coaster wrote: ↑Mon Jul 01, 2019 5:59 pm
I have already been down this route and fitted an Atomic Ultimate reed valve to my Avanti TT3 a couple or so years ago. I had to remove some finage but not too much and I cut windows in the piston and cut a 3mm D section from the bottom of the inlet side of the skirt using an old TS1 Matica piston as a template. I did however have a LOT of trouble with frame clearance as the Avanti barrel puts the manifold further out. I already had an R1 shock (195mm) and fitted offset cones but I could only just get the shock on. Ended up grinding away at the reed manifold until I had about 1mm clearance. Scooter ran like dream for a couple of hundred miles, did the Derby 150 on it but then I blew a hole in the piston due to an air leak on the ScootRs manifold in one of the bolt holes.
Right, I looked on Rons website and I too saw the TS1 pistons so duly ordered one. Ron then emailed me to ask if it was for my Avanti and to give him a ring which I did. He said the TS1 pistons would not work due to the compression height and I would need to use a standard Avanti piston but without cutting holes/windows. Ron said the hole would eaken the piston and cause the skirt to crack. He did say it would be ok to cut a 3mm D section out of the piston skirt. Ron is of the opinion that the windows are not necessary.
I did all that but I have to say it didn't run like it had previously with the windows
The only other thing I changed at the same time was fitting a 60/110 crank which I know would have affected the port timings but I think it more likely its the lack of piston windows. I WAS going to take the piston pout and cut the windows but then I blew hole in the new piston despite my belief that Id fixed the air leak (leak down test carried out)
I don't really undertand the concern about weakening the skirt tbh, the GT240 uses a TS250 piston modified by Richard Taylor which seems to be without such issues
Before the advent of 'special to tuned Lambretta' pistons came along complete with their 'special' price tag to suit, there was little option other than to look to the motorcycle, watercraft, snow-sleds & kart industry for selection of something that could be utilised.
That is still a very good route to consider, bearing in mind the quality & costs.
When there was not much option, then modifying pistons, such as the Suzuki TS250 by adding ports or cutouts proved to be reliable.
There are also options, as long as the correct ring material is available, to utilise pistons intended for iron or sleeved bores in nikasil cylinders & vice versa.
That's what I have done, as long as the clearance allows, several times. It also helps to have an account with a major two stroke piston supplier.....
Currently, I am building a TS1 70 mm bore with a Yamaha DT250 piston simply as a change to the Suzuki item. The compression height is 1.0 mm different, but the DT has a window on the inlet side. I have queried with the Gran Turismo 'team' as to why the DT piston was not selected rather than the TS but from what I gather, any questions as to the way things are done, is taken as a challenge or criticism.....