Help regarding cylinders / new pistons rd400

[rd400greenhorn]

So, got these 2 set of cylinders measured. Fresh bore but can i get pistons to fit?

1 set measures 64,90 mm
1 set measures 66,10 mm

Anyone?
Or do I need to rebore and hone 🤷‍♂️

[Yamaha 179]

Wossner sells RZ pistons sized 64.19mm and 66.44mm.  I don't know about Wiseco but I think they sell 65.00mm pistons also.
Lyn Garland

[rd400greenhorn]

Thanks.
So would they fit, i Mean wirth tolerances?

[Dvsrd]

64.90 must be a worn out 64.75 nominal bore. No direct fit piston to my knowledge.
66.10 must be a somewhat worn 66.00 nominal bore. 0.10 or more in clearance may be too much.
If it is possible to bore to 66.25 or higher, and pistons are available, it may still have some life left.
Have you measured at several locations, and fore-aft as well as side to side?

[rodneya]

Where did you measure the bores, and what did you use to measure?

[Alain2]

You will need rebore.

Why were the cylinders freshly bored to these seizes?

[SoCal250]

^^ what Alain2 said

The normal procedure is to acquire the pistons, and then provide them to the shop performing the boring so the cylinders can be bored/honed to the correct clearances for the new pistons.

[rd400greenhorn]

Well you dont really remove materiel with a hone and the cylinders came with pistons in, gone now, so thought maybe i can judt find new pistons and be done with it.
Unfortunately not. They were measured with digital gauge, at s pro workshop.

[hayduke]

My dusty memory is 66mm is the max overbore on RD400's, Wiseco offers a 66mm piston, which may be about right for your 66.1 cylinders. The 64.9mm cylinders are probably too loose for a 64.75mm piston and will probably need to be bored to 65.25, as a 65mm bore may not get all the cylinder wall

2¢

[Alain2]

My dusty memory is 66mm is the max overbore on RD400's, Wiseco offers a 66mm piston, which may be about right for your 66.1 cylinders.

A 66.10mm bore is already too big for a 66mm Wiseco piston.

[1976RD400C]

Wiseco Pro Lite RZ350 pistons have a 66.25 and 66.50 size. They work well in a RD400.