PTO shims and shaft speed

AA Ford Discussion Group relating to the repair and restoration of your AA Ford.
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Posts: 51
Joined: July 8th, 2017, 7:14 am
Body Type: 82B
Model Year: 1931

PTO shims and shaft speed

Post by wild4679 » October 26th, 2018, 6:15 am

I will install a PTO that was purchased separately from the transmission. How does one know how many shims to install between the PTO case and the transmission?

Also, this PTO will direct drive a vintage Deming water pump. I need to know the PTO RPM at low idle and full idle of the engine. A friend on this AA site advised me that by counting teeth in the transmission gears and the PTO gear, the output speed of the PTO will be 71% of the crankshaft speed of the engine. However, he would and I would like some verification to our theory. Does anyone know the answer?

Bob C
Posts: 1322
Joined: April 24th, 2003, 11:50 am
Body Type: Mail truck, Stake tr
Model Year: 1931
Location: SO CAL

Re: PTO shims and shaft speed

Post by Bob C » October 26th, 2018, 8:40 am

Looks like it may depend on the manufacture of the PTO. St.Paul PTO shows 1062RPM per 1000RPM of engine
and Muncie PTO shows 59 percent of engine speed.


Posts: 648
Joined: August 11th, 2013, 4:25 pm
Body Type: 82-A Platform
Model Year: 1928
Location: Hamilton, Mich.

Re: PTO shims and shaft speed

Post by KimVanOrder » October 26th, 2018, 4:13 pm

counting the teeth is the sure way to know the gear ratio. Go with that. Shim so that the teeth just have clearance. Plastigage will do the trick.
Dec. '28 AA

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