The original G-50 ring gear part # 950 116 143 01 were flat similar to 915 and 930.
The updated ring gear 950 116 143 05 had a extended 1.5 mm lip to aid in poor starter engagement and fixed 99% of the engagement problems.
For that last 1 % like yours you will need to measure the dimensions shown in TSB 8708 Starter motor/gear damage and you will most likely find it was not machined correctly from the factory, dimensions should be 115mm +- 0.2
I have only seen this problem of incorrect machining tolerances 3-4 times in the past 35 + years and it is very rare and mainly on the early 87’s.
You don’t have to disassemble the gearbox and any competent machine shop can handle this.
Please post the dimensions you come up for the mounting flange, just curious what it is and if this is actually your problem.
Richard
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