Scooterotica

Just built an engine & on the first start I could hear a rattle at tickover picking up as I revved it.
The noise is coming from the barrel area. Engine spec is standard cast jet 200 new re-bore to match a 67.0mm asso piston.......60mil crank with standard base gasket + thick type(jet) head gasket.....20mm carb.....standard exhaust. The exhaust port is 45mm wide with u bend modified to suit. Took the barrel of & turned the engine over by hand via the flywheel & there is no noise or anything unusual. On checking the ring gap I can get a 0.4mm feeler gauge (16thou) in & piston to bore a 0.2 feeler(8 thou) in.
Could this cause 'piston slap' & hopefully be rectified with a bigger piston/re-bore?
What I did notice when I pulled the piston out the barrel there was a fair coating of petrol/oil still on the surfaces.

all depends wher you place the feeler gauge higher up the piston bigger the clearance lower down its tighter,

Muppet,

Put the feeler in with piston at tdc position getting 0.2 mm. The ring I tried at the top of the barrel 0.4 mm & the bottom getting the same reading.

mesure at the piston skirt just below the pin bosses put piston in barrel from the top but upside down and from the bottom piston right way up and mesure away from ports; 0.2 big but would be right for the tapered top of the piston to pot; put one in mine last year mesured 0.08 for piston;the rings were 0.3,

Muppet,

You said it was a rebore? Do you know who did it and when it was done? A feeler gauge is going to be rather inaccurate. If you're concerned about the clearance though, i would take the piston and barrel to a shop to have it checked. Is it possible that it's the cylinder cowling, flywheel cowling, or something like that rattling?

This is by no means 100% fool proof, but i've been boring my own barrels out recently with a lathe that i bought. Still on the learning curve. I have one suzuki 190 running, but i've messed a few up and over shot the clearance twice. In my experience, even with cheap knock off pistons, once you get to 0.008" clearance or more, your ring end gap starts getting far too big, since the two are related. I would think that with a decent piston, if your end gap were 0.4mm, then the bore shouldn't be that far out, otherwise with a good 0.002" clearance the rings might overlap. One of my barrels ended up tapered, with a little over 0.008"(don't remember exactly how far out it was) bore clearance at the top. The ring gap was up and over 1mm oops

Again, it doesn't tell you for certain. If it's in question, get it checked.

4 thou will rattle a fair bit. 8 thou is going to be very noisy.

Just tried an Indian standard barrel I had spare........same noise. If this wasn't a new build (with the barrel now taken out of the equation)it sounds like a big end shot. Going to have a look inside the chaincase this weekend.

Checked under chain case & everything fine.
Took off the st4 barrel & noticed at BDC the piston skirt has marked the crankcase mouth. This must be the source of what I thought was 'piston slap'.
At BDC the piston is just touching the crankcase mouth. Two things...What modifications need to be done to sort this mods to the piston or casing.Or just fit a standard crank .

Is this a usual modification for a 60 stroke crank or does it just depend on the individual engine casing?
I've already had to machine out the crankcase area because the con rod was just marking the casing.

eibarlad wrote:Is this a usual modification for a 60 stroke crank or does it just depend on the individual engine casing?
I've already had to machine out the crankcase area because the con rod was just marking the casing.

Have heard of this before with 60mm cranks, what kind of main casing is it?