[BruceWilkie]

Well... I'm in the process of gathering parts to put a motor together. I have a crank from a 389 that was built way back in 62 and never run. (Ive had this thing 12 yrs now) Decided to clean the crank as it was starting to get a bit hazed in spots. I never noticed that what I thought was just a stain of dried up oil is actually a shallow radiused groove in the center of the main journals.

Groove is same width and location as bearing groove and a bit shallower than the bearing groove. It is nicely radiused. I understand what that groove can offer in terms of oiling. However... is this going to create a problem as far as durability?

[J.C.you]

Not sure about a cast crank, yours steel? But i have a sd 389 crank with the same grooves. I believe there was a 421 sd thread awhile back with a crank with the same type grooves. It must have been fairly common back in those days to do the mod. I would venture to guess the mod came out of one machine shop, or (maybe originated from pontiac sd engineering department possibly?)

[cfmcnc]

That full groove main journal will deplete the hydrodynamic oil wedge between the journal and the bearing,Not a good idea!.Bill C.

[Tom Vaught]

As Bill says, the groove is basically a leak path that never allows the Hydrodynamic wedge to fully develop on the crank surface.

Tom Vaught

[Half-Inch Stud]

Original engineer has since gone quiet, long ago. Reasoning was to promote positive. Pressure to the rods (yea, you know). So, thought I was being clever to run Lower-grooved MainBearings in the 1980s (Nunzie said "ya you cando that but not needed"), for the same reason on film load carrying ability HALVED or more. Did that, tore it down and found some shiny Lowers.

So, I went with custom home 6-pack bearing mods; ramps, 80percent width pinch relief, and some sweat. Ran well. Pined away for P-Dudes 3/4bearings as ideal. Most. Recent build, and spare have as-bought bearings chucked-in. HIS

Yawn, smooth crank journal would be best fatigue strength. 3.25Main journal may not are though since deflections are nil. Could go for running it.

[BruceWilkie]

Could I get back some(most/all?) of the hydrodynamic wedge with bearing mods?? Shallow chamfer(s) across the bearing to near edge of bearing??

Seems like the groove itself would send more oil to the rods and that might not be a bad thing.

J.C.you, Sorry for the novel... but...

the crank is a 59 and correct number with armasteel logo thats been worked. Its set up for internal balance and smoothed up very nice... hard to find parting lines or square edges anywhere... it even has a bit of ring to it. It has .010/.010 bearings on it. Had smoothed up early SD rods and polished/blended domed pistons(JE) you can see reflections in. There are no valve notches, the cam was the old E2 solid grind(about .420 gross lift.) and 1960 head castings that were cleaned and polished ports and chambers. Setup for pushrod oiling had super light pushrods and chevy rockers. Also had a Harvey aluminum flywheel on it. Guy that had this motor built got killed in an accident before he got to use it. (New Years 1963) Eventually the car ended up in a field on relatives property. The car still had numbers match 420A tripower in it and the built motor was on the ground and heavily taped up. Pan rotted and had a mouse ghetto inside.(packed everywhere) The chambers were fine as the rockers had been backed off. Ports, valves only minor rust. All oil film had turned to wax. I almost didnt take the motor when I bought the car but figured their might be some parts I might want. The block is a 2 bolt but has all the same strength features as the 420A(same casting was used in all 59's) Bores measure right at 4.12 and minor rust should clean up with a light hone. Glad I threw it on the trailer. It was a few months before I decided to see what was under all the tape.

[Jack Gifford]

Originally Posted by J.C.you

... (maybe originated from pontiac sd engineering department possibly?)

My SD389 crank has no grooves in the main journals.

[lust4speed]

So you are talking a grove around the complete circumference? My first thought is that it might be similar to scoring a piece of glass so it breaks on the line. The nice radius probably might negate the problem, but even with a full smooth radius the smaller diameter of the groove will be the weak area of the crank. Instead of torsional loading being spread evenly across the journal, the flex is all going to occur in the radius.

[Half-Inch Stud]

Tore apart a 1959 "425A" 389; The ArmaSteel cranks Main journals had 2 fine grooves at mid-journal, spaced about 100mils. Seemed the grooves were not deep or wide enough to do anything useful for the Rod pressure. I forget if it was cross drilled. That junk motor had good bearings..and sold the crank to a "parts collector".

[GOSFAST]

I believe only some early 406" Ford's had the mains grooved?

For some years now on our very high HP builds, mostly "blown" BB's, we've been modifying all 5 mains to improve on the "hydro-dynamic-wedge" effect and we have the proof that it really works!

In most app's the groove won't be detrimental to the overall performance. Under extremely high loads the picture changes some!

It was for this reason we started the brg mods (pictured below)! It originally began on the front main only on BB's with the "Gilmer" belt drives. When we established the increased load carrying capacity available we carried it over to all 5 mains!

Thanks, Gary in N.Y.

P.S. Here's a shot or two of the finished mains. We've also run the "old" nylon timing chain setup in some high end (1200+ HP) "blown" roller builds!

[J.C.you]

Originally Posted by Tom Vaught

As Bill says, the groove is basically a leak path that never allows the Hydrodynamic wedge to fully develop on the crank surface.

Tom Vaught

The original thought behind the grooved main must have been a resevoir of oil in extreme applications? You are saying the wedge will not be present when the groove is full of oil, under pressure and centrifugal force pushing oil against the bearing? Or this wedge is dimished in the area affected?

