SQUISH measurement at TDC 1962 Arrow

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Trev Sellars
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SQUISH measurement at TDC 1962 Arrow

Post by Trev Sellars »

Hi, I am just going to change over from low compression heads to high compression ones.
The "squish on the old heads just removed was over 2mm, seems high, no wonder it was a low compression! Mind you, it had 2 head gaskets fitted.
Can anyone advise on what the gap at TDC between piston and head should be, "The squish". Suzuki GT250 Pistons fitted.
I am not looking for max power, just a compromise really of decent performance and reliability. I was thinking an average of around 1.2mm would be o.k. (0.047)" but am open to more suggestions.
I will profile the squish band on the High compression heads, to match the shape of the piston, hopefully, this will aid the burn process.
Thanks for your help.
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Re: SQUISH measurement at TDC 1962 Arrow

Post by Simon.Gardiner »

Trev - have you done a search of the 2-stroke section? 'suzuki' will bring up some refs to using GT250 pistons.
From what I can see some people 'adjust' things, some people don't (but you might also need to check that the piston skirts will clear the flywheels.)

If you've not found them already Brian H's comments here (and again on page 4 of the thread) might be useful
viewtopic.php?f=20&t=8604&hilit=suzuki&start=10
And there's quite a discussion in this one:
viewtopic.php?f=20&t=7579&p=51739&hilit=suzuki#p51739

(Dunno if any of that is enough help.)

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Trev Sellars
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Re: SQUISH measurement at TDC 1962 Arrow

Post by Trev Sellars »

Phew, the 2nd topic has a lot of detailed information.
Looks as if I am on the right track at least and the min squish would appear to be 0.040", so my Rough calculation of 0.047 would appear to be about the right side of disaster! I'll report back and let you know.
Thanks again for your help.
Interestingly I have found 2 types of High compression heads, see pics attached. The squish band on one is of a much larger dia, to aid flow of gases to the plug. Are they both original heads, or has the bottom one been skimmed?
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ariel hc head.jpg
ariel cyl head 2.jpg
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grahamhibbs
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Re: SQUISH measurement at TDC 1962 Arrow

Post by grahamhibbs »

Looks to me like the bottom one has been skimmed,
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Re: SQUISH measurement at TDC 1962 Arrow

Post by bh.sprint »

Hi,

After talking to Ron from Fahron Eng. he suggested 40 thou as a minimum, keeping in mind the Suzuli pistons have a steeper dome angle. I checked those on my Leader and they were just over 50 thou, so I reduced it to 46-47 ish, the bike does run well. On the Arrow I used 40 thou and there did seem to be a slight increase in liveliness from the larger gap of 48 ish thou . Brian H.
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Re: SQUISH measurement at TDC 1962 Arrow

Post by Trev Sellars »

Hi and thanks for your advice.
Graham, how many Ariels does a man need?

Opening up the gap will make it rev a bit more, up to a point, so I will try and settle at 0.47 ish. No doubt e5 fuel will be the order of the day, due to the higher compression and a bigger main jet too. Currently on a 130 main on a concentric 622.
Did find a nasty surprise lurking in the R/H barrel. I wondered why the R/H plug was oiled up more than the L/H one.
Managed to hone most it out and the piston + ring have been cleaned up. I have raced with worse.
All going back together tonight, so here's hoping! The previous owner had the barrels re conditioned but the port windows have not been let in, (edges not radiused), so I think some sharp bits may of lodged in the wall of the piston. Shame really as he did a lovely job on the bike and it has only done 2500 miles since total rebuild. It did need a de coke anyway, so nothing really lost. The piston looks clean because I had just scraped it clean!
The joys of a 2 stroke!
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right barrel (2).JPG
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Re: SQUISH measurement at TDC 1962 Arrow

Post by ian williams »

i remember from when i was younger when japanese 2 strokes were tuned in abundance that to get a more power we raised the exhaust port 1mm .
remember doing my mates rd250 one day and it went very well with a pair of microns added as well as exhaust ports raised by 1mm

i have a yamaha 175 i did myself back then but i went even higher 2mm with the exhaust port and it ended up being all top end power and revs , it is like a sack of spuds low down until it picks up and then tears off like mad with front wheel in air .

i have an ariel racer with a 58 leader donor engine that was tuned back in the early 1970s and is pretty much done the same as the comprehensive tuning article featured in the 1965? motorcycle mechanics magazine feature , along with expansion chambers made to a design drawing that there was a drawing kicking around for .
it has high comp heads on it and engine runs nicely low down and really goes like stink when revved and only has a 626 concentric carb on it .
i tried a bigger carb but it wasnt having it .
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Re: SQUISH measurement at TDC 1962 Arrow

Post by Roger James »

Don't forget Ariel did produce two different High compression heads from 1964, one with a narrower squish band for the smaller diameter Arrow 200 piston.
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