Thread: Advice on Broken keyway for Crank

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Ok I have a serious problem. While doing some maintance on my super charger I noticed the crank pulley was loose. Meaning I could turn it by hand after the blower belts and all were removed. So I tore it all apart and found that the key is broken in the keyway. Now the engine was acutally running fine prior to this, there must have been enough friction to hold it all together. (Car is just driven on the street- no racing)

1. Anyway I'm wondering if this repairable for an engine that's making 600+ Hp with a 6-71 Blower or should I just be replacing the crank?

2. The crank is internally balanced, if I buy a new internal balance crank doesn't the entire assembly need to be balanced with it?

3. What crank would you reccommend, forged, cast etc, and could I put in a 4.00 stroker crank instead?

4. How can I prevent this from happening? And what happened?

http://www.rlmdigital.com/photohost/Key1.jpg
http://www.rlmdigital.com/photohost/Key2.jpg
http://www.rlmdigital.com/photohost/Pulley.jpg
http://www.rlmdigital.com/photohost/Crank.jpg

Thanks for your help, I'm sort of new to street rodding and learning as I go. :-)

Car info:
'28 Ford Model A convertable
402 Chevrolet Big Block, GMC 6-71 blower, Muncie trans

Denny is probably right on the dampner being loose. I have heard of the key giving up on others that claimed they had it tight though. I've always worried that mine was going to give me problems at some point in time. My worry came after I was told that, after I had built the engine, I should have had a double keyway done and it would prevent problems that single would have on blown engines. Three of the serious engine builders I know do this to their cranks. I always like to get the information before I install and complete the engine . Repairing it depends on the keyway slot damage. If it is useable, I'd do the second keyway IMHO.

I would knock both the key remnants out and check the keyways on the crank snout. From the pics, it looks like the keys just sheared in half. You might just get by with new keys and a balancer, if its keyway was distorted.

The keyways on my 406 were "crunched" on one side by the keys. I was able to put new keys in and fill the gap with JB Weld. After a day, I gingerly replaced the balancer and bolted it to spec. It's held for two years.

From the pics it looks like you may be lucky. If you don't want to tear down and cut extra keyways just take them out, try driving straight down on one end and spin it out,both keys, you'll probably need to prick punch the area to raise some metal because the ballancer has spun,clean thoroughly add locktite,and install with a proper tool, forget the hammer and block of wood. Locktite the retainer bolt too. Hank

Denny's fix is certainly viable. We have used it on small rotating machinery up to 2 MW with success. The washer is called a Belleville. You want the preload specs on it so as to avoid flattening it out completely.


Good Luck, Kitz

sorry no way in hell . when they spin the crank go under size you will need to take the crank out and get the nose spray weld reground to size and you will want a new hub and get the crank cut for the 6.00 clock key way. there needs to be a press on the hub have seen this happens on 454. have been there and done this .sorry to tell you but you do not want to half ass this if it comes rigth off bad things happen. and i have seen that to. there is alot of torsion getting drove thru the hub it need to be right chevys do not use bolts to hold the hub on it is press the crank bolt on engines that do not use press have a much bigger front bolt in the hub chevy is a small 1/2 not like many of other engines that have much bigger bolts .and there is less than a 1/4 inch a round the hub for this bell washer to work i do not think you can get enough tq on the 1/2 bolt to hold the hub on and drive a 3 inch belt and 420 cube blower ??on the 4 inch crank you would need a set of custom pistons about $800to900 for the pistons it would get you close to 427 but i do not know how big your bore is and it would need $$$mallory metal in the crank or run 454 fly wheel and damper or hub .if you are looking for a new crank i have a new gm L88 steel crank if needed

you can use a one long key way at6 o clock or get one more woodruff added between the two stock ones and they sell 8630 alloy steel heat-treated rockwell c40-50 or do all the above

Nobody has addressed the fix yet. Here's a copy of a recommendation from Blower Drive Service.....

When rebuilding, cranks should not be less than a 10/10 grind with 2 keys 3/16's" and ¼ " 180 degrees from each other and should have all the trick work as you would for any high performance engine.

Hey, if he puts a single key back in it, it's just gonna shear again. Roots blowers require 2 keys.

Originally posted by techinspector1
Nobody has addressed the fix yet. Here's a copy of a recommendation from Blower Drive Service.....

When rebuilding, cranks should not be less than a 10/10 grind with 2 keys 3/16's" and ¼ " 180 degrees from each other and should have all the trick work as you would for any high performance engine.

this is crap a 10/ 10 give me a brake who is the jack ass that put that in i did not want to go here but what the hell my car has one key way with ALOT of press the two key way spilts the press

well my mud truck friends weld one on when they tell me about it i plug my earsand tech i did not mean you. but come on 10/10 crank that is crap like some of bds bull like you need to run a hub on a roots blower more crap look at what lunati said about runing no damper on a blower engine and what do you think will break first on a small block chevy two big keys cut in the small crank nose or a 20/20 crank i did adress how to fix this do it right it has to have a press back on it

I didn't look at any pictures. I thought it was common knowledge that blowers require double keyways on the crank.

Whether or not he has to have two keys is a simple computation if he knows the torque output from the blower. We would also need to know what material the crank and pulley is. The key can be made out any variety of materials that satisfy minimum strength requirements.

If the crank keyway is too trashed other options may have to be considered as well.

It is certainly not a unique problem however!

Kitz

[QUOTE]Originally posted by kitz
[B]I didn't look at any pictures. I thought it was common knowledge that blowers require double keyways on the crank.

this quote right here could be bebated stock key ways 4130 steel crank fluidampr 548 7500rpm over 4 years stock keyways

Hey Denny, sorry buddy, I must have glossed over your 2-key reply. Went back to re-read it and you have erased all your posts on this thread. What's up? I didn't mean to upset you. Still love ya buddy, kiss, kiss

NO i think he is mad at me