In what applications are fully grooved main bearings used then? The 302 bearings in my dirt motor are fully grooved. Reused them after a teardown this offseason. Beating this 4.5 stroke pretty hard!

[BruceWilkie]

Gary thanks for the bearing pictures. I was thinking possibly a V pattern from the center of the oil hole might do it as well. Maybe V the upper bearing from center then to area where your mod goes straight across. Probably harder to do though.

Seems to me with a full groove, oil feed to the rods would not get shut down as much as no crank groove and only half the main bearing grooved greatly restricts rod feed once oil hole on the journal hits the non grooved bearing.

As far as strength of the main journal... the groove is a bit shallower than the bearing groove and very smooth polished radius. Thinking the breaking point would be higher power/rpm than should be expected from a cast crank combo with 3.75 stroke.

[bignate]

Ok here is a question. If you have a steel crank that this "mod" was done to years ago, can you have the grove spray welded up and regrind the journal back to specs?

Thanks, Nate

[Dragncar]

The oiling of Pontiacs back in the 60s was a issue with the engineers. Some of their ideas were just plain wrong and they tried things to fix the problems. Some worked, some did not.
We now know there was no real reason to use the 3.25 main for any reason. But they did it anyway. Smokey U was very mad about it. His factory "421 SDs" were actually 389 SDs bored out. He wanted no part of the big mains. And he ground down the 421 cranks on his own dime. Ever watch the Nasscar races from the early 60s. Pontiacs made good power and ran well. But always seemed to blow up before long races ended.
oiling problems.
They tried very loose side clearances like the early SD rods had. Giant mains, oil pumps with tons of pressure. I suspect the groves in the cranks were just something they tried that did not do anything.

As far as I am concerned all the aftermarket blocks should come with 2.75 mains and the option of going to 3". Then the smart racers can grind their forged cranks down to 2.75" and run them.
It is a no cost option for IA2s and MR1s to be bored out to 3.25 (who in their right mind would want to do that) So the same thing should apply for 2.75 to 3.00".

[Singleton]

I have a SD421 "990" crank that has the fully grooved mains. This was apparently done on 11/19/62, the date scribed on the first counterweight. It was also stroked to 4.5" at this time, which I assume was not common for the time period. Inscribed next to the date is "Lees Custom Motors". I haven't been able to find any info on this machine shop.

Tony

[BruceWilkie]

Did some diggin today. In my brothers old Mopar race bulletins dated 1973 it states you can groove the stock crank to improve rod oiling. Says to use a .060" radius .060" deep.

Says do not run hemi grooved bearings with this modification. Interesting! The Mopar runs a smaller main journal and larger rod journal than Pontiac.

Without measuring... my grooves look close to .060" radius and just may be about that deep. I'll have to measure. I'll take pics while I'm at it.

Singleton... cast or forged could be welded but it will likely affect heat treat. Best to contact Moldex or other crank company to do the job right. Worth it IMO with a rare piece like you have.

[Half-Inch Stud]

Originally Posted by Dragncar

...We now know there was no real reason to use the 3.25 main for any reason...

I'm not convinced yet, because 3" Mains depends on how much cabbage a fella stuffed into the crankshaft.

REMINDER: the CONN-Rods oil-shear rotates on nearly effective 6" Diameter, making the 3 vs 3.25" Mains MOOT for HP loss.

[J.C.you]

Originally Posted by BruceWilkie

Did some diggin today. In my brothers old Mopar race bulletins dated 1973 it states you can groove the stock crank to improve rod oiling. Says to use a .060" radius .060" deep.

Says do not run hemi grooved bearings with this modification. Interesting! The Mopar runs a smaller main journal and larger rod journal than Pontiac.

Without measuring... my grooves look close to .060" radius and just may be about that deep. I'll have to measure. I'll take pics while I'm at it.

Singleton... cast or forged could be welded but it will likely affect heat treat. Best to contact Moldex or other crank company to do the job right. Worth it IMO with a rare piece like you have.

Input on durability....ran my grooved 3.75 steel unit to the 600hp level on the dyno. 1944 bobweight and had the crank knife edged and lightened. The thing was a chunk before lightening around 76lbs i believe.

[Dragncar]

Its not about the HP loss with 3.25 mains.
Its about the extra centrifugal force "throwing" the oil off the bearing surface.
Its just a bad idea and one of Pontiacs ideas that did not work.

The blocks end up weaker and a cast 3" main crabk will hold up as well as a cast 3.25 main crank.
So why do it? Pontiac went cast in the early 60s to save a few bucks and the engineers thought they needed the extra size for strength.
The did not.
No reason on earth to have a 3.25 main.

[BruceWilkie]

Originally Posted by J.C.you

Input on durability....ran my grooved 3.75 steel unit to the 600hp level on the dyno. 1944 bobweight and had the crank knife edged and lightened. The thing was a chunk before lightening around 76lbs i believe.

I may just put a solid bearing up top drill an oil hole and fishmouth it sideways to help spread the oil. I think the full groove in the crank will be enough to keep the rods happy. I'm not expecting big power with only 8.3 compression stockish 4x heads NA for now. Just want to put something together for now and get that SBC out of my car. I'll put better heads and forged crank in it when I get ready to add the turbos.

The stock cast crank in the 59 420 A motor was around 76 lbs too. Maybe I'll weigh this one when I get out to take some pics and measure the groove dimensions.

That block looks familiar. Present 59 casting is a 2 bolt but still has all the beefy features. (better IMO as it allows splayed caps)