Transmissions - Transmission line size

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Transmission line size

Printed From: The Ford Torino Page
Category: Powertrain Specific Forum
Forum Name: Transmissions
Forum Description: For discussion relating to all Ford Transmissions
URL: https://forum.grantorinosport.org/forum_posts.asp?TID=20026
Printed Date: 26-March-2026 at 9:20AM
Software Version: Web Wiz Forums 12.06 - https://www.webwizforums.com

Topic: Transmission line size

Posted By: pyntre
Subject: Transmission line size
Date Posted: 08-February-2022 at 10:38AM

Hi ,

Can someone confirm the cooler line size on an Fmx trans .

Thinking they are both 5/16 “

Thanks

1975 Montego

Replies:

Posted By: 72 RS 351
Date Posted: 08-February-2022 at 2:42PM

Most older Fords had 5/16" lines, but 3/8" lines are much better. Ford learned and upgraded all trans lines sometime in the 1980's, the smaller lines restrict flow a little too much. I'd put 3/8" on anything, and the parts stores carry a variety of lengths with the fittings on each end. It's listed as fuel line, and they used to have it on display racks.

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Don
73 Ranchero "Sport 72 front end", floor shift/console, planning EFI 7000 rpm 351 stroker
73 Ranchero GT 351C-4V &4R70W for sale later.
92 Lincoln Mark VII SE GTC, OBDII 347/4R70W

Posted By: pyntre
Date Posted: 08-February-2022 at 3:12PM

thank you for the response and information!!! 👍

Posted By: Rockatansky
Date Posted: 13-February-2022 at 12:25PM

what good does upsizing the tubing if the internal orifice of the fittings is more like 1/4" ?

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72 GT Ute

Posted By: 72 RS 351
Date Posted: 13-February-2022 at 2:20PM

The orifice isn't always the same size for all fittings. The main fittings on the side of the transmissions differ for a 5/16" line versus the 3/8" line, not much but slightly.

The late late model transmissions require even more flow than the prior 3/8" lines and simple coolers. Those have to have noticeably bigger coolers, and the parts they use have less restrictions in the whole system. I read a little of that this past Fall when I was researching the 6R80 six speed. The notes warn of using lesser coolers or fittings etc, but that is a different trans of course. I think just keeping good flow in general is wise, so avoid smaller parts etc, for the trans cooling system. It cannot hurt.

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Don
73 Ranchero "Sport 72 front end", floor shift/console, planning EFI 7000 rpm 351 stroker
73 Ranchero GT 351C-4V &4R70W for sale later.
92 Lincoln Mark VII SE GTC, OBDII 347/4R70W

Posted By: californiajohnny
Date Posted: 14-February-2022 at 7:36AM

5/16" is standard across most cars... yes the internal fittings can be 1/4" so bigger tubing wont increase any flow....

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JOHN
74 GRAN TORINO S&H CLONE
74 VETTE CUSTOM
90 S10 BLAZER 4X4 LIFTED
77 CELICA CUSTOM
75 V8 MONZA SUPERCHARGED
79 COURIER VERT. SLAMMED
75 VEGA V6 5 SPD
70 CHEV C10 P/U
68 MUSTANG FB CONVERSION

Posted By: pyntre
Date Posted: 14-February-2022 at 7:55AM

This is not a performance build , but will be adding an external cooler to the system . I want to get the transmission system done and forget about it . Thanks for the info everyone!!

Posted By: 72 RS 351
Date Posted: 14-February-2022 at 7:55AM

All restrictions are restrictions. Small 5/16" line flows less than 3/8", regardless of the fitting orifice sizing.

It's the same as an exhaust, just because the joint at the collector is less than 2 1/8" in size, that does not mean that 2 1/8" pipe is sufficient for the whole system. You should make everything flow as much as possible, within reason. 3/8" lines are much better than 5/16" lines, always.

That's like being cheap building a house with lesser materials, hell 7/16" roofing OSB is good enough, why not. The next size thicker will last decades and not warp, but it'll cost an extra $500, let's be cheap with building that $200k house.

Posted By: pyntre
Date Posted: 14-February-2022 at 11:43AM

going to run -6 lines to everything, that’s 3/8”

Should be fine for this ditch runner !