Please print out and include this form with your parts



1986-87 Buick Grand National / 1989 Pontiac Turbo TA

Throttle Body & Plenum Machining Services

A 62 mm throttle body is one of the upgrades that helped my stock motor and turbo 1986 GN break into the 11's in 1997.

Flow Test Results

Stock TB flow is 480 cfm on the intake test, and 398 cfm on the "blow through" test.

A 62mm TB will flow 541 cfm on the intake test and 464 cfm on the exhaust test. These figures were on a 62mm TB with the shaft unmodified.  The Half Shaft mod will increase flow even more. 

Throttle Body Features





Machine throttle body to 62mm 

Bellmouth TB Entrance

Bore shaft seal pockets to accept Harwal seals

Install new shaft seals

Mfr custom fit polished 6061-T6 throttle plate

Modify shaft to imcrease flow equal to a 64mm TB (half shaft mod)

Chase all threads prior to reassembly 

GN Throttle Body Rebuilding

As above except stock bore

Powder Coating Info

 The bake process following powder coating softens the aluminum which causes a poor finish when bored/ The TB needs to come here first to be machined then I return it to you to have it powder coated and then it comes back here for the shaft seal installation and to have the throttle plate made & assembled.   

Plenum Features

Clean and bead blast inside and out

Remove  EGR Boss

Machine opening to 63mm

 Round over lower edge of entrance

Removed EGR Boss

Blend transition from enlarged opening to rear


Have an RJC Racing Powerplate under your plenum?

These modifications have no negative effect on the RJC Powerplates air distribuion 

AccuFab / Precision 70mm TB Rebuild Service

$80 incl shpg to a US address

Order Form

Print this out and include it when you send your parts.


FYI, The only available shaft seals for Buick GN throttle bodys measure 10 X 17 X 5mm. These are .010"too large! ANSI Standards for these seals specify a maximum press fit of .001" thousandth. When I receive your TB it will be machined to accept the Harwal seals.  

Unmachined seal recesses will ruin the seals

  Here's what happens when installing a Harwal seal without machining the seal recess.  

Guaranteed to leak.


Throttle Body Thread Sizes

TPS Threads - M4.5 X .75

This size screw is difficult to find

( http://www.corvetteforum.com/forums/c4-tech-performance/3106112-tps-bolts.html )

If stripped, you can re-tap to 8-32 

 Shaft Retaining Clip Threads - M4 X .70

Vacuum Block & IAC Boss Threads - SAE 10-32

Any questions please email stevemonroe@cox.net 

~ FYI ~

Here's a picture looking down into a stock TB mounted on a stock plenum. Note the mismatch 

84-85 Throttle Body Page