Help: Tapping M20 exhaust stud holes

[blood_sweat_&_gears]

I'm in the process of swapping a 325i head onto my 325e. I decided to tap the holes and get new studs as I was forced to hack off a few of the studs in order to get the stock exhaust off and turbo manifold on. The head is off an ’87 325i and uses the M8x40 studs. I have tapped 2 holes with a M8x1.25 tap but what concerns me is that though the new studs thread in very nicely they have a very small amount of play in them!!! Is there something I'm missing here? Every other M8 hole I've ever tapped on the car has fit perfectly. The studs are OEM so I doubt they are out of spec and the tap is from a decent tap set. Has anyone else had this problem? Will the heat expand that studs over time??? I will post pics if that helps.

[jeffnhiscars]

They shouldn't have any play at all. I take it you cut the old studs flush then drilled and tapped back to the original size. It's damn near impossible to resestablish the original hole. If your pilot is not perfectly centered or if doesn't cut at the precise depth of the original threads (i.e. You cut through the old threads instead of just chasing them) you are going to get slop.

The right way to do this is with a helicoil or the next size up in which case yourpbab
Yo have tomopennthe manifold holes as well.

[blood_sweat_&_gears]

They shouldn't have any play at all. I take it you cut the old studs flush then drilled and tapped back to the original size. It's damn near impossible to resestablish the original hole. If your pilot is not perfectly centered or if doesn't cut at the precise depth of the original threads (i.e. You cut through the old threads instead of just chasing them) you are going to get slop.

The right way to do this is with a helicoil or the next size up in which case yourpbab
Yo have tomopennthe manifold holes as well.

Sorry I wasn't clear. I didn't have to cut the studs flush. I cut the nuts off, removed the manifold, then had enough stud left sticking out to either vise grip the studs out or manage to get 2x nuts on and wrench them out. All the old studs were threaded out, nothing drilled. An chance the threads are fine... maybe M8x1.0?

[blood_sweat_&_gears]

Solved!

I was 99% I had the right tap based on my research and I did! After snugging the studs right up using the 2 (or 3) nut method, the movement goes away... not sure why I didn't try it before. Anyways for anyone that ends up here in the future ill quickly sum it up for you.

Studs: M8x40
Tap: M8x1.25

If you have a rounded nut grab a metal blade and a reciprocating saw and cut the nut flush with the manifold. Once the manifold is off you should be able to still fit 2x nuts on and remove the stud. If you have access to a torch (even a propane one will do) heat the SH%t out of the stud and lather it up with penetrating oil. I found this worked like a charm.

[AndrewBird]

If you aren't careful when chasing the holes with a tap, you can put side load on the tap and cut the threads deeper than you are supposed to, especially with soft cast aluminum like the cylinder head is made out of.

Taps also come in different classes designated by a GH number. Higher the number, the looser the tap. See here:

http://www.newmantools.com/taps/taptech.htm

[jeffnhiscars]

If you aren't careful when chasing the holes with a tap, you can put side load on the tap and cut the threads deeper than you are supposed to, especially with soft cast aluminum like the cylinder head is made out of.

Taps also come in different classes designated by a GH number. Higher the number, the looser the tap. See here:

http://www.newmantools.com/taps/taptech.htm

I feel in over my head now to ever buy another tap...lol...and I presume that's Newman from Seinfeld ? :)

To chase head bolts which variation is best ?

[AndrewBird]

They make thread chasers that don't cut. This is the easiest for those who want a simple solution.

But really, pretty much any tap you buy will be ok. I would go with the tightest thread class you can find and consider that good enough.

Metric threads have looser and looser tolerances as the diameter increases, so with larger bolts like head bolts, which tap you use isn't as critical. Incidentally it also lessens the number of taps available. I.E. - The lowest class M8 tap you can buy is a D3 or D4 (metric goes by D, not GH or just H like inch taps), depending on where you are looking and the type of tap.

[digger]

threaded holes for studs usually use the smaller end of the tolerance scale so the studs dont come out very easy