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Blower Sizes-101 (A "Visual" approach)
Here are some images of different blowers that may help to identify a blower that you may have.
Please keep in mind that this particular page has information mainly about Detroit Diesel GMC (General Motors Coach) blowers.
Some aftermarket companies like BDS, B & M, Bowers, Dyers, Littlefield, Hampton, Weiand, just to name a few
have built their own blowers and have taken blower building to the next level, but still base their design and got their "grass roots" from the GMC Detroit Diesel blowers.
 
Click the image to view large image
Here is a 6-71 "small bore" GMC blower. The center case is 15" long,(8-71's are 16" long center case only, not the whole blower) and in the second image you can see a
few differences from a small bore, and a large bore. The biggest "Viewable" difference is the "thickness" or "height" of the center beams
(the red arrow in the center of the pic) and the
"dimples" or "pockets" (red arrows at the top and bottom of the pic)in the ends of the blower.
If your blower
has tall flat beams that stand up in the case similar to these images, then it's a 6-71 small bore. But don't
always go by the "pockets" I noted.. Some small bores have solid faces on the top too like a large bore does.
The easiest way to tell is just look at the beams or "fins" if you will on the top. Fins? then its a small
bore, Round beams, then its a large bore (in most cases) hahah...no pun intended..
Now...just to confuse you a bit, if someone gets their hands on a virgin blower, and they convert their blower to "street use" then the "fins" or "vertical" beams in the centers of
the intakes are generally machined out. So this leaves you still somewhat wondering "what size is this thing?". Read on...
 
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Ok, these images show a little bit more on the large bore and small bore blowers.. See the "hour glass"
figure of the bores on the first image, see how the case has nice round contours on the skinniest part of
the "hour glass" shape. This is another way to tell that its a large bore in the event that you cannot just measure the bore.
If you
have the case taken apart this far though and have a set of calipers or a good measuring scale then you
can be for sure of what you have by measuring the bore. A 5.525 inch diameter bore is a small bore. A 5.800 inch diameter bore is
a large bore. Ok, the second image shows a stack of cases I have here in the shop. Once again the hour
glass figure is what I am looking at.
The two TOP blower housings are large bores..The bottom one is a small bore...see?
 
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Ok, Now for some differences in blowers all together.. You should already know how to tell the 6-71
differences in blower sizes from one another. So let's move on to some other sizes.
In these images, the first image has two blowers. From
left to right, a 6-71 small bore blower, and a 4-71 blower. The obvious way to tell is by the physical
size difference, but you can always
look at the bolt patterns on them..
A 6-71 has 8 bolt holes (4 on the left and 4 on the right) that hold it on the manifold, and 6 top
carb flange bolt holes. The 4-71 well, you get the point by looking at the images...
The second image is to show you the sizes from left to right a 4-71, a 3-71, and a 4-53 blower. Now..
the thing here is that a 4-71 and a 3-71 both have the same amount of bolt holes, but a 3-71 is really
short, and has NO center beam in the case on the top or bottom.
 
Click the image to view large image
In these images, the top left image is a 4-53 blower. The second top right image has a couple of 6V53 blowers.
The bottom center image is an entirely different blower type. They are Eatons, made for Ford and other auto manufacturers but mainly Ford. Eaton also
makes "superchargers" for other types of cars and trucks including the new Ford Lightning truck that came
with a Harley Davidson in the back.
These smaller blowers are a bit different than their bigger cousins above. The 4-53 and 6V53 are "pressure
oil fed" type blowers meaning they need pressurized oil from the engine to stay lubricated.
The Eatons on
the other hand are like the bigger "71"
series blowers that use oil in the blower bearing case in front, and have sealed bearings in the back.
Now, there is a note on that though... 2-71, 3-71, 4-71, 6-71, and 8-71's (the 71 series) of blowers are
crank case oil fed if they are on a diesel engine. The "conversion" to gas use is where all this changes, and just about all blowers go to
isolated oiling systems and sealed bearings all together.
 
Click the image to view large image
Ok, lastly, the "other" type of GMC blowers.. These are the most common of the GMC blowers. They are the "V 71" series of blowers. We will talk about the 6V71 and 8V71 here due to these two "V" series blowers being the ones you
will typically run into. The "V" signifies the layout in which the cylinders are with the Detroit Diesels. I.E... 6V71 is 3 cylinders per bank and 71 cubic inches per cylinder.
In the first image to the left we have a typical 6V71 GMC blower. Some of these have the "governor" like what you see in this picture attached to them. Some don't. Doesn't really matter though as the "core center sections" are the same on both.
The fastest way to tell what you are looking at on these without knowing anything about GMC part numbers is to look at the "fins" on the side of the case. Two sets of fins signifies a 6V71.
Ok, in the other image to the right we have an 8V71. The fastest way to tell this one, is that it has three sets of fins on the side.
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