1966 GTO 389 Fuel Pump Eccentric, Cam Thrust Plate

I searched the Forum and surprisingly not finding much on Fuel Pump Eccentrics.

I have heard these were a weak spot from production.

Mount tab on eccentric sheared. Research says Cam "walks" forward with decreased thrust/changing/decreasing RPM. Eccentric is mounted to Cam Sprocket and when Cam walks forward, the eccentric shears on inside face of timing chain cover.

I do see circular marring on inside of timing chain cover. Unsure if marring is recent/during this occurrence or past.

Research says other engines have Cam Thrust Plate to prevent Cam from walking forward but the Pontiac 389 does not utilize cam thrust plate. Anyone know?

Research says (Pontiac 389) design mount of distributor on passenger side of engine causes downward force on distributor shaft to translate to rearward force on cam shaft. And, decreasing RPM when slowing vehicle reduces this downward force, which reduces rearward force on cam shaft, allowing cam shaft to "walk" forward. With no Cam Thrust Plate, cam shaft walks the fuel pump eccentric into the timing chain cover, shearing the fuel pump eccentric mount tab, leaving operator to coast to shoulder.

[Edit: Other research says when the Distributor is mounted on passenger side of engine it engages the camshaft such that a COUNTERCLOCKWISE rotational force from the Distributor Shaft is imparted to the Camshaft, causing a FORWARD force on the camshaft and that Pontiacs use a Thrust Plate to restrain this forward force .... and consequently interrupt shearing of fuel pump eccentric on inside of timing chain cover. Contrary to other engine designs (Chevrolet?) where Distributor is mounted on driver side of engine and so impart a CLOCKWISE rotational force from the Distributor Shaft upon the Camshaft, causing REARWARD force which is naturally checked by (internal camshaft mount?).

I also have a concern for placement of the mounting tab on the fuel pump eccentric. The one upon first failure was nearer to the center of eccentric mount hole and so engaged the cam shaft sprocket at the perimeter of cam shaft sprocket mount hole, in a little square cutout.

All available eccentric replacements I have found have the tab just a bit further out from center so engage the cam shaft sprocket further out from center, in a circular dimple on the face of the cam shaft sprocket. I am concerned if increased centrifugal force from further-from-center displacement or ill-matched circular dimple to square tab caused what I believe is second failure (I have yet to muster energy to see if second stranding was caused by second failure of fuel pump eccentric. But, I am getting close!).

Finally, while trying to motivate myself to get going on it, I wondered if timing chain cover might have been shaved on a planer during past recondition, eliminating just enough clearance between fuel pump eccentric ... even if it does migrate a bit ... to cause eccentric tab shear.

Oh, and I do not want go with an electric fuel pump because I am still telling myself I enjoy too much the joys (like the car sitting in garage for months while I gather enthusiasm) of originality.
 
Last edited:

Raleigh251

New member
1
0
1
There should be a trust plate on the 389. There is a guy that makes a Pontiac Cam trust plate with roller Torrington bearings. These are great for all cams but are a must for roller cams. His name is Scott Sims he does the work for $285 his contact info: 812-350-8637 or simsb53@gmail.com. Or you could pick up a cheap stock one for $50.
 

cammerjeff

Member
454
8
18
Belleville MI
As far as I know the only GM Pushrod V8's that do not have thrust plates are Buick's. Every Pontiac V8 I have seen has a thrust plate behind the fuel pump eccentric.

Here is a link to a complete assembly on E-bay, you can see the trust plate

 
Your email address will not be publicly visible. We will only use it to contact you to confirm your post.
Top