Model B Crankshafts

C

Cajun

Senior Member
I broke a crankshaft between the number 1 cylinder and the front of the engine. I need to balance cost and being bullet proof.

The engine has full circulation (40 psi) and 7-1 compression. I limit RPM's to 3500.

What do you recommend on crankshaft for a Model B engine with full circulation oil pressure? Original B/Cast/Forged/Billet? Rods?
 
Hi Cajun, A division of Scat Crankshafts, I believe it is USA Crankshafts will make you a crank from a rough forging which has been manufactured in India. All the machine work is done here in the USA, I believe. I have not seen one of these, but Scat has a very good reputation. I think that this will cost you about $1250.00. If you ever need one for an A, be sure to look up Burlington Crankshaft. Good Luck!
 
I would first be interested in knowing "why" the crank broke. That engine was actually machined by Bob Meyers at Performance Machine in Greeneville, TN about a decade or so ago, and while it has had a hard life, Bob has built other short blocks for hill climbers (such as Ken Miller, Buck Welborn, et al) and other racers whose engines have seen equally hard lives. Bob's brother generally does the crank grinding is pretty knowledgeable in doing the centering and set-up too, so I would be intersted to see if it was a casting defect, harmonics, or craftsmanship that caused the failure.

Quite honestly, I think some of what you are asking really needs to be determined when you declare what budget range you are wanting to stay in.
 
i would inspect the crank where it broke to see if it had a crack in it and it just got worse over the years, was it an old crack or a new one, a real dark crack line means its an old one, a fairly lite crack line means its probably fairly new, is the babbit on the mains wore crooked if so it indicates a bent crank
 
A picture of the break is attached. It is between the #1 rod cap and the front main bearing.

There is what looks like a "J" on the crank near the break. I don't know what that is. I will remove the engine next week and will see more.

Thanks for your suggestions.
 

Attachments

  • Crank 1.jpg
    Crank 1.jpg
    74.6 KB · Views: 139
Cajun, The crank clearly broke at the bevel line on the crank web at the beginning of the bevel on the outside of the web, and the point that the rod pin joined with the web on the inside. A confluence of major stress points! This is one reason that Burlington Crankshaft, which makes replacement cranks for the A, does not bevel the outside of the crank web on their cranks. Chris at Burlington...
 
Cajun said:
Brent,

Could you put that in layman's terms?

Bill
Click to expand...

Not really!!
LOL.gif


In a nutshell, there are many vibrations inside of an engine. Some you can easily feel oscillations as it vibrates the vehicle, and other ocillations are not as easy to feel and these tiny frequency vibrations are affected by speed, load, counter dampening and all kinds of stuff WAY over my head or knowledge. What I think Chris is saying is anytime there is a sharp edge, there is a great chance a stress riser can form causing a crack. Also, certain RPMs of the engine cause the frequency vibrations to be stronger and thus it is likley that you hit one of the stages and it began to destruct the crankshaft. Crankshaft whipping and the lack of a properly positioned damper allowed it to continue to fracture. Anybody else want to chime in and help out?? :confused:

.
 
You say it has full circulation. Does that mean the crank was drilled for pressure oil from one journal to another? Is it possible the drilling weakened the crank? That would seem unlikely, unless the drilling came close to one edge of the crank web. Was it making noise before it broke, like loose bearings?
 
Cajun said:
I broke a crankshaft between the number 1 cylinder and the front of the engine. I need to balance cost and being bullet proof.

The engine has full circulation (40 psi) and 7-1 compression. I limit RPM's to 3500.

What do you recommend on crankshaft for a Model B engine with full circulation oil pressure? Original B/Cast/Forged/Billet? Rods?
Click to expand...

Cajun,
If 'bullet proof' wallet...'CROWER RODS' !
 
It's the same reason that soldiers break cadence while crossing a rope bridge so as not to cause waves that get bigger and bigger and destroying the bridge.

Boeing does lots of vibration analysis on engine wings so the wings don't fly apart in the air.

Cajun said:
Brent,

Could you put that in layman's terms?

Bill
Click to expand...
 
You must log in or register to reply here.
Back
Top