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Old 03-21-2020, 06:59 PM   #1
rhphotord
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Default Radiator cap pressure AV8 build

Maybe a dumb question but what pressure cap should I be running on my mildly built flattie using a new aluminum radiator? Thanks
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Old 03-21-2020, 07:52 PM   #2
aussie merc
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Default Re: Radiator cap pressure AV8 build

depends on a lot of things, hose quaility , how long ago was the engine assembled and what gaskets where used , do you have a heater [remember its a radiator just works in revese] how old is it and when was it last looked at plus lots of others with this in mind seven pound is generaly a safe bet but with modern hoses [2ply] modern gaskets updated heatercore 15 psi + is not unreasonable
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Old 03-21-2020, 07:59 PM   #3
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Default Re: Radiator cap pressure AV8 build

Yeah, but the water pumps in our old fords don't take that sorta pressure....Does your AV8 even need a pressure cap? I'd try for the least amount and go from there....
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Old 03-21-2020, 08:14 PM   #4
FortyNiner
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Default Re: Radiator cap pressure AV8 build

I'm using a 7 pound cap on a fresh rebuild. No leaks.
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Old 03-21-2020, 10:55 PM   #5
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Default Re: Radiator cap pressure AV8 build

I run a 7 lb on mine as well. Works great. Just keep in mind that your antifreeze is going to find its own level so if you fill the radiator to the top, it might leak out a little bit but don’t keep filling it because it will be OK
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Old 03-22-2020, 09:31 AM   #6
rhphotord
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Default Re: Radiator cap pressure AV8 build

Great information thanks. The engine is a new rebuild,modern hoses,and the rebuilt pumps have a tendency to weep after running up to temp. I was considering water pump replacement but thought pressure build up may be forcing fluid past the pump seals. The engine doesn't overheat although runs in the 185-195 degree range. I'll try going to a lower temp cap. Thanks all
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Old 03-22-2020, 02:17 PM   #7
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Default Re: Radiator cap pressure AV8 build

Really no need to use a high pressure cap in the temperature range that flatheads run in. Sounds like your headed in the right direction, 185-195 is a great heat range.
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