A few questions/advice needed - piston clearances

[servetakid]

I plan on borring my 200cc Spanish cylinder to 225 and having the ports reworked. But I am trying to get my head round bore/piston clearences, I have a piston that is 70.2mm. After looking in stickies guide, it recommends that you start with a clearance of 0.075 for tuned cylinders. So would it be a safe bet boring the cylinder out to 70.3?

The piston I have is a GPM one, are these any good? Or should I avoid?


Cheers, :fb:

[Adam_Winstone]

GPM pistons are not good for anything more than a mild road tune and don't widen the exhaust port much or you will break rings. I managed to buy a job lot of these pistons many years ago and have done many, many thousands of miles on 175s and 225s fitted with these, however, the rings are no better than the original Borgo items (early GPM are worse) and will break if you give them reason to do so.

TBH these pistons are really only good for little more than a straight rebore and 2mm higher on the exhaust port + the short inlet side will increase your inlet timing anyway, with no widening to inlet or exhaust. This will result in a 'standard' motor feel/ride but with the ability to pull 4.8 2-up, all day, every day with a clubman and sensible size carb.

If you want to tune the barrel then you are better looking to change to a decent thin ring conversion piston.

Adam

PS - Get the tuning wrong and you will be pushing it home.... I have

[tavspeed]

hi,i have what i think is a servetta jet 200 barell, piston and head,are there any marks,notches etc on the barrell or head that will confirm it is spanish,cheers all.

[servetakid]

GPM pistons are not good for anything more than a mild road tune and don't widen the exhaust port much or you will break rings. I managed to buy a job lot of these pistons many years ago and have done many, many thousands of miles on 175s and 225s fitted with these, however, the rings are no better than the original Borgo items (early GPM are worse) and will break if you give them reason to do so.

TBH these pistons are really only good for little more than a straight rebore and 2mm higher on the exhaust port + the short inlet side will increase your inlet timing anyway, with no widening to inlet or exhaust. This will result in a 'standard' motor feel/ride but with the ability to pull 4.8 2-up, all day, every day with a clubman and sensible size carb.

If you want to tune the barrel then you are better looking to change to a decent thin ring conversion piston.

Adam

PS - Get the tuning wrong and you will be pushing it home.... I have

I may avoid the GPM piston if that is the case, I plan on using an NK exhaust and 30mm carb. So I am not sure if it'll be to the job. Any advice on the piston/bore clearances?

[tavspeed]

dont take this as gospel but .075 is 7 1/2 thou so should give you 3 3/4 thou each side of the piston,3 thou per side is the ideal after a rebore,so .075 on the diameter sounds about right for a tuned cylinder.

[soulsurfer]

dont take this as gospel but .075 is 7 1/2 thou so should give you 3 3/4 thou each side of the piston,3 thou per side is the ideal after a rebore,so .075 on the diameter sounds about right for a tuned cylinder.

I think it's 0.075mm which is about 3 thousandths of an inch in my calculations

[servetakid]

I think I'll probably use an asso piston as they have a good reputation. So if I use a 70mm piston, I need to bore the cylinder to 70.075mm or would it be safer to go with 70.1mm?

[Avantone]

I think I'll probably use an asso piston as they have a good reputation. So if I use a 70mm piston, I need to bore the cylinder to 70.075mm or would it be safer to go with 70.1mm?

You don't need to complicate it. When you take the piston and cylinder to the machine shop, tell them what clearance you want and they'll do the rest.

I've personally run at 2 thou @ 70mm with no issues. 3 seems popular, but anymore just feels slappy.

[Terrible Taff]

Reboring of cylinders is subject to many options being thought through!

It is normal to have the cylinder bored close to the piston size, then port edges chamfered then honed to it's finished size ie; that of the nominal bore size of the piston being used.
This means a piston size of 70.2mm should require the cylinder bore to be 70.2mm also.
The clearance is within the cam grinding shape of the piston, this is normally measured approximatly 10mm up from the base of the skirt using a micrometer & a bore gauge.

In the past I have used 0.001" clearance per 1" of bore.
So 70.2mm / 25.4 = 2.76" x 0.001" = 0.00276" ~ Close as damm it to 0.003" overall clearance between the piston skirt & the cylinder bore surface.

Now when tuning up your scooter, the engine's bore clearance will vary subject to many things!
The engine's compression ratioyou intend to use, the exhaust sytem (Clubman, PM, JL, NK, Taffspeed, MB Devtour etc) the type of piston (Asso, GPM, Meteor, Honda, Yamaha etc) the fuel & oil ratio used (95 unleaded or 98 Supergreen)!

Remember that the engines final opperating TEMPERATURE will determin if the engine has the correct clearance.
If too tight the piston will nip up with the tell tail marks showing on the 4 main bosses around the gudeon area.

Please note that the quality of the cast iron used in different cylinders varies a great deal.
The Indian cylinder is one of the hardest & gives good wear resistance, pity that some after market linered alloy cylinders use a softer grade cast iron & wear out quicker.

Best regards

Ian Frankland
Taffspeed
07794 003276

[servetakid]

Thanks for all the advice. I know its a bit like how long is a piece of string with these things...

However, I am having some second thoughts about doing all the work. I am very tempted to get a rapido 225 race kit. This seems to be what I will be creating in the end. Can any one point le to a thread with peoples set ups etc with this kit. I have searched but can't find the kind of info I need as works like race are filtered out...