I have a Thun (?) crankset and bb on the bike I'm fixing up and I'm having trouble getting the cranks off. I have a crank puller (not sure what model) that works on my Shimano Sora cranks on my cross bike but won't fit this bike. There is dustcap on these cranks...the fixing bolt is the only thing so there is no way to screw the puller in. The other part of the crank puller is too large to screw into the inner piece.

Are there different sized crank pullers out there? What size should I be looking for?

If you are going to junk these cranks, and have no intention of ever using them again, you can bang them off with a BFH. (big fuc_king hammer).
Or you can ride the bike with no crank bolts installed, the cranks will quickly loosen.

i had to get an odd sized puller for my stronglight cranks.

I've heard an automotive rod puller can be used as a crank puller.

2 Narrow peices of metal and a chisel. put the to peices between the crank and the bb and hamer the chigel between them. Top then bottom. Had to do it this way a few times didnt damage anything.

."..the fixing bolt is the only thing so there is no way to screw the puller in."
Are you not able to remove the 'fixing bolt'? If not maybe a thinner walled socket? Either way, no puller is going to work if the cranks are still bolted on.

if you have access to a rubber hammer then a few solid blows with that'll knock em right off.

I guess I explained "fixing bolt" wrong. I removed bolt that holds on the cranks with no problem. My problem is the crank puller won't screw into the inner part where there is usually some dustcap threads.

I think I'll just go the BFH way...

Operation BFH was a success. The crank and chainring were one piece...what the hell kinda machine is this?!?!

Now...to buy a money wrench to get that other bb cup off.

Thanks for all your help!

Yep. BFH's solve almost any problem.

Ah, are these a sort of cottered cranks? The non-drive side has a bolt that release a pinch in the crank, then it can slide off the serated spindle which is part of the drive side crank, right? The cups are pressed inside the BB shell and the bearings are located outside of them with a small locknut on the NDS holding the whole mess together?

Nope, like a regular bb but with a weird Italian twist.

Oh, I git it. Regular square taper just with the ring swaged to the crank.