250-500cc Squish numbers on a 360 with KTM rod kit

[Darrel78]

Using the KTM 360 rod and assembling the top end on my rebuild. I placed the piston in the cylinder with the head installed and recorded a baseline number for the piston resting against the cylinder head. I then installed the piston on the rod with no rings or base gasket and installed the cylinder/head assembly and recorded a clearance of 0.003". This seems good news as the piston doesn't strike the cylinder head. I ended up using two base gaskets and after torquing the cylinder to specs I ended up with a total clearance number of 0.022". Let's see how well this works!

 

[justintendo]

 

[2premo]

Using the KTM 360 rod and assembling the top end on my rebuild. I placed the piston in the cylinder with the head installed and recorded a baseline number for the piston resting against the cylinder head. I then installed the piston on the rod with no rings or base gasket and installed the cylinder/head assembly and recorded a clearance of 0.003". This seems good news as the piston doesn't strike the cylinder head. I ended up using two base gaskets and after torquing the cylinder to specs I ended up with a total clearance number of 0.022". Let's see how well this works!

that might be a bit balsy, they,, being the maker recommend almost 5 times more
.022 vs. .102
figure any play and the piston can move a tick more, say the bearings loosen up about .005 and you add heat expansion, leaves very little room for error
i personally would double it to be sure

 

[justintendo]

that might be a bit balsy, they,, being the maker recommend almost 5 times more
.022 vs. .102
figure any play and the piston can move a tick more, say the bearings loosen up about .005 and you add heat expansion, leaves very little room for error
i personally would double it to be sure

or being being revved to max rpm...sometimes its about more than just whether it hits or not. theres heat and pressure going on. .022 is really tight

 

[justintendo]

that might be a bit balsy, they,, being the maker recommend almost 5 times more
.022 vs. .102
figure any play and the piston can move a tick more, say the bearings loosen up about .005 and you add heat expansion, leaves very little room for error
i personally would double it to be sure

they actually recommend even more than that, after the operation is done it says to add another .5mm there at the end...

 

[Darrel78]

Reading through that operation certainly suggest a large specification. I used two of the 0.5 mm gaskets. This should come in at 1mm or about 0.040". Mine went to 0.022 after torque was applied. Re-reading the procedure would seem to suggest we want to end up with a bit less than 1/8 " (~0.112" or so). I wonder perhaps the difference in a Wossner piston verses a stock piston with respect to piston crown radius and angles? The top edge of this piston doesn't quite cover up the wear markings at the cylinder/piston junction, it is slightly lower in the bore than the previous piston. The old piston had a "H" like the Husky logo cast inside the crown (bottom side) so I'm assuming it was a stock unit. I completed the rebuild and fired the bike up; 2nd kick. I've two heat cycles on it. My plans are to give it another couple of heat cycles and then take it for a short ride. It may be interesting to pull the head off in a bit and check for scavenging patterns on the piston head. I'll also check the compression numbers to see where we are there. Doesn't seem any more difficult to kick than before.
Thanks again for all the help and information on this forum; you guys ROCK!!

 

[Motosportz]

Have your head cut so it is right. Not something to mess with.

 

[Darrel78]

Thanks, Kelly; I never even considered this option. I'll compare the two piston head radius to determine if there is a difference and then simply match my head radius to the current piston setup. Do you suppose we are dealing with a constant radius for this application? I've a radius cutting attachment for my lathe so this may be a straightforward task.

 

[Motosportz]

http://twostrokeperformance.com.au/measuring-squish-clearance/

yes I believe it is a constant radius but I would be real careful to get it right. Do some research. Should not be hard but needs to be right.

 

[juicypips]

There is no point of a squish clearance being 2.6mm the band isnt even working.
I halved mine and thats still playing way safe in my opinion,yes heat expands piston cylinder and head sooo it should all be same clearance when hot.

Yes you should see scavenging with that squish clearance you also may incur pre ignition pinking, this can hole pistons in no time.
Looking forward to your pictures and experiance.
Thats my stock piston @50 thou ~ 1.27 mm

 

[justintendo]

There is no point of a squish clearance being 2.6mm the band isnt even working.
I halved mine and thats still playing way safe in my opinion,yes heat expands piston cylinder and head sooo it should all be same clearance when hot.

Yes you should see scavenging with that squish clearance you also may incur pre ignition pinking, this can hole pistons in no time.
Looking forward to your pictures and experiance.View attachment 55569
Thats my stock piston @50 thou ~ 1.27 mm

perhaps the extra clearance gives longer life? isnt this 360 seizing or wearing its rings really quickly?

 

[juicypips]

No i seized the 93 cylinder that had a cast liner the 50thou is on the 2002 nikasil liner and original piston, rings have done like 100 odd hours its all good tho.

Have new piston for 93 now but will install that when i get the time, with a tad more clearance

 

[justintendo]

i see, didnt know if that was still going on. not sure i have the guts to go quite as low as you have but i think i could lose a gasket. i bet the 360 can be a real animal if blueprinted

 

[juicypips]

Its a beast at the moment the extra squish with lectron is awesome the lowdown grunt mixed with top end that just digs in on dirt is flipping fantastic, get 70+mph on the 02 cylinder in sixth with 15/48 i dont want to go back to lower fromt sprockets cus the front end never stays down.
In the gnarr 2nd is perfect. Rarely get her out of fourth on track its just wicked fun to ride.
Ive said it before an i will say it again thanks kelly lectron truly is worth the money.

 

[justintendo]

yes the lectron is a nice piece of kit, as you blokes like to say.

 

[Darrel78]

This is good to hear, just today ordered my lectron from Kelly!

 

[Darrel78]

Juicypips, would you happen to have a picture of the cylinder head that goes along with the well scavenged piston picture?

 

[Darrel78]

Well, pulled the head off tonight to check the squish numbers via the solder crush method. After the piston wouldn't even touch solder with 0.093 diameter I realize I mis-read my calipers. ( pictures show my initial reading was 1.130 while the final reading was 1.251 so B-A is then 0.121 and not the 0.022 as originally reported. I offer my sincere apologies for my mistake.
Compressing a 0.05mm gasket seems to yield about 0.010 or so. I've two of these on my bike and it seems I could go with one to be in the squish ball park. Again, my apologies for the mistake. This was a very interesting exercise.

 

[Darrel78]

Forgot to add my compression pressure comes in at 150psi using an automotive compression gauge.

 

[juicypips]

You can see the scavenging minorly on your piston already, sounds dam close to my 1.27mm you could go more but im happy where i am. What oil mix you running looks clean sorry if youve posted already.
No pics of head but i can whip it off and post some up, snapped clutch cable on enduro on sunday 6th lap in so bikes gunna be out of action for a while...