Stock & O/S Pistons RD400

[Milan]

Have you found the stock pistons with different parts numbers to be the same for the RD400:

1A1-11630

versus

2R9-11630

USA versus UK

?

[Milan]

Parts books list these std pistons:
3J7-11631
2R9-11631
1A1-11631

Rings:
3J7-11601
1A1-11610

Same for oversize ones with different suffixes, same prefixes tho

Does anyone know if these are then interchangeable using the 2R9 heads?

What about the 2V0 pistons 2V0-11631

( all numbers came out of various downloaded parts lists )

[scully]

The 1A1 pistons have a notch on the exhaust side of skirt. The later models 2vo 2r9 and 3j7 do not have the notch.
I believe otherwise, interchangeable.

[Milan]

So the 1A1's with the notch will work.

I have a parts list, that at the end also lists 1A1 in the repair kit section.
Any idea what the notch is for


M

[scully]

In theory, to bleed off some crank case pressure to help with surging.

[SoCal250]

Not sure about 3J7 and 2R9 parts but there are differences between 1A1 and 2V0 as mentioned. Also 1A1 & 2V0 rings are different (and partially interchangable).

1976-78 RD400 (1A1) pistons have a small cutout at the bottom of the skirt on the exhaust side for emissions reasons that actually let exhaust gasses into the crankcase at TDC. The Daytona (2V0) did not have this cutout because it used vacuum-controlled external exhaust butterfly valves to accomplish the same result. (credit: Ed Erlenbach)

Note on 400 rings:
1A1-11610-xx-xx are for 1A1 pistons. 1A1-11601-xx-xx ringsets are for 2V0 pistons.  2V0 pistons use two identical 1.2mm rings that are the same as the 1A1 piston top ring, and no ring expander on the bottom ring. (credit: Chuck Quenzler)

[Milan]

Attached is a shot of the 400E parts.

Lists 2r9 pistons for part 4-27

Lists 1A1-11610 rings for part 4-28

So 2R9 pistons use the 1A1-11610 rings according to this

Another book shows 3J7 pistons using the 3J7-11601 rings.

So do the 3J7 rings work on the 2R9 or 1A1 pistons



I have a piston kit that came with 2R9 pistons and 1A1-11601 rings not the 11610?
( so these rings may not be the right ones )

Another 3J7 piston kit came with 3J7 rings.

M