Recommendation for tap fit hole tolerance in 4140 pre-hard steel

Hi everyone,
I would like to ask if anyone can please recommend a tap fit hole Min. & Max. tolerance for a 3/8" hardened steel dowel (.3751" Min to .3753"
max OD) ? The pin will be tapped into 4140 Pre-hard steel.
The pin is being tapped through a bearing housing, 1" OD X 1.5" long. The bearing housing fits between two upright supports and there is one drawn cup needle bearing pressed into each of the upright supports. I want to insert the dowel pin through a bearing at one end, and then tap it through a hole in the 4140 housing and on through to the bearing at the other upright support. The housing and pin will rotate together, and the pin rotates in the bearings.
I will probably build simple fixture to locate and hold the housing as I tap the pin through. I want it to be fairly easy to tap the pin through, so assembly goes smooth, but I don't want the housing to rotate on the pin, the pin and housing must rotate together in the bearings.
In order to get a nice easy tap fit, can I have a very small amount of interference to do I need some clearance ? I was going to shoot for a hole size of .3748" Min. to .3751" Max.
I'm just curious what's usually recommended for an easy tap fit.
Thanks John
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On Thu, 29 Nov 2007 17:32:18 -0800 (PST), John2005

How about a cold pin sized to just slide in a warm hole? 35 degC difference would give a slight interference fit.
Brian W
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John, I think a light press fit would be .0001" interference. Given the hard materials. If you get a hole that's .3748" and a pin that's .3753" you end up with a half thou interference, that will require some pushing.
Machinery's Handbook 19th calls for a shaft of 1/4-13/32 to be: "Easy Driving Fit" to be .00028-.00047 interference. "Close Driving Fit" at .00047-.00083 "Forced Fit" .00083-.00146 Dave
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Thanks for the feedback Brian and Mechanical Magic,
I have Machinery's handbook on PDF and I searched for "tap fit" and found nothing. Thanks for the info from the handbook Dave.
.0005" interference is very close the to the max end of an easy driving fit and the minimum start of a close driving fit. I've never heard of "close driving fit", I guess it just the mid point of an easy tap fit and a press fit.
I guess I could rig up a press for this thing but it's still best for me to keep interference to a minimum.
Thanks again guys, John
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John, "fits, forced" in the index.
I have never heard of a "tap fit" when referencing the force need to install a pin. But, then I've never seen "close driving fit" either. I have been known to install dowel pins in a drill press, or mill. Need to install a spacer deep in the chuck so the force is taken there, not by tightening the chuck really tight. Dave

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