Don’t Junk That Case, Repair A Loose Motor Case Bearing Insert

Published In The July 2014 Issue Of Cycle Source

Article & Photos By: Chris Condon


We are going to show you how to repair a loose motor case bearing insert. Rather than junk your case, this modification will make it better than the original factory setup. James Best of Dudley’s Basement in Central Islip, N.Y. is going to walk us through his repair. James started by clamping the case down then indicated the pinion race to find center. Once center was found additional clamps were used to insure stability. The other case half was bolted down using the factory case dowel pins. Next he checked the cylinder decks to insure that they were square. Make sure to double check before you start the next step because there’s no turning back after this. James then bored out the original steel insert until there was no steel visible. Checking with an Intrimik he was left with a 2.950 clean hole. Taking a length of 4340 with a 4 ¼” diameter, he cut it down to approximately 3 1/4” long. Using the lathe he cut the slug down until it had an OD .003 larger than the hole in the case to insure a pressed fit. He then machined the step in the OD that aligns the alternator stator. He proceeds to machine the ID .040 smaller than what the finished ID will be and cuts the Timken bearing snap ring groove in the ID of the insert. Next James flipped the insert in the lathe and faced the back off, leaving a .125 thick flange. He then counter bored the case .125 to accommodate the flange on the insert. Afterwards he chamfered the edge of the hole and using a sharpie, he marked out the oil return hole and the alternator plug location. Once all the cutting and milling is done it’s time to clean everything off with denatured alcohol. When everything is cleaned and dried he placed the insert on dry ice for an hour and heated the case up to 250 degrees. He cleans off the insert quickly and coats it with Green bearing mount Locktite. Now it’s time to press it into the case. He uses a press to hold the insert down in place while the temperatures normalize. He then puts the case in a 5C collet indexer. Using a .1495 drill bit he drilled 5 holes and countersunk them to 82 degrees then tapped the holes for the 10/24 screws. He lays out an abstract screw pattern so as to not interfere with the location of where the alternator plug and the alternator mounting screws will come through. The last step is to put the case in a Jig Bore and bore .040 out of the insert for the Timken bearing and the seal to fit in. Make sure to use the hand drill to drill out the oil feed and return passage using the appropriate drill sizes. You now have a case insert that will last you a lifetime and you don’t have to go out and spend a grand on new cases. That money can be better spent on more horsepower and that makes for one happy customer! Disclaimer: If you don’t know your way around the machine shop don’t try to be a hero. It’s very easy to turn your cases into junk if you make even the slightest mistake. You can call James at 631-851-9005 and mail him your cases and it will be done right.


Clamp down, indicate pinion race center, and install more clamps


Cleaned case hole measures 2.950


Bolt case together, bore out old insert until all steel removed


Rough-cut 4 ¼”diameter piece of 4340 to 3 ¼”long


Cut insert down to .003 larger than ID of bored hole in the case


Press insert into case and hold down until temperatures normalize


Apply Green permanent Locktite and torque screws.


Clamp in lathe and cut 1/8” counter bore, warm the case, freeze the insert and apply Green bearing mount Loctite


Drill five 10/24 screw holes into flange and chamfer


Case reinstalled in Jig Bore and ID line bored to final size for bearing races and seal

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