View Full Version : Odd crankshaft question
01-07-2005, 10:20:00 AM
I have a 327 that is not drilled for a balancer bolt in the end of the crank. I need to drill and tap it for a 7/16-20 bolt, but the question is how deep is the bolt hole?
Anyone got a 350 crank handy that they could check on how deep this hole should be?
Any tips on how others may of drilled this hole while the crank is in the motor?
01-07-2005, 10:44:00 AM
I would drill it deep enough to allow about 1 inch of threads.
01-07-2005, 10:58:00 AM
If you have a really good balancer, you won't need that bolt....just about ALL the older 327 cranks (I never heard of any with it) don't have any holes in them for a bolt. I had messed with one from a 68 and it was 'w/o'...
01-07-2005, 11:24:00 AM
I don't know how tight you twist the little motor, but I'd feel better knowing it had a bolt holding the balancer on. Would be my luck that it would decide to let go when reaching over to the carb linkage for a quick rev.
01-07-2005, 11:29:00 AM
I wanted the bolt hole to make it easier to remove and install the balancer in the near future for a cam swap. The original one is toast, have an 8" one from a ZZ4 to replace it with.
01-07-2005, 12:24:00 PM
Are you sure the ZZ4 balancer is compatible with the 327 crank?
01-07-2005, 12:48:00 PM
Should be, it's the same balancer that they used on the 70-74 LT-1 350's. I understood that all balancers would fit on a SBC crank, just the hub dia. and thickness were different.
01-07-2005, 01:02:00 PM
Just make sure you drill it straight. Use cutting oil on the drill and on the tap. You will need a bottom cutting tap after you get the standard tap started. Back off every 1/4 turn and keep plenty of oil on the cutting surfaces. You want a 1/2" -20 thread tap and the proper 29/64ths drill bit. I would drill down 1.5 inches into the snout of the crank (put a piece of tape around the bit at 1.5 inches so you know the depth), and try to thread that all the way down if possible. The more threads the better. It has to hold 65 lb ft of torque on the wrench.
Whatever you do, don't force the tap because they are brittle and will break off. Drilling out a tap is damn near impossible.
I had to punch my crank to an un-godly 3/4" -10 size to repair it with a steel insert. No fun in the car, trust me. Always use a balancer puller and a balancer installer. Never trust the threads to pull the balancer on or you will be sorry.
01-07-2005, 01:22:00 PM
Thanks, 73454. I believe that the small blocks use a 7/16-20 bolt which would need a 25/64" drill. You may be thinking about your Big Block.
Luckily, my motor is on some storage skid legs and not in the car.
Maybe someone will have a SBC crank laying around that they could stick a pencil down in and measure how deep the hole really is.
01-07-2005, 01:44:00 PM
Hole depth is 1.562"
01-07-2005, 05:52:00 PM
<font face="Arial,Verdana" size="2">Originally posted by BlueZ/28:
Hole depth is 1.562" </font>
Or basically 1-9/16. I do believe it is a 7/16-24 thread.
01-07-2005, 07:02:00 PM
Ya know, this brings up an odd question. For decades we've been told DO NOT USE A HAMMER to install the balancer. How many of these old non-tapped cranks are there still around,, and how the hell do they think the balancer ws put on in the first place!!! I was 30 before I used anything other than a 2x4 and a BFH to install a balancer. OK so that was a few decades ago too, but still....... http://www.nastyz28.com/ubb/wink.gif
The standard depth is 1-9/16" deep, 7/16-20tpi, and you need a case hardened bolt 2-1/4" long, and LARGE, THICK washer.
01-07-2005, 08:32:00 PM
Okay, so 1-9/16" it is! Thanks, guys. Summit sells a dampner bolt and washer for $5, Moroso for $9 and ARP for like $18. So I'll prob get a Summit one. Since they only sell 7/16-20 bolts I was pretty sure that's what it was.
When I first put this motor together 20 years ago I think I used an ingot of lead (from casting bullets) and a BFH. Thought it might be easier/better this way.
01-07-2005, 09:43:00 PM
Sorry... I was thinking big block bolt size. http://nastyz28.com/ubb/icons/embarassed.gif
01-07-2005, 10:56:00 PM
I would have tapped her oversize and ran a stud bolt AND nutted the damper on before I woulda drilled her out THAT big..
I can see where that musta been a heck of a job...
As for beating the dampers on, I figure GM has a little press that bolts across the front of the crank to the four early motor front mount holes and they just pushed it on real easy!!