Largest possible throttle body bore

[xj bmx]

Since i got called back to work, i want to bore out my TB, what is the largest diamiter i can go without it breaking something?

Just want to know so i can have our lazer guy write program for a larger butterfly.

[zxtwou2]

how well can your laser machine articulate? just wondering, because if you laser cut a plate with flat edges, it will never seal right. the edges on the butterfly plate are angled as they get further from the pivot axis...they are also very slightly eccentric and not perfectly round IIRC.

[Beodude123]

58mm is pretty close. People have done it out to 60.5 or 61. 58mm is pretty good for close to stock though, so it works pretty well.

[mechtech]

xj - Get a spare TB for this from a boneyard.
This is really not a mod for normal street engines . Maybe for really built ones.
It is the last, not the first thing to do.

[zxtwou2]

the throttle bodies i'm putting on are 38mm diameter...but there are 4 of them

[02 5SPD]

I had a friend bore one to 58. Much more than that and you start getting close to the bearings the rod for the butterfly go through.

[JonsZX2SR]

Somewhere between 57.25 and 58.0 mm is the largest you can bore the TB depending on the specific casting. I would bore and measure looking for thin spots. When you get down a wall thickness of 1.0-1.25 mm in the worst spot i would stop.

You also have to check for porosity, although JB weld or aluminum filled epoxy can fill pits and pores. After you are finished fine boring and polishing measure the final ID and make the throttle plate accordingly.

Cuttting an oval plate is the wrong way to make the throttle plate, you should use the method that is done with actual manufacturing. make a fixture with the right inclination angle (In this case 7.8-8.0 degrees and sandwich a brass plate in the fixture. If you machine the plate at this angle you will get an oval shape with the machined diameter across the short dimension, a longer distance 90 degrees to the short axis and the correct angle at the edges.

If you want to go into production you can sandwich 20 or so plates in the fixture and make 20 or so TB's this way.

I made a TB with a 57.6 mm bore using a '96 Contour Zetec TB and made the matching plate as described above.

[02 5SPD]

On the one I had my friend do the original bore horribly was off-center, and he centered it out when he bored it. IIRC he want off the holes you use to mount it to the IM to center it.

[powder]

On the one I had my friend do the original bore horribly was off-center, and he centered it out when he bored it. IIRC he want off the holes you use to mount it to the IM to center it.

The holes are sporadic, you can't use them to align the bore. Easiest and best way is to just dial in the bore, and use a boring head on a mill, or a boring bar on a lathe to machine it.

[02 5SPD]

I'll have to ask him what he what he used to center it. I do know he did a good job at centering it out.

[xj bmx]

johnszx2, why dident you go for the full bore? it looks like there is still a I.D change.

[Beodude123]

You wouldn't be able to make a stock throttle body be straight though. There still has to be some angle from the top to the plate.