C2 BB Fan Blade

[Robert T.]

Can someone help clarify a discrepency in the 66 Judging Manual in reguards to the configuration of a 1966 Big Block Fan blade? The JM says the fan should be 17.0 inch in dia. with flat head rivets. I thought it was suppose to be 17.5 in. Since I believe that the 17.0 is wrong, what about the rivet type. Is flat head correct? Thanks for any help.

Bob

[Michael H.]

Re: C2 BB Fan Blade

Bob,

I just inspected what I believe to be the original fan blade assy on my 66 425 HP car. The rivets are flat head, as the manual describes. However, I disagree with the 17.0" dimension that they show. I believe the dim should be 17.5". The 1966 printing of the parts book also lists the dim as 17.5" for the 3888366 blade.

My fan is still attached to the car so it's not really possible to measure the OL diameter dim but I did measure from the edge of the large center circle to the tip of a blade and that dim is 7 9/16". Hope this helps.

[Wayne M.]

I'd vote for 17" (pics front and back)

My Oct '66 P&A30 also shows 17.5", five blades for #3888366, but I took a tape measure and I believe it's closer to 17", and not 1/2" more. This is taking a stiff tape to the nearest point to the tips of the blades that are almost opposite, and through the center of the hub. To be real accurate, you'd need a stiff length of spring steel and form a hoop that all blades just touch; then divide the circumference by pi.

Notice the "H", "FRONT" and "66" (last 2 digits of part # ?)
Click on second link and notice the "W" stamping on one of the spiders on the backside.

[Michael H.]

Re: I'd vote for 17" (pics front and back)

Wayne,

Can you measure the diameter of the center hole? If that dim is known, we can use it, plus the dim that I posted, times two, to come up with a true diameter. The number I came up with for the blade length is 7 9/16" from the edge of the center hole to the tip of the blade.

So, the dim would be 7 9/16", times two, which would be 15 1/8". Add this to the diameter of the center hole.

[Robert T.]

Re: I'd vote for 17" (pics front and back)

These are some great responses so far. I really appreciate you guys going to look at your fans and even posting pictures. What a great forum. Thanks. It still sounds like it could go either way reguarding the diameter, 17 vs 17.5. It does sound like the flat heads are correct though.

Thanks Bob

[Wayne M.]

1/2(2.377"dia center hole)+7.375"=8.564x2=17.127"

... outer blade circle diameter. I measured the length of one blade from the edge of the center hole through the centerline of the 3 radial rivets to the tip. The hole was measured with a dial caliper and took 3 readings. Could measure a few more blades to get an average, though.

[Michael H.]

Re: 1/2(2.377"dia center hole)+7.375"=8.564x2=17.1

That's interesting Wayne. Your blade is 7 3/8" and mine is 7 9/16". Using your center hole dim of 2.377, my OL dia comes out to exactly 17.5" and yours comes out to 17 1/8".

A 65 396 blade is supposed to be 17.0 and a 63-67 327 is supposed to be 17 1/8". I dunno.

[Jack H.]

Measuring the diameter of fans can be tricky...

On these early fans, look closely and note there is NO mechanism to achieve balance! On later era fans, you'll find balancing weights attached to individual blades, here/there, as necessary. So, it's a case of one of three situations:

(1) All of these early fans were dynamically 'perfect' (give me a BREAK!)
(2) Designers didn't give a 'hoot' about rotational balance (unlikely at best)
(3) Fans were balanced in a 'devious' manner...

A couple of years ago, I shot this question to a retired friend who'd made his living balancing fans for various commercial applications. He picked up a Corvette fan, looked at it and immediately chuckled. He suggested I take a micrometer and measure the overall length of each blade on the fan and go compute the variance.

So, for grins, I did the exercise and came up with some surprising results. I was seeing differences in individual blade length that were WELL beyond what I'd consider 'normal' for blades being formed from sheet steel stock by the same stamping die!

If I'm right, fans were individually balanced after assembly by 'shaving' material from individual blade tips on an as required basis. If so, the exercise of measuring overall fan diameter by 'eyeballing' a ruler, tip to tip, through the visual center of the fan hub is going to give erratic results!

[Wayne M.]

