Technical Big block crank snout on small block crank?

As you may know, small block chevy crank snouts can't really handle the stress from a roots supercharger.

What's involved in putting a big block crank snout on a small block crank?

1) If boost is kept at 5lbs. or under, would this then be okay for a stock crank snout?

2) Do centrifugal blowers put less stress on the crank snout than a roots would?

3) Would smaller roots type blowers such as B+M/Weiand 144 put less stress on the crank vs. a 6-71?

 

I don't have all the answers for you, but..... I have a 144 Weiand on my land yacht, and wing the hell out of it. Stocker set up=1 key=factory.You could have the crank double indexed for security. Depends on where your going with this..... Street, and a lil pedal down time is fine, winging it out with hard track time, or stoplight draggin' I might go with the after mentioned. These thing have been done since the dinosaurs.........And- to my knowledge, which is very nill, no such animal as the big block to small block thing..... Just hammer down!

 

Machine a second key way in the crank and you should be fine.

 

If your boost is only around 5lbs, there is no need for a crank snout change, or a double key way. The small wieand blowers use a ribbed belt and tensioner, no stress on crank. I have one for sale.

 

"What's involved in putting a big block crank snout on a small block crank?"
You order a SB crank machined with a BB snout.

 

When running a 471-671 blower with gilmer belt, belt tension adjustment is pretty critical. Lots of crank snouts have been snapped by the belt being way to tight.

 

Have a crank made with the big block front end on it.
If you are just driving around town, a second key is fine on a stock dia. crank.
If you plan on pushing it, a billet crank might be well worth the money.

Another idea to think about, a girdle. A bearing is placed in the lower pulley and a "framework" is bolted to the front four, early motor-mount holes in the front of the block with a support going into the bearing/lower pulley. Been around a long time.

Mike

 

Really depends on what you plan to do with the car. On the street with the occasional blast through the gears I'd leave the crank stock. If you are going to run a blower drive that uses a harmonic balancer rather than a steel hub for the lower pulley then I'd recommend a steel balancer but still I think the single key will be fine on the street at 5# boost.
I toolmaker I worked with years ago snapped two SBC steel crank snouts with his V6-71 running too tight a belt, but some guys never learn. Listen to Mad Mikey, belt tension is critical.
If you have machine shop access and the required skills and really feel the need to run the large snout you could face off the front main, drill and tap the crank for a LH thread like maybe 5/8 or 3/4 and weld a snout on and then finish grind it. Now you need to accurately locate the keyway in relation to the number one throw, then bore the crank gear and broach it for the bigger snout. Make sure it is a steel gear, not the typical powder metal variety. Oh, don't forget the larger seal needed too. That requires a modified stock front cover, or finding an old Weiand or Moon or some such aluminum cover machined for the big seal.
There is a lot to a big snout, whether you buy or modify a crank yourself. You know what the monkey said when he pissed in the cash register? "This is gonna run into money!"

 

I broke 2 forged crank snouts with a 144 B&M at only 2-3# boost. But, that was after many thousand miles at the torque peak. That's where you get the most crank flex and load shock, especially lugging it at lower RPM's and heavy loads.

They always break off at the bottom of the hole in the snout. You can drill and tap through the snout all the way into the pork chop and use a long grade 8 automotive head bolt to hold things together. Or you can buy a real expensive custom crank with a BBC snout on it, and a custom timing set and cover. Or put a crank support bearing on it if you have room.

Also, NEVER HAMMER THE DAMPER ONTO THE CRANK!!!

 

Most of the NASCAR cranks come with a "big-block' snout already, so a used crank from a race team would be an ideal way to avoid some (now un-needed, & rather expensive)machine work. especially if you can live with a short stroke motor. They use only top-quality cranks to begin with, & you might also consider the Jessel-style timing set they also use(for your project). Not saying the snout addition can 't or shouldn't be done, but WHY when there are better answers readily available?? They also use the smallblock front cover with the bigblock seal. You'll need one. This is common enough that a timing set is offered: "SBC with BBC Balancer"

 

One source, http://competitionplus.com/drag-rac...nk-announces-new-small-block-chevy-crankshaft
You don't just stick the crank in a lathe and cut off the old snout and weld on a new one.
You are looking at 945 for a crank from them though http://www.ohiocrank.com/chevysb_cranks.html

 

I saw one from Howards Machine shop that the crank had the snout welded up like you would when repairing a rod or main journal then it was turned to big block specs. Anybody who repairs crankshafts should be able to do this task. Howards is no longer open. As stated in other post the belt tension and aligment is very critical I hope this helps.. Bobby.

