Questions regarding lower rear rubber cab mount bushings for 91 Chevy K1500 Pickup

[john2007]

Hi everyone,

I purchased some new lower rear cab mount bushings or cushions for my 91 Chevy K1500 Pickup with a standard two door cab. The rubber bushings have what looks like a thin flat plastic ring bonded on one end. On the other end, there is no plastic ring, but the outer edge of the bushing has a small fillet or radius.

What I need t know is, which end is up? Do I install the plastic ring against the frame bracket, or does it go against the round steel retainer / washer that is directly under the head of the cab mount bolts ?

Also, does anyone know where I can purchase just two lower polyurethane bushings or is it always necessary to buy a whole bushing kit for $50.0 or $60 bucks ?

I would appreciate any feedback,

Thanks
John

[Bob K]

I have the same truck only a 1993 model year. I also have a shop manual, so I got it out to be sure I give you the correct info. To my regret it does not specify the assembly direction. However for the radiator support they have a detailed drawing that shows the flat side up and the radius side to the retainer (large washer).

As far as the bushing you ask about, I don’t remember them being more than one piece. You may be looking at a broken part.

Bob K

[john2007]

Thanks for your feedback Bob & for checking your manual.

I went back to the dealer and they pulled up a drawing and it does indeed appear that the end of the bushing with the flat plastic ring bonded to it, is installed against the frame bracket and the end with the radius on the outer edge is installed against the round steel retainer that is between the rubber bushing and the head of the cab mount bolts.

As far as the bushing you ask about, I don’t remember them being more than one piece. You may be looking at a broken part.

I was not implying that the bushing itself was in more than one piece. What I meant was, I was looking for a bushing made from polyurethane which would be tougher than a stock rubber bushing. However, I was not able to purchase just one or two bushings that I needed, I had to buy a whole kit of body bushings for the whole truck for $50 or $60 bucks, instead of just buying the two bushings I needed for $8.00 each, so I passed on polyurethane.

I measured the bushing with dial calipers and If the stock GM rubber bushing wears again, I'm going to see if I can find a machine shop to machine or turn two polyurethane bushings on a lathe for me.

It's interesting that both the bushing OD and ID has a slight taper from the top to bottom. Here are the dimensions of the stock GM bushing.

Outside diameter at the top side with the plastic ring = 3.098" Inch.

Outside diameter at the bottom of the bushing that mates with the round steel retainer under the cab mount bolt head = 3.042" inch

Inside Diameter at top side with flat plastic ring = 2.415" inch

Inside diameter at the bottom that mates with the retainer ring = 2.36" inches

The total thickness or height including the bonded plastic end ring = 1.091" inches.

The thickness of the plastic end ring is = .084" inches

Lower radius on the outer edge of the bushing = 0.138" inch

I don't know if the plastic end ring is some type of composite or not, but it actually wore into the steel frame bracket a little bit, so it's tough whatever it is. If I make one out of polyurethane, I guess I will just forget about the plastic end ring.

Thanks again,
John

[Bob K]

I don’t think the ring wore into the frame bracket, I think the frame bracket got wet and the water was held there for a long time causing accelerated rust in that area. I switched out my frame last fall and found most of the brackets rusted away.

I went to a pick-a-part scrap yard and got all four rubber bushings for $10. The part number is cast into the rubber so you can look around and get the right ones.


Bob K

[john2007]

Hi Bob,

I don’t think the ring wore into the frame bracket, I think the frame bracket got wet and the water was held there for a long time causing accelerated rust in that area

No, it was not rust. This was a brand new 1/4" thick bracket I had a fab shop make for me, which I bolted to the frame through existing holes in the frame. Somehow, the plastic (or perhaps a composite or phenolic) ring, wore into the steel. It's only been on there for a year, and it was well painted and coated with cavity wax rust inhibitor. There was no rust.

I don't know exactly what type of steel the bracket was made from but it was tough to drill with brand new titanium coated bits.

I was surprised to see this wear, I thought it might be a streak of some rust inhibitor I had sprayed on when I installed the bracket, but I ran a chisel over it and there was a slight groove there which appeared to be caused by the plastic ring wearing into the steel, strange as it might sound.

I know that some plastic self lubricating bushings can significantly wear their steel shafts after they have been used for some time.

John

[john2007]

Actually, the groove may have been caused by the fact that when I replaced the bushings the first time, due to some modifications I made, I should have added a 1/2" thick spacer between the end of the center steel tube on the upper bushing and the lower retainer that goes under the cab mount bolt head. Since I did not add the spacer, when I torqued the cab mount bolts down, it compressed the lower rubber bushings 1/2" thinner than they were supposed to be compressed, which is why they only lasted a year.

When I replaced the lower bushings this time, I added the 1/2" thick spacers and everything looks right and the lower bushings are not over-compressed.

Since the original bushings were compressed 1/2" thinner than they were supposed to be, there was a fair amount of force where the plastic ring meets the steel frame bracket. The groove I saw may not have been from wear but may have possibly been a slight dent from the compressive force.

Either way, it seems unlikely & I was surprised to see the groove.

John