FreeSpirit

Can anyone confirm what the part numbers are for the fan switch and knock sensor, for a 305 TBI?

According to Rockauto, the fan switch is ACDELCO Part # 154315 and the knock sensor is STANDARD MOTOR PRODUCTS Part # KS2. (http://www.rockauto.com/catalog/rafr...&parttype=4312)

Neither of those parts fit the engine in my car. The thread sizes are different.

It's funny because the parts store that sold me the KS2 sold me ACDELCO Part # D1855B as a fan switch, which looks like the Z28 fan switch (based on what that rockauto link says and the pictures). That fan switch fits my car, but the knock sensor does not.

So can anyone out there confirm what the part numbers are for a 305 TBI? Up until this point I thought I had a 305 TBI, but it's possible someone had swapped the block a while ago, and for some odd reason kept the TBI top end. Because it's definitely slow like a 305 TBI would be, and the intake is definitely a 305 TBI intake.

sofakingdom

iTrader: (1)

The knock sensor is ¼" pipe thread; has been since 1955. That's about 5/8" outer diameter. Doesn't matter if the motor has been swapped. The threads might need to be chased, but they're all the same size.

Fan switch however, goes in a hole in the head; and that HAS changed. It was originally ½" pipe (about 7/8" OD), then on about 1980 the factory shrank the hole in the head to 3/8" pipe (about ¾" OD). If you have that situation, i.e. your motor has early heads with the large hole, there is no stock fan switch that will work. Instead, get the right switch for your car, and go to a hardware store or even a home improvement store, and get a brass 3/8"-½" bushing.

FreeSpirit

Well here's a picture of the difference in thread sizes of the knock sensors.

I didn't take a picture of the two different fan switches, but you can even kinda see in the Rockauto pictures how the RS fan switch has a bigger size thread than the Z28 fan switch.

sofakingdom

iTrader: (1)

Those look exactly the same to me. ¼" pipe. Exactly what they have been since those wizard engineers at GM designed into their blocks, knowing that electronically controlled fuel injection was coming someday, and wanting to prepare for it; but they didn't want those evil minions and henchmen at Frod & Xler to suspect what they were up to, and disguised the ¼" pipe knock sensor hole as the coolant drain. In 1955.

As I also said, they changed the size of the hole that the fan switch goes in, sometime around 1980; the hole went from ½" pipe (about 7/8" dia) to 3/8" pipe (smaller, about ¾" dia). Z28, RS, truck, Vette, Caprice, doesn't matter; 80-back (more or less) = ½" pipe, 80-up (more or less) = 3/8" pipe.

These are the facts. That's all there is to it. Whatever you think you are seeing in the pic, if it doesn't agree with those facts, then it's not right.

FreeSpirit

It's been a while since I tried, but neither me or my friend who is a mechanic by trade could thread in the knock sensor. Besides the picture, we both could see it was different size in person. Maybe the PO stripped the threads and retapped the hole?

The fan switch I just tried a few days ago, and it was different size thread as well. But the new one I got was a bigger size thread, not a smaller one. I even measured it with a tape measurer to make sure. So it wouldn't be that I have an old block with the larger size thread, when the new switch is the one with the larger thread.

This is the fan switch I have in there right now: ACDELCO Part # D1855B. This is the new one that didn't fit: ACDELCO Part # 154315

The fan switch problem is easily solved, I can just order ACDELCO Part # D1855B. The knock sensor is the question mark.