is it possible to have a narrow bolt pattern off an older 4-speed toploader with a six bolt bellhousing? I dont want to fork out the $300+ for the scattersheild. Can I drill and tap my existing three speed bellhousing, which has the wide bolt pattern?
Can you post some pictures of what you have and what you're looking to do? If it's a mild setup, I'd say yes. You just have to be exacting, otherwise you'll wear out pilot and input shaft bearings fast.
If it was me I'll be taking it to the machine shop and get a price. Like dacofa said it has to be centered perfect or it will eat more dollars in parts than it would cost to have it done right the first time. I'm sure they are not gonna be using a drill press to do this job. Most likely it will be done on a mill.
If you use the tranny as a guide to start the holes you will have a nice center point at each bolt hole. make shure your drill just slips in the hole and then carefully put pressure on the bit as it turns slowly. That is the key, turn the bit slowly and use lots of pressure. after you have "started" each hole then you pull the tranny back off and in the center of the bore is a nice place to drill with a smaller drill. 1/4, then 3/8, then your tap drill. start the tap using the tranny again. then after it is started run it down carfully with plenty of cutting oil (use aluminum cutting oil if it is an aluminum bell housing). When you have two opposite holes done then put the tranny on and make sure the other holes line up. If you drill one hole at a time then you can line them each up with the tranny and the bolts in place that you have already done. It takes a little time and you have to work carefully but you can do it if you have any mechanical aptitude at all.
If you don't want to risk it, or you have no aptitude at all then a machine shop can do it for a price that will be worth it to you.
If your 3 speed bellhousing is iron, you might be able to re-drill it. Good chance the iron bellhousing is thick enough (where you want the new bolt holes) to accept threading, if it is aluminum then there probably won't be enough metal where you need it. Your three speed trans should have both bolt patterns, if not you will have to get a 4 spd case that has both bolt patterns so you can bolt it to the bell housing and transfer the location of the inner bolt holes to the bell housing. If you have a drill press that is big enough you can drill and tap the new holes, if not you will have to have some one to do it.
i have done this just took the trans case and put it in the big hole like i was installing it to the floor of the shop marked the holes like in the previous post then removed ,drilled,replaced etc till the holes were there. Then took out the tig welder and some shore t1 aluminum dowels and welded them over the holes on the back side and redrilled the holes and tapped them ... Have about 5000 miles so far with no problems