hands, I realized that a slight clamping distance would be necessary. I had to flat sand the shell a bit after going to .036" on the insert. Good to see you mention this.
    moto_surfer likes this.
  10. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    You are right about the different variables in a head. The only thing I didn't like about the 300 head was that the squish band to inner chamber transition has too much radius. It creates an area that is not quite squish band and not quite inner chamber. Better to have a more defined edge, but that can't be created without a whole bunch of work. Main chamber was OK and typical of most bikes. My plug inserts into the head about 1/2 thread too much. I have never worried excessively about squish velocity and never had a problem, even on my high reving road racers. What I am hoping to point out is that anyone can correct their squish band without too much difficulty and get some modest engine gains from it. Picklito, I can't believe you sanded yours down. You get the superman award!!! Can you explain how you flat sanded yours. I might take mine down another 0.005". I looked in some old notes and used to wrap 3 smaller ID solders together in a spiral to measure large squish gaps. Beats buying a couple sizes of solder. Cam.
  11. hhdwtmtw He who dies with the most toys wins...

    Location:
    Melbourne, Australia
    Husqvarna Motorcycle:
    '14 TE300, '11 TE449, '10 TE250
    Other Motorcycles:
    Royal Enfield Continental GT 650
    I can vouch for Dave at Two Stroke Performance here in Oz (http://twostrokeperformance.com.au).
    I had my TE300 head machined for the correct squish band and combustion chamber profile. The result was 30% better fuel range and a much smoother engine.
  12. Picklito Husqvarna
    Pro Class

    Location:
    Washington
    Husqvarna Motorcycle:
    430, 430, 430, ,400, 175
    Other Motorcycles:
    KTM250xc, KTM500mx, KTM440exc
    . I have a 3" thick granite flat plate that makes flat sanding very easy. Took it slowly and mic'd the perimeter to make sure it stayed square. It did! Started at .048", test ran at .040", then another sand and test at .036. Planning to try .032.
    moto_surfer likes this.
  13. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    What grit did you use? Cam.
  14. Picklito Husqvarna
    Pro Class

    Location:
    Washington
    Husqvarna Motorcycle:
    430, 430, 430, ,400, 175
    Other Motorcycles:
    KTM250xc, KTM500mx, KTM440exc
    Started with 80. Finished with 220. Thought the 220 would still be a little too "open" but the head surface was very nice so I left it. I'll see if I still have the pics.
  15. Picklito Husqvarna
    Pro Class

    Location:
    Washington
    Husqvarna Motorcycle:
    430, 430, 430, ,400, 175
    Other Motorcycles:
    KTM250xc, KTM500mx, KTM440exc
    Here's the head at 220 grit finish. Looks rough in the pic! But it was very nice for real.

    IMG_7980.jpg
    John Bunker likes this.
  16. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    That looks really nice! I may have to try that. My 200 is at 0.035" and runs perfect on 94 pump gas. I would think your 150 should be able run the 0.032" easily. I was sitting on one in the shop today. Nice bike and so light. Cam.

    ps just looked it up and on my 2cyl 350cc road racer I was running 0.022" squish, that's on a 175cc cylinder up to 11,500 rpm.
    John Bunker likes this.
  17. Picklito Husqvarna
    Pro Class

    Location:
    Washington
    Husqvarna Motorcycle:
    430, 430, 430, ,400, 175
    Other Motorcycles:
    KTM250xc, KTM500mx, KTM440exc
    On the current 125/150 it's pretty low risk cuz a replacement insert is only ~$30.
  18. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    Thats cheap. For $30 you can test a few. You can also put a thin shim base gasket in. 0.005" wont effect port height much. As an alternative to sanding a guy could also use a lathe file. Basically a green giant version of a normal file. A buddy used to deck 350 block with on and it did a good job. Cam
  19. KXcam22 Husqvarna
    AA Class

    Location:
    Kamloops, BC, Canada
    Husqvarna Motorcycle:
    2011 SM630, 2017 300XC
    Other Motorcycles:
    08FZR6;07CRF450;98CBR900RR;02KTM200
    Retest: With a squish clearance of 1.27mm and without the muffler plug my compression was 197psi, cold at 1100 ft. That's more like it. I expected close to 200 psi since the squish clearance could be tighter. Cam.
  20. John Bunker Husqvarna
    A Class

    Location:
    Mid Michigan
    Husqvarna Motorcycle:
    TE150 2017
    Other Motorcycles:
    XR250 Honda 1985

    .05 seems like a lot, is this reading before you shaved the head?