Hey, RD400 guys...

[85RZwade]

Thanks guys, we're closing in on this. It sounds like we need to identify his cylinder head; it should be a C, but who knows? If it is a C, Chuck, would you recommend using the stock C cylinders with it or the F cylinders pictured above?
He's a very experienced, skilled rider close to the century mark. The bike will be street ridden, not raced and has stock carbs and airbox (except, I imagine, jetting) and DG pipes. I don't know what ignition he has, so I assume it's OEM.

[SUPERTUNE]

F cylinders will bolt on and work with C,D, & E heads. Just need to make sure you port and raise the exhaust port to help lower the compression ratio. The F cylinders can be a little harder to kick start as they don't have compression bleed holes above the exhaust port. The F models did have a different kick starter gear ratio to help the F model starting.
The hickup will be fitting pipes up. Look at the DG pipe flanges and see if slotted or round hole?
F cylinder has a wider exhaust stud spacing. (look in my JL pipe mod for spec's)
The exhaust flange is also a slightly different angle and will need some pipe hanger work to bolt on the frame sub mounts in the rear.
Chuck

[85RZwade]

Thank you for the information, Chuck. You are a remarkable resource for this community and a generous person. I see why people speak highly of you