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PostPosted: Fri Aug 24, 2018 5:58 am 
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To the on ramp

Joined: Fri Aug 11, 2017 7:09 am
Posts: 260
Country: england
Bikes owned: several gt250 ramairs
i've not seen this done for many years, but one way of tuning 2 stroke engines was to file a trapeze shape out of the top of the piston, level with the transfer ports. deeper at the outside and tapering up to nothing. this supposedly opened the transfer ports earlier and closed them later. reason i ask i twofold. 1. has anybody ever run an engine with these cut-outs? i've done 2 pistons for a gt250 but never ran them, so can't comment. 2. when the piston on the gt350 is at bdc the top of the piston is about 2mm above the bottom of the [barrel] transfer port. sort of half open stable door syndrome again :) any thoughts, ideas? easy enough to do, but don't fancy wrecking 2 ts185 pistons just to try something. last time i saw this done was on an old t500 ;) in about 1977.
cheers, dd.

_________________
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.


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PostPosted: Fri Aug 24, 2018 6:44 am 
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Road race school

Joined: Tue Dec 16, 2008 11:37 am
Posts: 898
Location: Trowbridge UK
Country: UK
Bikes owned: T500R, SV1000S, TS125, Seeley T500
Funny you should mention an "old T500" dollydog. I was doing this to my race pistons back in the day, it allows you to experiment with pushing the porting with only a new pair of pistons to replace if it does not work. I only lowered the side of the piston by 1mm.

In an article by Stan Stevens in Classic Motorcycle Mechanics this process was mention wrt the Yam TY250 trials bike. Yamaha had done this from the factory with the lowered cut outs actually part of the piston casting. Easier to do it with pistons than re-port barrels.

Cheers Geoff


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PostPosted: Fri Aug 24, 2018 7:04 am 
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To the on ramp

Joined: Fri Aug 11, 2017 7:09 am
Posts: 260
Country: england
Bikes owned: several gt250 ramairs
thanks geoff :D and being as the barrels cost me about £400 with the new liners and the pistons were £59 a pair, including post from japan, it does seem the cheapest way of going about it. the only thing the last article i read on it [clymer manual] was to stay 2mm away from the top ring land to avoid this breaking off. i've got some work to do on the pistons anyway, such as radiusing where the transfer ports meet the pistons, rounding the corners off and radiusing the skirts to [supposedly] cut through the oil on the liner better and throw more up onto the little end and underneath the top of the piston. it's all good fun :D
cheers, dd.

_________________
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.


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PostPosted: Fri Aug 24, 2018 10:10 am 
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Road race school

Joined: Tue Dec 16, 2008 11:37 am
Posts: 898
Location: Trowbridge UK
Country: UK
Bikes owned: T500R, SV1000S, TS125, Seeley T500
Hi DD,

I would suggest that if you are looking to get the best from your special GT350 engine to keep it std. However spend some time cleaning up the transfers tunnels, matching the c/cases to the transfer opening in the base of the barrel, cutting the base gasket so it does not intrude into the passages and matching the cut-out in the piston to match the barrel etc. In effect blue printing the barrels and cases. I did this this to my TS125 when I rebuilt the engine, no porting dimensions were changed. Even with my fat arse on-board this engine pulls easily to the red line in 6th, the throttle response from 6000rpm to the red line is hooligan inducing :twisted: .

Cheers Geoff


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PostPosted: Sat Aug 25, 2018 9:11 pm 
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Around the block

Joined: Tue Jul 03, 2018 4:32 pm
Posts: 76
Country: Usa
Bikes owned: '74 GT250
Vito's uses this idea with the yamaha banshee and blaster cutting a notch on the exhaust side of the piston to effectively change the exhaust duration. As an inexpensive way of testing exhaust/transfer durations its fine But I don't recommend running pistons like this long term. I'm sure you know the purpose of the squish band around the outer portion of the combustion chamber to prevent detonation? The tight clearance forces the fuel to the center of the combustion chamber helping in good burn but it also prevents the fuel from igniting on the outer edges of the chamber preventing detonation.
Its my belief that having these notches in the piston 1. hinder the squish band from effectively forcing all the fuel to the center of the chamber and probably causes a loss of performance but more importantly 2. could allow fuel to ignite in these areas causing detonation which as we know can cause catastrophic engine damage.

These are just my observations and they could be unwarranted but we engine builders can deduce from the current port timing and port area among other things if a raise of the transfers is warranted and can use a right angle pencil grinder to carefully raise the transfer roof a bit at a time until we have it where we need it.

A fast and inexpensive mod I used to do when I was a kid was to use a few extra base gaskets and get my uncle to shave the difference from the top of the cylinder with the milling machine effectively raising all the ports while maintaining or sometimes even adding a little compression. Its not the right way to do things but it is a fast mod that will usually result in a good boost in horsepower.


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