74 Midget Kingpin to bushing clearance

[Dadandson]

I am about to begin rebuilding the front suspension on my midget, too hot to drive in the summer in AZ, and that will include replacing the bushings and possibly the kingpins. Does anyone know the clearance between the bushings and kingpin? I do not have a reamer and I will machine the bushing ID's here at my shop. I am guessing around .002" total, .001" per side.

 

[GB1]

That maybe a little tight IMHO, Peter C would know.

Pat

 

[tosoutherncars]

FYI... common wisdom seems to be that the kingpins wear, and the bushings don't (which seems contrary to metallurgic common sense, but there you go.) You may want to try fitting your new kingpins to your existing knuckle, and see whether you have any slop.

It has been suggested by some that grit may become embedded in the bushings, causing wear in the new kingpins, but I have yet to see any evidence of this. Most rebuilds seem to consist of new (rubber or poly) bushings throughout (lower, upper / trunnion) and new kingpins, with existing bushings kept in place after inspection. Of course, if you are able to press in and line bore the bushings, that's great. I'm not sure how effective it would be to bore them, and then press them in - wouldn't that change (if only minutely) the ID?

 

[Dug]

Hello,
Even if it sounds simple to pre-bore the bushings, taking into consideration the amount of O.D. press for “over boring” of the I.D. for clearance, It’s iffy at best. Reaming them in place is best. You’re about to rely on the bushing bores (in the swivel axle) being perfectly inline at the time of manufacturing.
It’s truly best to do them installed. Clearance amount? I’d go for more like 0.0025/ 0.003. It’s grease that’s filling the space and not oil. Using 0.0025/ 0.003 for a small amount of misalignment spread over the distance given and some kingpin taper (measure from top to bottom of each pins bearing surface and again at 90) you “might” find a small amount present on new pins.
I like using adjustable reamers that have a mandrel that can be sized to the unfinished and finished I.D of the other bushing. This way you can account for any cutter deflection and have an alignment tool to boot.
Not pushing anything personal here, but my experience is from providing a service for reaming swivel axles for three shops when needed for better than 20+ years and have found some interesting things over those years.
Below is information for others if they read this

First thing is to magnaflux the spindle and then check them for straightness. I then place a long shaft sized to the I.D. of the bores in the swivel axle that have 0.002 clearances to check their alignment before installing new bushings. All this gives you a good overall ideal to the swivel axles condition.

Sorry for the long answer but it has been shortened. I know you are involved in machining as well so if you make yourself up a shaft to check the bores alignment you will be able to maybe have better success if you proceed by pre-boring. Heck for us it’s no big thing to make more bushings anyway….
“dug”

 

[Morris]

Machinists are cool.

 

[jlaird]

Do't forget both of the bearings top and bottom are different sizes.

 

[Dadandson]

I was going to press in new bushings and then size the bore using a Jig Grinder for alignment and a hone for size. I am going to spin the kingpins in a O.D. grinder and true them up by grinding if needed. I may be able to salvage the old kingpins by grinding .005-.010 off the O.D.'s. I will use .003 total, .0015 per side, for the clearance to allow grease on the pins. Sound right?

 

[Dug]

Yes,
If everything could be perfectly aligned you could do with only 0.002 total clearance but it’s not practical. You know as well as I that 0.001 clearances per side on the two sizes would be great as far as a “free” fit but there wouldn’t be ample room for the grease and the application isn’t correct for this close of fit.
I end up “truing” the new kingpins as a standard practice. New pins are a little tapered at the big end close to the radius more times than not.
I've never used old bits as everyone supplies the full kit new for fitting. Have fun with the shims or making a thicker oillite thrust washer when shimming the end float of the assembly to the upper trunnion.
"dug"