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How I made my 42mm spica ITB's

3314 Views 3 Replies 2 Participants Last post by  Caroni
Some want to see how I did it.
So here it is.

First I cut a block to the same angle that the buterflys set at closed.
Then I mount a chunck of brass that I ruf cut and pre drilled.

Then I milled it. the mill does a circle but due to the angle of the block the part ends up oblong and the edge also has the same angle on it.

This is the old on top of the new one

now it is together (BTW I also made a new rod and put in all new ball bearings )

This is the back side where I cut a air passage so I will have a good spot to tap for MAP


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Hey, I appreciated the pictures of your set up but got busy and never thanked you for the post. I imagine that you are using a CNC Mill to do the cutting. I have a manual "Mill/Drill" that is not as accurate, rigid... as what I learned on and my skills certainly don't lend themselves to cutting perfect circles by hand. I will either hit up my brother-in-law who has a custom shop in the Bay Area or try to do it on a lathe as was suggested by Jim K. I wish I had a CNC mill though.:(

I think I found a picture of your exhaust manifold on the web when I was searching for Spica manifold pictures that shows the vacuum ports attached to the back. Great idea. I was going to tap each port runner separately where the boss is but your idea is cleaner.

Thanks again,

I did not come of with that. I also saw it I think that was Jim's set up if my memory is right.

on a non NC I think you could use a bore-bar. I did not have one at the time. but I got one at
if you get a bore head and a lefthand cutter you can cut the out side with it. all you have to do it line it up and move the Z down.

with a right hand borebar you cut the inside. like the ITB its self.

it can be tricky as it can start to chatter if you try to cut too much. but it will give the best finish.
the lathe would also give a good finish. After I did this I found out you can get the buterflys premade easy.
unless it is some strange size. I just did not think to look for them at the time.
Sometimes I wonder about my brain!

Thanks for the tip. Actually I have a 2" boring bar holder. I was planning to use it for the boring out of the throttle bores. It just never occurred to me to use them for the outside cut. Cool!:) (Can you tell i don't do this for a living?)

Do you have a source for the pre made butterflies? Depending on the price I'd have to think about buying versus making. I already have lots to make and the plates would be relatively simple, thanks to your suggestion, but if inexpensive enough purchasing might make sense.


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