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rt 225 piston choice

Need help with a tuning kit, how do you tune your scooter, which kit should I choose, and all general tuning and modifcations questions are for in here.

Re: rt 225 piston choice

Postby rossclark » Wed Sep 11, 2019 4:22 pm

Adam_Winstone wrote:^... Hi guys,

Thanks for the replies.

I'm using one of the 72mm pistons (39mm compression height), which I am about to machine the head for, and have noted just how 'out' the AFR head's squish band profile is. Whilst I'm aware that these are not a great match for the standard dome profile, I'm surprised to see just how far out they are!

I machined a few of these previously, always reprofiling the squish to suit whatever piston I've used, but I'm wondering just what I can get away with as the high capacity means that I'll be increasing the combustion chamber volume to deliver the compression ratio that I'm after... and the more I remove to reprofile = the more that I then need to work out of the bowl.

If this is the standard profile piston then there must be a lot of people out there with the profile being a mile out... yet I don't read lots of threads about it, nor hear lots of people talking about running issues.

Cheers,

Adam


How do you go about checking the profile?
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Re: rt 225 piston choice

Postby Adam_Winstone » Wed Sep 11, 2019 7:59 pm

Each to their own, and I've no doubt that others use more accurate methods, but my 'Heath Robinson' way is to cut a cardboard piston profile and then place it centrally across the bore so that I can see how it marries up to the squish profile. As in my current case, there is clearly a mismatch of the 2 profiles, as when the edge of cardboard template is placed against the edges of the squish profile of the head, the centre of the piston dome is several mm away from the inner edge, where the squish band meets the combustion chamber. This is a 'divergent squish band' scenario:

https://www.scootering.com/scootering-c ... r-top-end/

I can conclude from this simple examination that my 2 profiles are miles out, that it will be impossible to get a uniform squish clearance and that my assembly would benefit from machining the squish profile of the head to match piston crown profile.

Adam
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Re: rt 225 piston choice

Postby rossclark » Wed Sep 11, 2019 8:26 pm

Thanks for that Adam, so you're aiming for a more parallel squish band?
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Re: rt 225 piston choice

Postby Adam_Winstone » Wed Sep 11, 2019 9:36 pm

Yes.

There is plenty written about the pros and cons for each scenario (illustrated in the link that I posted) but parallel is the usual target. If parallel and of an appropriate squish clearance, from piston crown then the squish functions as it should, keeping combustion charge centred in the combustion chamber, and stops combustion from happening in the squish band region, which stops 'end gases' from igniting and heating the very edge of the piston crown, so allowing it to run cooler.

There's plenty of decent info available on line, much more than I could do justice to in my layman's terms, so feel free to google it if you want more info. Jennings 2-Stroke Tuning is a good source of info.

Adam
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