K75 tank filler cap bolts snapped

[Jules]

I posted previously about a replacement (2nd hand) tank I bought for my K75 that turned out to be the wrong tank. I now have the right tank, but... Two of the four petrol cap bolts are snapped off when the PO stripped the tank to sell all the parts seperately. One bolt is snapped off flush and the other is almost flush (maybe 0.5mm proud).

Is there a recommended method to remove them? I've never removed steel bolts from aluminium before. Whenever I've drilled bolts out, it's always been steel in steel (or iron). Is the drill just going to bounce off the bolt and go for the aluminium? Not sure I could get the tank under a drill press as I only have access to a small bench drill.

Hints, tips, and suggestions appreciated!

[The Mighty Gryphon]

I'll be watching the replies with interest as my 100 has one screw that went in harder than the others.  Might be cross threaded, but I'm afraid to pull it out to check.

[johnny]

greetings...

if me... id f it up with left twist drill or dremel or extractor...

so... i would take it to a machine shop and have them do it...

j o

[drut]

For what its worth,my experiences in drilling out steel bolts in aluminium,best results:carefully flatten snapped stud with grinder if proud(petrol tanks generally safest when full of water),use center punch to create start point the use small drill bit as pilot & work up over.If you get it right you can actually drill the hole close enough to pull the thread of the original bolt out like a piece of wire,if you get it slightly wrong you can tap,or drill hole larger and use thread insert(helicoil).Easy outs(l/h flute extractors),in my experience rarely work well in stubborn bolts and if you snap one are a PITA to deal with.

[Jules]

Quick update on where I got to this weekend. Tank was flushed with soapy water and then dried out before any work commenced. I used a small grinder wheel to flatten the remains of the bolts to a flat surface. Then used a centre punch to mark the centre.

I offered up the tank to the drill press, but the drill didn't have the reach, so had to resort to a cordless. I started off with a 1mm HSS drill and drilled all the way through both bolts. Then enlarged the hole with a 2mm HSS drill, before finally trying a left handed thread 3mm HSS. This went all the way through one, but snapped inside the other. Neither bolt unscrewed. :(

I managed to extract the broken tip with a small pair of tweezers. That's as far as I got at my friends place (where the drill press is). I tried to knock out the superficial dent in the tank whilst there too, but didn't get very far with it. It's only a small dent, so will probably fill it before I paint the tank.

I've ordered a 3.5mm HSS drill and an M4x0.7 tap to hopefully finish the job.

Back at my place I tried to do a compression check on the engine, only to find I have no 12mm adapter on my compression tester (only 14mm). So ordered an adapter off eBay.

[vintagemilano]

Nothing wrong with giving up on the threaded holes altogether and popping a helicoil in the spots you need. I'm not totally familiar with how much meat the tank has in this area but it it works out you will have a better threaded hole than original. Just follow the instructions on the package

[Jules]

Thanks, I may still go down the helicoil route, it depends how badly I mess up tapping the hole!

I tried one of those dirt cheap paintless dent repair (PDR) kits off eBay that includes a hot glue gun, various pads you glue to the dent and a metal brace that goes over the dent that you pull on the back of the pad from (the pad has a bolt coming out the back of it). It worked surprisingly well. However it broke before I was finished getting the dent out of the tank. The pad you glue to the dent is made of plastic and there was a big bubble in the moulding where it snapped. However, it was so cheap I was prepared to write the cost off as a proof of concept as I had never used one of these dent pullers before. I'll pick up a better quality example and try again.