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.
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
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 firstname.lastname@example.org
~ FYI ~
Here's a picture looking down into a stock TB mounted on a stock plenum. Note the mismatch
84-85 Throttle Body Page