RZ/RD 350 & Misc. 2-Stroke Tech BBS

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PostPosted: Wed May 16, 2018 3:44 pm 
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Hi
After torquing down my freshly rebuilt top end, I found the squish'es to be 1.3 and 1.8mm.

1. is it normal that the squish'es varies that much between the cylinders?
2.How do I best go about setting squish to ~0.8 (is what I think it should be?) when I have to compensate somehow for these differences?

Thanks for reading

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PostPosted: Wed May 16, 2018 5:27 pm 
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That is quite a large variance between cylinders. Base gaskets can reduce the squish but you then have to worry about the cylinder head volumes and increased compression. The ports will also change height if you are using significant thickness. Excess compression is generally corrected by machining the domes. Not saying you have a warped head but it is something to at least take a look for. The tolerance between all the various parts is something close to a mm thus why the squish was set so wide. At the tightest end you had a close to perfect squish without damaging the motors.


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PostPosted: Wed May 16, 2018 7:22 pm 
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By dropping one of the gaskets to 0.25 the squishes would at least be equal, but I have no idea if that is significant for compression. Will have investigate som more.
But still I need to shave the head another 0.5 from 1.3 to achieve 0.8. For ome of the cylinders that will ne a significant change of 0.75 total. This was all easier on my single cyl Husqy 2 stroke enduro ;-)

Another thought; could the piston crowns be 0.5 mm off each other? A swap would tell I guess....

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PostPosted: Wed May 16, 2018 7:27 pm 
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always check cylinders have the same height.

you say you checked squish after the engine was completly rebuilt ? so you just stuck one bend soldering iron in ? that to can lead to misreading : depending on where you stick it, the piston can flip back/forth giving different readings.
best is to measure at 4 points at the same time, at 45° to the piston pin.
this is one of the big advantages of O-ringed heads : you tape 2 sodering's on the piston, torque down the head, measure and open it up again . With a normal headgaskett this becomes expensive :smt003 . Then use 2 soldering's at 180° back and front.

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Last edited by JanBros on Thu May 17, 2018 7:04 pm, edited 1 time in total.

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PostPosted: Thu May 17, 2018 3:56 am 
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Yes JanBros you are right, torqued it down straight after rebuild. I will take you advice on how to measure.
About the head gaskets- can they not be reused after torqueing even if the engine has not been run yet? (I am thinking not heated from running yet)?

Also some guys are talking about splitting the 3 layers of head gaskets. Do or don't ?

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PostPosted: Thu May 17, 2018 6:33 am 
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Evan is on it. If you get the squish( once evened out) down to ideal, so 0,8 on a good crank, compression will be way too high untill you machine the domes. Easier on a central plug single.
Brings much easier.
Heck I'd strongly consider getting it flat and running standard.

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PostPosted: Thu May 17, 2018 11:33 am 
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If the gasket has not been heated I reuse them. I've even reused old gaskets by using cleaning surfaces and gasket and applying Permatex 80697 Copper Spray-A-Gasket. If you choose to do that your on your own I disavow any poor results. However it did work for me for thousands of miles on my RZ350 with a used OEM gasket.


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PostPosted: Thu May 17, 2018 1:02 pm 
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Always check the squish with a cross and measure at all 4 points. make sure oyu are going right to the edge as well since squish is measured a fraction from the edge of the cylinder.

re-using gaskets: if you did a rebuild, you have an old head gasket. Use that while you are checking the squish and tightening head. I have tightened and re-torqued a head gasket without issue but of course you are increasing chance of a failure.

Personally I would not separate a head gasket but it is totally up to you. Think you increase chance of a leak.

I would not expect a new piston manufacture to be out on the pin location by a large portion of a mm but feel free to swap pistons and let us know.

If everything is stock and installed properly, your motor will run fine as it sits. Stock squish on 2-strokes was generally mid 1-2mm due to the safety margin. These bikes were still fun and ran well.

If you decide you want it optimal you should: cc your head, shim the base gaskets or shave the cylinders or head. Machine head if compression/head volume found too low after prior steps.


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PostPosted: Fri May 18, 2018 3:58 am 
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Thanks for all your highly appreciated input. Again, alot of details and stuff to consider. Also will stock up on some spare gaskets just in case. Sometimes I think people are wrong when saying "two-strokes are so simple"!. There is really alot to be aware of.

I am now leaning towards running as is through the summer, and do the optimization steps during winter. I am also really curious to how a stock setup will compare to an optimized squish motor in the end.

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PostPosted: Fri May 18, 2018 4:02 am 
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Everyone is talking about head mods. Rod length is another issue, along with weight. Who matched the crank parts, checked them? Many other factors to consider before you start modding heads.
People often over think things. Two strokes are tough to tune, but you have to try and keep things simple.

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PostPosted: Fri May 18, 2018 2:22 pm 
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Compared to an 16 valve head they really are quite simple. no cam timing, grinding valves or shiming cams.

Rod length i believe is one of the items considered 'variable' as well as piston pin position resulting in the wide squish.

I would second your decision to ride it and deal with the mods during the winter. You are looking at a small marginal improvement with corrected squish vs. not riding.


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PostPosted: Fri May 18, 2018 8:38 pm 
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minimal squish isn't only about more performance, it has as much to do with better piston cooling (or better : less heating up).

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PostPosted: Fri May 18, 2018 10:09 pm 
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I certainly agree everything should run better with a blueprinted motor but if the question is get it correct vs. riding I would choose the latter. Still find it a bit weird they are so much different but sure yamaha had a tolerance somewhere?


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PostPosted: Fri May 18, 2018 10:22 pm 
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30 odd years, could easily be someone swapped out a barrel , first pair skimmed to try cure blown gasket. Replacement barrel not skimmed. Maybe. On my aircooled Stinger resto the only engine mod I did was to skim squish down to 0.8 then open chambers till they were the same as stock. No more load on the old engine but better cooling and less waste gas.

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PostPosted: Sat May 19, 2018 3:32 pm 
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.8 wow, I'm racing 2 rz350's so interesting stuff, how wide is the band? I know mine I'd 9mm wide and I'm at .050 on the squish, I refuse to seize 2 strokes any more


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