1975 Rd350 Cafe Rebuild

[RNC28]

Hey guys, new to this forum. Currently restoring a cafe'd out '75 rd350, the overall goal for this build was to have a well set up, more than average power Rd. Cylinders were mismatched and ovaled out so i had them bored out to 65.5mm, Ports chamfered, Prox Banshee pistons with the tang cut, Cylinder top milled to fit 400 headgaskets. Problem is that is sounds real slappy near the top end after the rebuild. Been told the piston clearance was set to 0.0025in. It's running 32:1 premix, dual stage reeds, 400 carbs and uni foams, squish at 0.060in ish. Any advice would be highly appreciated.

Video's too large too attach - https://photos.app.goo.gl/Lx5ZAyiATWfm2tft5

 

[irk miller]

At what point in running it are you hearing the piston slap? Is the engine at least 180 degrees?

 

[RNC28]

Right after start up and after it’s warmed up as well, don’t think it gets quieter once warm. I did find a good amount of side to side play in the big end of the rod on the left side looks like the bearing is good (no up/down play)but I can fit a 0.6mm feeler between the rod side and web, right side is at 0.3mm.

 

[teazer]

are the pistons hitting the head gasket by any chance? That sounds horrible.

 

[RNC28]

are the pistons hitting the head gasket by any chance? That sounds horrible.

Don’t think it’s hitting the head gasket, measured squish with them on at 0.060in, even added a base gasket to give it a bit more room at the top, still the same noise. I’m really not sure what the noise is, I’ve rebuild 3-4 of these engines and never had a problem

 

[Luugo86]

That lash on the connecting rod seems like a huge amount of play for an engine that revs so high. I'm not an expert on these 2-stroke engines like many others here but that would be my starting point. Do the piston skirts have any scoring on them ?

 

[Rider52]

Measure deflection at the little end of the rods. The recommended side deflection specs are 0.36 to 0.98 mm. A huge range but when you take into account you are moving the little end to check deflection maybe not so big!

You can also measure the distance between the conrod and the thrust washers and the tolerance is 0.45mm (min) to 0.95mm (max).

Referencing your comments about about the piston clear being 0.0025in, I would check to make sure. A whole lot of slapping noises are piston related. Are your pistons showing any scoring?

 

[RNC28]

Measure deflection at the little end of the rods. The recommended side deflection specs are 0.36 to 0.98 mm. A huge range but when you take into account you are moving the little end to check deflection maybe not so big!

You can also measure the distance between the conrod and the thrust washers and the tolerance is 0.45mm (min) to 0.95mm (max).

Referencing your comments about about the piston clear being 0.0025in, I would check to make sure. A whole lot of slapping noises are piston related. Are your pistons showing any scoring?

Thanks for the info, will measure small end play (doesn’t feel bad at all though), the Conrod to thrust was averaging 0.65mm on the left and 0.28mm on the right. Cylinders were scored above the intake and exhaust after a couple min of running, not bad enough to catch a nail though. The Prox ring gaps were a bit larger than oem spec, but I’ve got no way to make them smaller and I’ve read that it’s kinda normal on oversize pistons. Also heard that dual stage reeds make more noise, any way that could add to the noise? Thanks y’all

 

[teazer]

I cannot see from that photo but are the ports appropriately chamfered?

 

[irk miller]

Looks to me like you're rubbing above your ports.
You have to account for the piston swelling. Doesn't look like the ports are chamfered, either.

 

[RNC28]

Yup, I realized that the machine shop didn't chamfer the upper lip of the ports so I had to go around with a circular file and clean up every port.