250 fan belt help needed please

[Love 70 Chevys]

My truck has the same exact P/S pump and alternator stock positions as shown in the diagram. Could anyone help me with the cross section and length of the v-belt? A part number would be even better! A question I have is why are all of the pulleys double except for the P/S pump? Does the alternator pulley have enough wrap? I have seen pictures on-line where it looks line one long belt wraps the double pulleys twice. I am confused and obviously in need of help. Thanks in advance for any advice.

[RichardJ]

In the diagram you have posted, an alternator belt would wrap around the crank pulley and the Alt pulley only. The PS belt wraps around the second groove of the double Alt pulley just to keep it out of the way. I assume they didn't show the Alt belt in order to show the PS belt more clearly.

My setup is similar, but the Alt belt is behind the PS belt. I used a 250 PS bracket, but made modifications to get it to work on the 292, but you can see how the double Alt pulley is used.

The AC crank pulley is a bolt on. It has the advantage of trapping the outer ring of the Harmonic Balancer should it fail and it will eventually.
I installed the outside pulley for insurance, long before I added the AC. I had previously dropped two balancer rings and didn't want to do that again.

[Jason Banks]

I can't look right now...but when I did mine I ordered a Gates 7500 Gates 7508 and Gates 7512 belt.

I'm pretty sure I used the 7500 and can confirm that tomorrow.

[Love 70 Chevys]

Thanks for the help guys! Both replies helped to solidify this in my mind. My set-up will require only one belt, just like the diagram. I was confused as to why there was a second set of pulleys, but it is for AC trucks, which mine is not. I have a gates 7400 belt here that I was able to cut and measure what was missing and it came out to exactly the O.C. of a 7500 with the alternator in the middle of its adjustment. Would have taken me a long time to get to the answer without the great advice and pictures!

[RichardJ]

The diagram in the first post is only showing one belt, because they were highlighting the power steering components.

If you look at the belt contact with the Alternator pulley, you can see that there is not anything close to what is needed to spin the Alternator. The belt would just rub against the alt pulley and squeal.

The single belt running across the water pump pulley would also be less than needed. The belt has to turn the WP and the fan. Even with the belt as tight as you could get it, it would probably squeal every time the engine was accelerated.

If you look closely at the diagram, the alternator pulley is a double groove pulley.
A single belt will work without PS and give enough pulley wrap.

[Love 70 Chevys]

I see what you are saying. That makes sense. So this is where I think I end up. I'll have one belt (Gates 7500) just like the diagram shows on the inside. In front of that I will end up with another belt that runs from the water pump pulley to the alternator. I tried going from the crank pulley to the alternator like your picture shows, but my alternator sits up a little "higher" and there is no tangent line between the two without the belt running into the water pump pulley. Everything clears if I go from WP pulley to the alt. The first belt ties the crank pulley to the WP pulley so I think it should still be fine as far as getting drive. Unfortunately, I have to hope there is a belt length that just about nails the required length between the WP pulley and alt pulley perfectly as there is very little adjustment with the PS pulley and that is the only thing that can move. Something that seems so simple can get complicated in a hurry can't it! Would like to get more of your thoughts on the topic, you have some good experience here.

[Love 70 Chevys]

Picture:

[Love 70 Chevys]

OK. Finally there. Never should have taken me this long. Found this diagram in a buddy's GM factory assembly manual for the 1969 el camino project he is working on. 250's went in those also. Here it is. Ended up using Dayco belts. Mostly because they had a really good website that went into all the configurations even for 1970 C-10's and their recommendations matched up with my measurements. P/N 15500 for the belt in the back that goes to the power steering, alternator, crank pulley and water pump pulley. 15400 for the front belt that goes to the alternator, crank pulley and water pump pulley.