View Full Version : blower pulley press fit question?

07-19-2015, 03:45 PM
I am trying to use a late model case with Gtp rotors and snout in a carbureted application for my '56 ford pick up with a 312.

The Pontiac snout is longer than ours, and will help get the pulleys where I need them while centering the blower over the engine better. I purchased a custom made drive mandrel/hub for the SC shaft to use bolt on interchangeable pulleys, similar to the cobra/Shelby/lightning guys. The Pontiac snouts drive shaft is also bigger than ours, and setup for a press fit, not a keyed slip fit. The shaft diameter is .780" the steel pulley that came off it is .770" , or a .010" press fit. My mandrel/hub is aluminum, and I will need to open it up to the correct press for the Pontiac shaft. My big question is does the aluminum material change the amount of press needed? How much press should I aim for when machining/honing? I am thinking that the aluminum will need a tighter press (ie, a SMALLER hole in the hub), but .010" seems pretty tight already. Anybody got any experience with this can help me out?



07-19-2015, 10:35 PM
do you have a pic of your setup?

07-20-2015, 12:59 AM
Not yet. Still not developed enough to really call a set up :D. Just basically a late SC case with GTP rotors and snout (longer). I have an innovators west balancer, a cog crank pulley from a 5.0, and a roots style blower and tensioner pulleys (what the hub is set up for). I have either a 4bbl intake, or an Offy 6X2bbl that will need a plate/box adaptor to mount the blower. It will be basically up side down (blower outlet directly into intake), with no inter cooler (may go with meth/H2O injection). Carb will be mounted behind/above the blower with a simple box with a square bore top plate. Carb will be a mech secondary with boost ref power valve.

I have been informed that a .010 press is crazy, and should be expecting no more than a thou or .002 absolute max. I measured the shaft with mic's, and the bore of the pulley with calipers (all I have), and believe one or the other may not be accurate (I measured an m112 shaft and pulley, and the GTP shaft and pulley, and cross checked the calipers to the mic's and came up with the same #s 5 or 6 times on both :confused:)

I'll try to snap some pics when time allows


07-20-2015, 07:06 AM
Running some quick numbers you only need 0.0004" interference. I wouldn't use calipers for this you will need a set of micrometers

Though read this https://www.engineersedge.com/class_v.htm

07-20-2015, 07:10 AM
It's not a tapered shaft is it?

07-20-2015, 01:01 PM
Bowez, thanks, I finally found that page last night too. All the other engineering links were for equations that made my eyes go crossed :eek:

820, I am planning on checking that very thing as soon as I can get to the garage. I could only get the calipers in one side of the pulley, and every shaft measurement was taken in roughly the same spot in the center of the shaft.


07-20-2015, 01:32 PM
Get any pictures?

07-20-2015, 02:32 PM
OK. 820, we have a winner. The hole in the pulley DOES appear to be tapered. Hard to measure with the dish, I cant get the calipers into the hole straight.

Now. I have pictures from my phone, BUT I am a techno-idiot. The jpeg files r 3MB, the site will only allow 400KB files to be uploaded? How can I either add larger files, or reduce the size of the pic file I take? I have never uploaded from my comp to the site b4.


07-20-2015, 03:00 PM

OK. Resized the pics and tried again.

One shows what I hope will work as drive set-up. I may have to go to a jack shaft arrangement like we run on the SC's, but I hope not, it would make things MUCH more complicated. The blower pulley is a "standard" roots style M8 pulley, so size is easily adjusted, good, because I think the overdrive with this combo would be WAY to much!

Another shot shows the case/snout/hub/pulley arrangement. The pulley is just bolted to the end of the shaft, as I need to resize the hub hole. This shot also shows the factory Pontiac pulley (large) and a factory Lightning M112 pulley (smaller).

The last shot shows the Thunderbird 312 with the 6X2bbl intake I MAY use. Use your imagination and remove the carbs, then a box will be fabbed to sit between the intake and blower. The box will be a trapiziod shape, basically "leaned" forward to get the pulleys lined up. If I can get the blower centered close enough, I may use a 4bbl intake instead, again this would make it simpler, and would get better distribution I think.


07-20-2015, 04:11 PM
how much boost are you planning on making, think the M90 will be too small for much over 6psi

07-20-2015, 05:02 PM
Luv'd the 312...

BTW, this is how Ford did it way back then:

07-20-2015, 07:10 PM
That cog belt will be loud. you will love it. I have a three inch open primary belt on one of my bikes, it sounds like I have a blower.

07-20-2015, 07:46 PM
Loud is GOOD:D

Luv those 55-7 fairlanes and birds! Can not, however afford a "real" one :rolleyes:

6psi would probably be ideal, I however am greedy :D Much over 10 psi and a y-block will give you problems, the combustion chamber is not terribly efficient, and will detonate beyond that. My original plan was a lightning M112 blower but decided it would be to much for the motor. There is also the fact that I really cant use any type of cooler with a draw thru set up, except the possibility of H2O/meth inj. I am also concerned about distribution.

This will all be going in my "old farm truck", also affectionately known as "Barney", NOT pushing for the Nth degree.63812

07-20-2015, 08:06 PM
The main reason I say use micrometers is calipers usually do not have the resolution necessary for these measurements. But unless you have your own lathe looks like you will need to take this to a machine shop. While .0004 is the average I'd settle for .001, mainly because of cost.

07-20-2015, 08:43 PM
Does the shaft taper, or just the pulley? I dont know if they are made that small or not but look for telescoping gauges to measure an inside bore. Or as suggested haul it to a machine shop.

07-21-2015, 04:10 AM
Yeah, I know calipers aren't gonna get it for a job like this. All I have :rolleyes: Bore mics are high on my list of tools to buy.

Got a buddy with a machine shop, it'll be on its way there as soon as I can find time to cross paths with him. He was expecting the taper as soon as explained the project, first question was does it have a bolt holding it too, or just the press? When I told him it had a bolt he said "well then it'll be tapered, I can't hone it it'll have to be machined". The shaft is not tapered, apparently the further you press it on the tighter it gets. I'm going to ask him to fit it on the tight side, more like .001 than .0005, then heat it to 450 or so in the oven (mamma LOVES when I do stuff like that :p) for a shrink fit if needed. If it cracks, or whatever I'm not out much, I'll just try again, and I'd rather error tight than have it spin on the shaft. Also concerned about the different expansion rates between the steel shaft and alum hub.

I have been wondering if I should disassemble the snout and have him machine it down to SC size and machine in a key way. The bearings and seals all APPEAR good, and I really don't want to tear it down. I also would be concerned with weakening the shaft, the hardness, how deep that goes ect. I'll ask his opinion on that when I drop it off.


07-21-2015, 07:24 PM
If he has the equipment to get you .0004, that will be plenty. If we get a 1" shaft at work that is .0003 over it can be beat/pressed on but requires a puller to get it off.