Makes sense this is probably what happened to mine.
Could you simply screw the male 20mm part into the block further so the filter doesn't screw past the threads into the gap? If the 20mm male threads are bottomed in the block, could you shave some threads off the 20mm side so the 3/4 threads and gap are closer to the block?
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There's actually plenty of internal thread to screw the adapter further into, but here's the rub. You see, over 30+ years of operation, the 20 x 1.5mm internally threaded portion that your oil filter bolt screws into gets pretty cruddy, and when cleaning up my F1 engine cases recently I had to run a threading tap thru it to clean all the crap out.
The threaded portion of the oil filter bolt is only 20mm, so the remaining 15mm (approx) of internal thread in the engine case fills with all kinds of goop that cooks itself into the unused threads over the years. With that kinda crap built up after the 20mm mark, It's really hard trying to screw a piece of threaded bar any further into the engine case, particularly when there's nothing that you can hook a wrench onto, so your fingers give up when you hit the crud barrier.
Fred's had the good fortune of having recently had one of my oil cooler adapters on his bike, and because my centre bolts threaded portions are around 25mm, the internally threaded portion in his engine case now has an additional 5mm "crud free zone", so he's been able to screw the threaded bar adapter that comes with the chrome oil filter in 5mm further, allowing him to screw the filter down tighter on it's seal, and hence, no leaks.
For those of you out there not fortunate enough to own one of my oil cooler adapters with the newly discovered benefit of an additional 5mm of crud removing thread on the centre bolt, you'll need to either buy a 20 x 1.5mm threading tap (expensive) to carefully clean the crap outta the threads, or a 20 x 1.5mm bolt (hard to find) or even an old rear axle. (the cheapest option, if you've got one) Cheers, Terry.