 

Dang, really. Long water pump? Running accessories? I've done a cast crank 350 that gets beat on regularly at 7-9#s depending on air temp and has been for at least a decade, much of that in daily driver status.

 

Another thing some overlook is the tune up, if you run on the ragged edge detonation enters into it too. It might not always be heard but it can be beating up the crank.
I have a forged crank but I know others that also run cast cranks successfully. It has to be a total package, parts, clearances, and tune up.

 

BITD before you could buy a big snout SBC crank that was a common and low cost method of salvaging a broken crank and/or beefing one up so it wouldn't fail. At the time the only alternative would have been a billet crank.

 

5LBs of boost is a waste of a blower IMO. And at that boost level, which you will not even see untill RPMs are near the top, a larger crank snout is simply not needed. Gary or Bill Dyer will tell you the same thing regarding 5Lbs boost and the crank snout.

 

Why can't they?

If built properly with the correct parts they should last.... There's lots of company's selling blower setups, parts and pieces for SBC's....wouldn't think they would still be in business selling if they all fail.

and if the cranks can't handle more than 5 psi I need to talk with my motor builder about setting me up to fail and break my motor

 

Yea! My 406, forged crank, 471, 10Lbs boost, raced, and street driven HARD! No problems with crank at all. I checked the bearings several times over winters, all good, crank still like the day it was put in. I know of several guys running SBC, built mildly, running over 5Lbs boost, and no issues .

 

Mikey.....there was sarcasism there

Eagle forged crank hidden in there plus made 18 psi on a dyno, my motor builder said i'm going to have a handful when i get her running

 

What is wrong with running a crankshaft support ?

 

No, it was in the bus in the avatar. Loaded it weighs 20,000# plus towing a car. One lasted about 110,000 miles and 11 years. Torque peak was 2800 RPM and thats where it's geared at. It's not supposed to work, but it did for quite a while. But then, bumble bees don't have enough wing area to fly either! It's a long story.

 

Nothing! If you have the room, and want one.

 

I Ran a 6:71 8mm set up on street /Race for over 10,000 hard miles on Cast crank ,single stock key way Stock cast harmonic balancer , (No crank support) stock balance rods short water pump 406 chevy 8 pds of boost , 3,300 pd car 6:40's 103 mph in 1/8 6 - 6,300 RPM's
Sift ..Never broke the key way or balancer , when I took engine apart, crank was cracked , but did not break. I guess LUCKY

 

Crazy. So a lot of torque tugging on probably a boat load of accessories, big alternator, pump for air brakes, etc? And most likely on a big stack of pulleys hanging out there. Long water pump might make it even worse with it all loaded so far out.

 

Actually, it had a short water pump with the 6 row serpentine blower drive pulley on the damper, it did have a 105A alternator but I only used one v-belt and it has power steering, no air compressor. It came with a big cast iron pulley on the crank and I found out that there are sheet metal pulleys out there in the salvage yards which I used.

The issue is and was, that the torque peak was 2800 RPM, not 4500 RPM. It was a low RPM torque motor. That has a tendency to break the snout off the crank just like a tractor puller when it bogs down. This motor never went over 3500 RPM.

There were times out west when I'd pull a 10% grade for 45 minutes to an hour getting up some of those mountain roads at full throttle. Like coming up out of Lewiston, ID on 195 toward Spokane, or I-70 west out of Denver. It would pull down to 2800 and stay there where it only had 25 degrees advance at full throttle. It didn't break immediately, but down the road a few months it did.

Also, I had 2 vacuum gauges on it, one under the carb and the other was a vacuum/boost gauge in the manifold. They were always about 4-5" different When the carb gauge went to zero the boost gauge went to about 4#.

With a draw through, roots style positive displacement blower, you always have pressure difference in the manifold, relative to vacuum in the carb. I only ran 3-4# boost, but it was enough to really wake it up!

I know it's not supposed to work, but it did for a long time. Try telling a bumble bee he's not supposed to fly! OK, that's enough, now back to hot rods!

 

They make a SBC blower hub with the BBC seal dia. that gives you extra material from the keyway slot to the seal area

 

don't run the big balancer , use the small early 283 hub . the blower belt and pulley absorbs the crankshaft whip so balancer is not needed

 

Make sure to use a longer "balancer bolt" and re-locate the threads all the way back inside the front bearing journal. That way when you torque that bolt/stud you are placing a compression load on the entire crank snout, which will make it much more resistant to cracking.

 

If you do decide to go with BBC snout I have a sbc timing cover with BBC seal that I'd love to cut loose. I don't know why I even have it.

 

32v : the 283 hub doesn't fit over the BBC hub.