Piston options for 186cc reed valve cylinder?

[bigmuff]

Admiral - you need to find out what the compression height is for the RD 350 piston. Then you can work out what rod length / crank / cylinder machining best suits your intended set-up and pocket.

RD350 piston has 36mm compression height.

[corrado]

According to Grampian the RD350 piston is 28mm from top of gudgeon pin hole to edge of piston crown, dome height = 3.5 mm.
Mitaka TS185E for Suzuki conversion is 25mm.
Standard Lambretta piston = 31mm. Dome height on standard piston looks more than 3.5mm so presume that you'd need to reprofile a std head to suit the RD piston?

Don't treat those measurements as gospel, I could be lying.

[corrado]

Admiral - you need to find out what the compression height is for the RD 350 piston. Then you can work out what rod length / crank / cylinder machining best suits your intended set-up and pocket.

RD350 piston has 36mm compression height.

That would confirm what Grampian list it as ... 28mm + 8mm [half of gudgeon pin measurement] = 36mm.

[soullad]

Ok, so matched with a standard 107mm rod you'll be 3mm below a standard cylinder top, with a 110 rod you'll be level and with a 115 you'll be 5 mm over, or 6mmover with a tv length 116 rod.

N.B for a 110 rod, you'll need the , about £45 from ebay,

[ArKaTxU]

Ok, but this is ok with a TV barrel?
Thanks guys!

[bigmuff]

I dont think it matters what barrel you use. A std lambretta has a combined con rod length and piston compression height of 146mm (107mm conrod + 39mm compresion height). Std TV 175 is 116mm + 30mm = 146mm. This is the measurement you need to get if you dont want to take anything off bottom of barrel. So a 110mm conrod and a 36mm compression height piston (RD350) = 146mm. Simples.
I think the exhaust and transfer port timings remain the same. Inlet timing would be controlled by the reed valve.

[Muppet]

Better with a Tv 175 barrel than using any bored out 150 barrel; shorten the barrel and use a standard length rod; or stroke it out to 60mm and shorten by approx 2mm, if you reed valve it make the inlet timing as big as possible and port back through the base of the barrel through the case below the inlet; also cut the barrel wall back here to give an extra feed 20-25mm wide removing half the length of the spigot as reed valves are a restriction if they can't breath well,

muppet.

[guygrrr]

if you reed valve it make the inlet timing as big as possible and port back through the base of the barrel through the case below the inlet; also cut the barrel wall back here to give an extra feed 20-25mm wide removing half the length of the spigot as reed valves are a restriction if they can't breath well,

muppet.

That's pretty interesting. Do you have any photos?

[ArKaTxU]

Thank you very much for all the info guys!

[jaffa1]

Hi, Iam planning to do a similar conversion using a brand new S.R. 185 barrell (110mm),and M.B. reed inlet but using the Yamaha Y.P.V.S.350 piston.This too has the required 64mm bore, a 16 mm gudgeon pin,66mm skirt length, and 36 mm crown height.I was hoping to use a crank with a 115mm Rotax rod,(cos i've got one in excellent condition and will give good service).Having taken in all of the advice on here, am I correct in thinking that I will need to pack the base of the barrell by 5mm to use this rod? Rod is 8mm longer than standard, but compression height of piston is 3mm less than standard, leaving 5mm outstanding.Is it worth the hassle or should I just go with the Suzuki piston(tried and tested)Any advice would be greatly appreciated, thanks.