The bolt you are talking about is one of the item #50's here, right? https://www.partzilla.com/catalog/honda/atv/1986/trx250-a-fourtrax-250/right-crankcase-coverMy Honda Fourtrax didn't have oil changed for 2 years because it only had about 2-4 hours on it as I only use it to get to the duck blind on flat dirt levy roads 2 mi r/t. When removing the oil filter bolts - all was fine and I put in a new filter but when replaced one of the bolts just spun and wouldn't tighten. I took it out and found out that the treads from the case were on the bolt. I measured the hole and it has more threads inside the case, maybe 1/4 inch. So I figure maybe I can get a longer bolt. This is a 6 sided 8mm bolt head w/about 3/16 in. or more shaft with 11/16 in. of treads under the head. Where do I get a longer bolt and does anyone know mm size to order?
Final question: if this doesn't work can a helicoil be put in? And how do you do that as I have worked on hot rods for years but never did a helicoil job.
If so, that is an M6 by 16mm long bolt. Just go to local hardware or big box store and pick up a M6 x 20mm (or so). But if you do put a longer bolt in and it engages with whatever threads are left in the case just fine, make sure that the bottom of the bolt head actually bottoms on the cover. Otherwise it won't seal well (you probably knew that but sometimes we have to state the obvious).
You can also pull out the remaining good bolt to size up for thread pitch and length.