Wanted HD8 dashpot assembly

(Include at least one picture)

Picture is borrowed from ebay

Description: looking for HD8 dashpot and piston matched set. My 420 rear assembly has to have 90w oil while the front has 30 wt motor oil. I’ve replaced damper with no increase of resistance, so the oil got me through the season. Now that winter is here I’d like to correct this problem. I think the rod the damper fits in is too big for it to function correctly.

Asking price (if selling):

Location:USA west of Chicago

Contact information: Private message

Cost of shipping (if selling):

Willing to ship worldwide?

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If your current assembly has the appropriate 5-6 second drop time - which is functionally much more important - then you’ll be better off fettling the damper piston. It’s apparently a couple of thou shy of optimum. Likely curable with a short piece of shrink tube though that might increase the diameter too much. Still, worth a shot. If your drop time is less then the problem isn’t the damper and sourcing a replacement matched set is appropriate, but before you buy you should verify drop time is within spec.

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How can the oily brass piston wear the steel tube it runs in? Very strange

Wondering if it was machined too large on the ID.

Has to be if changing the damper doesnt help lol. I’m fascinated you even found that.

I don’t think gear oil at 90w is that much thicker than 30w engine oil btw.


Oh, it most assuredly is, at least on this side of the pond.

The oil helped. It’s slightly heavier. Enough to balance the carbs off idle. Has anyone tried to swap the steel rods in the piston.

they’re cast in right. I would not want to try that. I’d rather have someone machine a new damper assembly and at that point you would be better off with a new unit if you’re lucky.

Seems like an unusual problem, one I’ve not seen before. Can you fill us in on some of the details? Is the drop time normal? I gather that you are trying to correct the rear piston from rising too quickly. Are you sure that it’s not a slow front piston due to wear? How about the springs? I know that you are a savvy mechanic and have probably already considered these things, but it’s what comes to my mind.

I had an issue with SU carb dampers several years ago. They of course only influence how quickly and evenly the dashpot piston rises, not how quickly it falls. In my case I had two pistons that were sluggish to rise on applying the throttle. The opposite of the OP’s issue. I cured the imbalance by taking off, iirc, about three thou of diameter from the brass plungers. Then the three pistons rose perfectly in sync. Discussed in this thread.

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Drop test good. Springs good. Just uneven rise from the damper. The damper assemblies replaced with no change. Oil weight helped. If I can I have a piston arriving soon. I’ll have to do drop test again. This is one nagging thing on my to do winter work list.