Radiator cap replacement for non stock aluminum radiator

[74Scooter]

So I was fitting my new fiberglass T/A hood today and ran into an issue I seriously did not expect at all. After 1k adjustments to the hinges and such the front would just not settle. Quite a few head scratches later I discovered it's the billet radiator cap. I ran to the auto store and picked up a Murray 16lb that is the replacement for the stock rad. Clearance issues went away and the hood sit's flat now. But the new rad cap just does not seem to sit well.

I pull it off and can see rings on both rubber flats where it is engaging the rad at the bottom of the spring and under the cap. On they Orielys site they reference "cap depth" which seem to range generally from 1-1/4 to 1-1/2. The caps on Summit don't reference a depth at all. Just got this heap back together and I would love to avoid spraying rad fluid all over... any radiator experts available to give fitment advise?

 

[Locomotion]

I was always under the impression that radiator caps were, for the most part, "standard", at least in the engagement area. But side by side comparisons and measurements won't be possible all the time in order to find the right one. As a last resort, you may be able to "slot" the radiator mounting holes 1/4"-1/2" to lower the radiator.

 

[74Scooter]

I was always under the impression that radiator caps were, for the most part, "standard", at least in the engagement area. But side by side comparisons and measurements won't be possible all the time in order to find the right one. As a last resort, you may be able to "slot" the radiator mounting holes 1/4"-1/2" to lower the radiator.

Yup.. I was already considering that before I test fitted a standard flat cap and got the clearance I needed from it. Honestly I'm not THAT attached to the billet cap and if I could get a good ole Stant replacement that fit & worked I'd be more than fine with it. One concern was the size of the engagement tangs on the replacement cap were quite a bit smaller than the original... see below. Hey.. everyone needs a lil tang right? lol. If I hold both up side by side the height seems correct. Guess I could spend $4 and fly out to Summit and get one of their generic 16lb caps to see if it sits the same.

 

[pishta]

Aftermarket AL rad is probably a Chevy spec neck. My Mazda takes a cap that is called out by Mazda to have the correct depth so it will vent properly to the overflow outlet and not vent out the cap. They are not all the same. I have a 67 spec Mustang Aftermarket AL rad in my '65 and they call out a Chevy cap too!

 

[74Scooter]

Aftermarket AL rad is probably a Chevy spec neck. My Mazda takes a cap that is called out by Mazda to have the correct depth so it will vent properly to the overflow outlet and not vent out the cap. They are not all the same. I have a 67 spec Mustang Aftermarket AL rad in my '65 and they call out a Chevy cap too!

Chebby... why not right? Last thing I thought I'd be spending any cycles on today is rad caps. Is there a spec? This is what I'm working with...

 

[pishta]

straight edge across top to the sealing flange inside where the rubber pressure ring would sit inside. That is your depth. see this and plug in your depth
Radiator and Expansion Tank Caps | Gates Corporation
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[74Scooter]

straight edge across top to the sealing flange inside where the rubber pressure ring would sit inside. That is your depth. see this and plug in your depth
Radiator and Expansion Tank Caps | Gates Corporation
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See that's the odd thing.... I drop the back side of my caliper in there and I get about .785.

When I click the gates link to the closest by depth.. the spec for the caps is far less than that.

 

[pishta]

They are on springs, and compress.....maybe the higher (depth) 2 would fit?

 

[74Scooter]

Not real happy with the shorter tangs on the replacement I found. So I cut the old cap out of the billet housing. Compared photos online and it's a Stant piece rated at 16lbs and has J8 stamped into it. No part number. These things do not appear to be available retail.. but it sure looks like everyone making a billet or custom 16lb radiator cap has access to them. Checked all part numbers starting with "R" and the closest I could come is the R31 hex top that seems to be discontinued. Not looking to spend a lot of money on a custom cap just so I can peel the Stant out of it.

Just in case anyone ever runs into the same situation... the Stant cap you need can be found hiding under the plastic on these. The top photo cap runs about $6... so naturally they are out of stock everywhere. The bottom cap runs $15 and is in stock... extra $10 for a sticker I'm going to remove... lol.

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