A silly hard top question.

[Dino in Reno]

The hard top on my 67.5 sounds hollow. Are they?
If so, would filling the empty space with expanding foam be worth it for sound and temp insuation?

[theunz]

Lot’s of different hardtop manufacturers made Datsun tops so I can’t say for sure about yours, but could you somehow measure the thickness of yours. I’ve always thought mine was single layer, but since it hasn’t been on the car in years I’m not a hundred percent sure. Even if yours is hollow how would you fill it? I think expanding foam would cause bulges. Glueing a soft lining to the underside would be a lot easier and probably just as effective.

[Bwk2000]

The hard top on my 67.5 sounds hollow. Are they?
If so, would filling the empty space with expanding foam be worth it for sound and temp insuation?

I’m with Mike on this. I think you would risk deforming or cracking the hardtop with expanding foam (if there is a void to be filled on your particular hardtop).

Probably get better noise reduction results by just replacing all the hardtop seals and ensuring the clamps are properly adjusted.

[Dino in Reno]

Thanks for the replies. There seems to be about 1” of space on the top of the top. I’ll do a test using plastic “to-go” boxes to see if the pressure builds up. The inner surface flexes to a push. Made me think it would add to road noise.
The top is a Custom Hardtops from Long Beach.

[fj20spl311]

Almost all the hardtops I have encountered are made of two pieces sandwiched together. I have cut a few up and they had some expanding foam, but not a lot in them.
I would be careful not to fill it too full.
Just a few strips of foam should dampen the flexing.

[FergO2k]

Custom Hardtops are made similar to other aftermarket hardtops.
There is an external skin ( "pebble-grained" ) and an inner skin, and they were bonded/glassed together then trimmed. The tops are all hollow.
Any single skin top is going to have noticeable ribs on the inside, one on fwd latch location, then one just aft of the door line, then probably one horizontal brace around the cockpit ledge. Without bracing, the single skin tops (which have acrylic/plexi back windows that are pressure sealed in) would be so flexible that the windows would pop out, and often.
Trying to remember when I refurb'd Stan's factory top, but I think it was single skin, had a brace at the door line, and a rod (similar to rear view mirror rod) that went top-bottom right down the middle of the rear window to keep the opening from flexing.
Some of the Dynamat / Sound Blaster products stuck on inner skin would be best bet for quieting it down.
Doesn't Alvin's have carpeting one the inner side?
sorry, don't mean to make this yet another WWAD (Wha Would Alvin Do?) kind of message! Hah!