Hey guys, i just did my 12 bolt swap but my diff was already broken in so this is for a friends. Just put new gears and new eaton posi in a 12 bolt going in a 68 elky. What is the proper break in for new gears and posi? Any recommendations on gear oil for a new rear end, posi additive? I have heard that you should take it easy for a little while and vary your speeds and such on the highway, but just how much is necessary and is this the best way to do it. Thanks.

Daniel

Daniel,
I have always used Richmond's synthetic gear oil. As far as break in goes, you should do whatever your gear's manufacturer recommends. Here is Richmond gears recommendation:
"A new ring and pinion installation, especially a high numeric ratio with new bearings, can cause an excessive heat buildup in the rear end and cause softening of the gear teeth and bearings if a break in is not performed.

Street vehicles should be driven at normal street driving speed for approximately 10 miles, then stop and let cool for 30 minutes. Do this 2 to 3 times. Towing vehicles need approximately 200-300 miles of normal street driving before being used for towing.

On circle track race cars make approximately 6 to 8 laps at slow speed, then let cool for 30 minutes. Make 6 to 8 more laps at slow speed, then 2 to 3 laps at full speed, the let cool again for 30 minutes.

Drag cars need only an initial run-in since they are driven short distances and heat is not normally a problem with proper lube and
backlash allowance.

NOTE: If after the above break in is performed, overheating of the rear end is suspected, repeat the final portion of the
break in procedures."

Hope this helps,

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100