Measured all 5 blades - & another theory

like Jack H. has suggested, and they're all within about 30 thou or less of equal length. But now I have to say that my blades are not quite 7_3/8ths, but about a fraction of a millimeter less. So now using the center diameter as you suggest (why didn't I think of that !!) I compute overall diameter at 17.048".

This fan is off my late 396. Now I strongly believe it is not the orignal, because this car had an "incident" with a school bus in Marshall Texas, around 1970. Pictures of the aftermath lead me to believe that it's original fan may not have survived. So if the then owner ordered another 3872792 (396 with 17.00+/- .06 per GM drawing in Noland's vol 2, pg 286), but original service stock was long gone, even though the part # was still available in P&A30's, my theory is that to replenish service stock, GM was then using the '66-67 center section (spider) with shorter blades, to satisfy the needs of the 396 replacements; otherwise the fan might hit the shroud with the full '66/67 diameter. Is this possibly what the "W" coding on the backside might represent ? or the "H" on the front ?

Now I've measured the blades alone (the piece that's riveted to the spider) and I get 5.612 +/- 0.002, with dial caliper, on all 5 (again through the centerline of the 3 rivets). This compares favorably with the blade dimension shown in by Noland of 5.68", and still computes to overall 17" +1/8th,-zero.

Any thoughts appreciated.

[Michael H.]

Re: Measured all 5 blades - & another theory

Wayne,

I have to think that you may indeed have a correct 65 396 blade assy. The part was still available at least into the mid 70's because I remember ordering, and receiving, these in at least 1975 or 76. The dim that you show are definitely in the range of the correct 17.125" that this blade was supposed to be according to print.

I just measured the blade (only, not the web) of the fan on my 66 and as near as I can with an 8" caliper, come up with 5.790". I know this isn't accurate because I can't get to the front of the blade to get a true measurement, which is likely slightly larger than my number. Quite a bit larger than the 5.680" of the one you have.

I also mounted a dial indicator on the fan shroud and checked the actual length comparrison of all the blades and I'm seeing a max variation of .023". I don't think that the blade tips were ground for balance, but if they were, it couldn't have been very much. In fact, I still don't know how they were balanced to begin with, with blades scattered all over the hub, as far as clock position is concerned.

I think we need more input from people that happen to have the blade off their 65 66 or 67 BB car. Bet that 66-67 blade is 17.5".

[Joe L.]

Re: C2 BB Fan Blade

Bob------

Sometimes, GM convention for the measurement of things is different than the intuitive oir otherwise conventional method. However, the GM blueprint spec for the GM #3888366 fan blade calls for a 17.5" dimension from the center of the hole to the end of the blades.

In any event, the 3888366 fan blade is usually identified by these stampings on the front of the center section of the blade------"FRONT" and "66". Other blade assemblies have the "FRONT" stamped on them but only the 3888366 has the "66". So, if you have that, you know it's the correct blade assembly regardless of how you measure the blades.

[Robert T.]

Re: C2 BB Fan Blade

Thanks again for all the great reponses. I think I'm pretty well convinced that 17.5 with flat head rivets is correct. What started all this for me was the pursuit of an original 66 BB fan. I just called on one that was advertised at 17.5, with the "Front" stamping, but no 66, and no "W" on the back side. Also it had rounded rivet heads. Just wondering if it really is a correct one. Looking in the JM only confused me more. Even though it is the correct 17.5 it sounds like without the W or the flat rivets that it would not judge very well.

Thanks again everybody. It's great to have this kind of expertise and references just an email away.

Bob

[Wayne M.]

And thanks to you, Bob

We go onto a topic that is well worth discussing, and where there's some degree of uncertainty.

[Tracy C.]

help me understand something Jack..

I've struggled to understand how a fan with asymmetrical blade clocking can be "balanced". If all blades were spaced equally, I would understand, but because they are not..this equation doesn't compute in my feeble mind.

I know the blades are clocked like they are to eliminate harmonic resonance. Somehow the balancing concept just flys over my head.

tc

[Mike M.]

Re: help me understand something Jack..

tc: the issue at hand, which is "over my head", is best handled by going to the refrigerator,grabbin a beast, hittin the easy chair, and forgetin about balancing fans.mikie