Rotax 122 - Measuring Squish?

FlubbaBubba · Borekit Bruiser Joined: 31 Dec 2014 Karma :

Can anybody tell me how to measure the squish on a stock rotax 122 engine?

Its got a new mitaka piston in an old mitaka cylinder and head which has around 4000miles on it.

Can i use solder to measure the squish and my manual says its supposed to have 1.5mm squish on it.

Does it have to be 1.5mm or just in the ball park?

Any help would be much appreciated, thanks.

Nobby the Bastard · Harley Gaydar Joined: 16 Aug 2013 Karma :

Squish band, you mean.

Not squish
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Pete. · Super Spammer Joined: 22 Aug 2006 Karma :

Rolled up plasticine on the piston crown. Properly done it's called 'claying'.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy

redeem ouzzer · World Chat Champion Joined: 06 Oct 2015 Karma :

You have to build it up and then compress the solder, then strip it to retrieve the solder ring so you can vernier it. Arsehole but only way to tell. No less then 0.8mm for pump fuel unless you like fucked Pistons from detonation.

I'm not sure what Nobby is on about but then again I'm not sure how familiar he is with tuning 2t's.
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Pete. · Super Spammer Joined: 22 Aug 2006 Karma :

I should think that 0.8mm squish would put the piston very close to contact with the head at full revs.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy

Hahadumball · World Chat Champion Joined: 07 Oct 2015 Karma :

Pete. · Super Spammer Joined: 22 Aug 2006 Karma :

1mm is the recommended minimum piston to head clearance for a steel conrod.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy

MarJay · But it's British! Joined: 15 Sep 2003 Karma :

I measured squish on my 350 YPVS by having a twisted up long piece of solder, poking it through the plughole until it touched the side of the cylinder, then turning the engine over. I did this four times on each cylinder, at each corner of the cylinder. I then used a vernier to measure the crushed solder.

I have a feeling on that bike it was about 3mm or so, and could have done with being reduced a bit.
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Old Git Racing · World Chat Champion Joined: 08 Aug 2009 Karma :

As Marjay says, solder down the plug hole is the easiest way. I wouldn't be going less than 1mm on a road bike engine, they just aren't durable enough without constant rebuilding of the crank, and fuel isn't good enough to prevent detonation. Even 1mm would be pushing it in my view.
My race only TZ could be built with varying squish gaps by changing the base gasket thickness. Running at .9 - 1mm was good for reliability and ok power, crank rebuild every 1000 miles. Running at .6 - .7mm gave more power, crank life down to 700 miles and pistons 250 miles instead of 300.

OGR.
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