What depth to bore/mill I.M. for 4.3 Throttle body?

[CPOM]

I was at a machine shop today and the guy asked me this very same question. I looked down the IM, and told him, "to the bottom" he says that is highly unlikely.

He showed me the longest "bit" he had, and it was about 2.75-3.5 inches long. I was expecting him to go 5 or 6 inches deep to the bottom.

Granted, my 3.1 is no race engine, but wouldn't it be self-defeating if i had him bore down a few inches only to leave a shelf, or lip of metal to kill incoming air?

Any metal CNC gurus have any terminology I can lay down to get him to bore down that deep?

BTW, I was quoted .5-1.5 hours @ 60/hr.

Thanks

CHRIS


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[RacerX11]

Has he looked at the intake yet? If I remember right from my 2.8 TBI manifold, each throttle bore goes down about 1-1/2 to 2", then it opens up into a plenum chamber before heading to the individual intake ports. Once you bore down enough to hit the open plenum, you stop, because you are no longer cutting anything. The 2.75-3.5 inch long mill bit should be long enough.

To be honest, you could probably do this machining operation at home using a brake cylinder hone. If I recall correctly, the 4.3 TBI isn't that much bigger than the 2.8 TBI. If you can find some coarse stones for a brake cylinder hone, you could just hone it out to the proper diameter with a drill. If the brake cylinder hone is too small, you might be able to fit a regular engine cylinder hone....it depends on the design. It will take longer to hone it out versus drilling, but you don't have to get it set up square on a mill/drill, and you don't have to pay the machinst $90.

Marty

[CPOM]

the open plenum only comes into play on the inside walls of each bore.

If you follow the bore down on the outer sides, the plenum is the same diameter all the way to the bottom.

Air entering the throttle bores goes forward and backward after it reaches the bottom.

As far as the inside walls are considered, there is a hole which equalizes the two bores, and below that, more material.

There are three options I see

1. bore 1 11/16" all the way to the IM "floor" of the common chamber
2. bore down 3" or so and that's it, leaving air to hit a "lip" created by the drilling.
3. bore down with a 10 degree taper, going from 1 11/16" to the standard 3.1 bore.

not sure what to do

[RacerX11]

You could just bore the hole as deep as their bit will allow (2"-3"), then smooth the lip out with a die grinder and head porting sanding roll on a long mandrel.

Marty

[geoffinbc]

I did the same thing and had no problems with any lip. Get him to drill as deep as possible. If there is any lip then smooth it in with a die grinder. I did not have a lip

[CPOM]

Thanks for the help guys.

The guy doesn't have a die grinder. But he's going to try to get a longer bit and bore all the way down.

I'll keep ya posted.