78 Fork Damper Rods

I have acquired two new (ish) damper rods for the forks on my 78 390 Auto build as I had one damaged one.
My question is that I was going to use just one to replace the damaged one. I measured the new ones today, and found they are slightly different.

The original 390WR ones are 271mm long, the new ones are 274mm long.

The originals have 4 oil holes. Three down one side and one at 180 deg opposite the middle hole.
These holes start at 6.47mm dia at the top, middle is 6mm dia, and the lower one is 5.5mm dia. The one on the other side is 5.95mm dia.

The new ones only have the three down one side. All at 5.98mm dia.

What were these new ones fitted to ? A CR or WR. How much difference in damping will there be ?

I guess I am going to have to use the matching pair, and I can enlarge the top hole to 6.5mm and drill the one on the opposite side. But I cannot make the lower hole smaller.

These all have the M8 thread in the bottom. I do have one odd one in my spares box that has an M10 in the bottom.

Any help would be nice

Not sure about the auto's, but WR's usually got last years suspension. So you may have 77 damping rods.

I agree. There were very small length and damping changes between 77 & 78. My advice? Use the SHINY ONES!!

The shiny ones probably work the best Looking at the parts book for 77, they used 15 12 303-01, then for 78 & 79 they used 15 12 343-01
I would imagine that the smaller holes (and only 3 of them) would stiffen the damping for a given weight of oil. It would also alter the damping curve.
I will fit them as they are, and see what it feels like....... or should I open them out and drill the extra hole ..... or better still, I'll go to the pub and think about it.

Don't start drilling till you've tried them, it is not always easy to undrill a hole!

I was thinking that Steve. I am going for option 3 at the moment ..... Pub
Life would be so much quicker if I had the fork tubes back from the platers ...... 5 weeks so far

Auto models always seemed to get the older stuff on everything but the engine updates. That is how it did not rise in price very much after the price drop in 77 or 78