Construction and other equipment

Help identify PTO

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Category: Allis Chalmers
Forum Name: Construction and other equipment
Forum Description: everything else with orange (or yellow) paint
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=210764
Printed Date: 17 Sep 2026 at 1:46pm
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Topic: Help identify PTO

Posted By: Mschwartz
Subject: Help identify PTO
Date Posted: 15 Apr 2026 at 12:14am

Picked up at an auction, similar bolt pattern to an M but is larger. Has HD6 written on it but I am pretty sure that is incorrect. Any help? Thanks, Mike uploads/25518/7C2E4132-6125-4B99-A55E-7796F7A3FB02.jpeg" rel="nofollow - uploads/25518/7C2E4132-6125-4B99-A55E-7796F7A3FB02.jpeg

Replies:

Posted By: Mschwartz
Date Posted: 15 Apr 2026 at 12:17am

another picture uploads/25518/F9477980-E9AD-4F50-8561-0F1659A98AEB.jpeg" rel="nofollow - uploads/25518/F9477980-E9AD-4F50-8561-0F1659A98AEB.jpeg

Posted By: Codger
Date Posted: 15 Apr 2026 at 6:26am

Looks to be a speed reducer/increaser used to drive something at a different output speed than input.

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That's All Folks!

Posted By: Mschwartz
Date Posted: 15 Apr 2026 at 10:48am

yes it is about a 3:1 reduction. I think that would get it close to being 540 pto speed. Output is 1 3/8 6 spline.

Posted By: Ian Beale
Date Posted: 15 Apr 2026 at 3:37pm

There was a cog box like that to get pto speed with IH TD6s

Posted By: Ray54
Date Posted: 16 Apr 2026 at 2:35pm

For what it is worth I measured the case on an HD5 today. Trapezoid shaped opening. The top 2 hole look 5 inches across. The lower holes are 7 1/2 inches. I think the up and down on them was 5 1/2 inches too. But ???? They are threaded 5/8 course thread. This is where the rear mounted hydraulic pump and control units like the Bee-Gee hydraulics were mounted. I am 99% certain the case of an HD 6 would be the same. As IH TD 9 and Caterpillar D 4 of the same vintage used the same mounting plate pateren.

For cable controls or a PTO unit like picture there are another set of holes lower in the case. So you would have 6 bolts rather than 4 holding it. They 9 1/2 inches apart and 9 1/2 inches from the top holes. Easier to think of them making a triangle with the top tip cut off.

Since the mounting hole where made the same size on 3 tractor brands, I wonder if there is a standard speed for the shaft coming from the transmission? For that matter I wonder if an HD 5 and HD 6 would be the same speed even.

I cannot remember what size the shaft is out of the AC transmission let alone what the splines would be. I have delt with more Cat stuff trying to mount Bee-Gee hydraulics. Just the one HD5 and that was almost 50 years ago. But I know Cat used an odd ball spline size. Always enough old Cat's at junkyard we would buy what ever shaft they had with splined coupling and have it made the right length. Never found anyone selling splined couplings the right size. And different models of D4 Cats needed different lengths of drive shaft from transmission out put to what ever you mounted on the back. So if an HD 5 and HD 6 used the same exact drive shaft I would be surprised. Another oddity of the crawler world the shaft out of the transmission on Cat D6 and D4 turn opposite directions. And for some reason believe the HD 5 and HD 9 are the same way. Many a person has taken a Bee-Gee off a D4 to put on a D6 and they don't work. Until you swape the control valve and tank return plumbing around. So if anyone uses the pictured PTO unit make sure which way things are turning before trying to power any equipment.

All things considered it would not be a real surprise to me if the bolt pattern on a model M crawler is the same as the HD 5&6. But that is all guessing as I am not sure of were any model M's are hiding today to look at one.