This is an update of information posted awhile back under "small block cam suggestions". As some of you know, I'm the unlucky owner of a Bill Mitchell sb 427 (4.125 bore x 4.00 stroke) With less than 50 miles on it, the engine developed a knock so loud it literally sounded like someone hitting the side of the block with a ball peen hammer. In less than 200 miles oil pressure went to zero and the knock remained. We tried different oil weights, but nothing helped. Once warm, the pressure went to zero and then the gauge acted like a tach. Blip the throttle, pressure comes up - to a max of 32psi, but right back down to 4-5psi at idle. Neither BM nor the "warranty" company would do anything at the time (the AG offices of TX, MI, and NY have since convinced them to behave a little more like a business and less like con-artists) so I was forced to tear the thing down in an attempt to find the source of the problem. I wanted a hydraulic roller setup and thought the paint job was crappyu anyway, so what the heck.

We're still kind of scratching our heads regarding the *exact* source of the noise, but here is what we found with the mic and caliper:
main clearances = .0038 to .0042
rod clearances = .0030 to .0032
cylinder clearances = .0055 to .0060
From a parts standpoint, the oil pump was a standard volume sbc melling. The oil pan is a Milodon 30908 - plenty of room, but the windage tray had been beat down so bad by the builder, it was useless - all the vanes were hammered shut. There was no sign of bearing damage - rods or mains, nor were there any broken rings, etc. Four of the piston skirts were scratched, but the cylinders were fine. The pistons have no distinctive marks - most likely a forged import. The rods are eagle h-beams and the crank is a scat forged. Cam is a crane hyd with speed pro lifters. Timing chain was garden variety $12.00 unit - same for the balancer.

What do you guys think? Are any of these clearances suspect?

Since the TX, MI, and NY AG offices are already coming down pretty hard on Mitchell and the "warranty" company (I have to put that in quotes because their real job is to think up any and every reason to delay and not pay), I had my last conversation today with Billy Mitchell (daddy's boy) regarding the tolerances we found. Surprise, surprise - the tolerances we reported were "exactly" on target for their specs. For a street motor, I asked? Sure, he said, you don't want it any tighter on the street - you'll spin bearings. In fact, he went on to tell me that GM, Edelbrock, and others do the same thing (he didn't know I had the GM spec right in front of me and it is NOT the same). When I referenced oil system flow and pressure studies done by Callies, Clevite and FM that refute his spec's he said none of those guys know what they're talking about because they don't build engines. Really? No kidding. So, I guess I've heard from the best of the best and all that R&D those other companies are doing can just give it up. If they are not checking with Bill Mitchell first, they're wasting their time.

But, back to the beast. This baby is going back together with a UD roller hyd, Crane pro timing chain, bored for new SRP's, proper clearances for street application and should run for years to come. comments/feedback appreciated.

Mike

Interesting experience. Thanks for sharing it with us. I just boughta how to Hot Rod SB MOPAR engines by now-retired bonafide m.e. with lots of performance experience and many years employed by Chrysler. His book says "very close" to 0.0025 on mains and "close" to 0.0025 on rods for most high performance racing engines. A little extra clearance makes lots of volume for the oil system to fill. I ain't an expert or hot rodder, but I'd say they're tryin' to blow smoke up your tail pipe.

My $0.02

Mike
imho those main & rods clearances are too loose to use a std volume pump as you can see by your low oil pressure.Before your bore your block,what does SRP recommend for piston to bore clearances??

I'll check with the machinist. He's kind of exclusive on SRP so I think he's got it down, but I need to ask that anyway.

Thanks,

Mike

Mike,
None of the clearances you have should be causing the oil pressure to drop to zero, nor should they be causing a knock. If the bearings were making the noise, you would see evidence of it in the bearings. The piston to wall clearance is just about right for most high expansion forged pistons like the SRP's etc. Heat one of the pistons up to about 200 deg in a pot of boiling water and then measure it...you will be surprised. Before you condemn the crank clearances...I suggest that you bolt a couple of rods together with bearings and also bolt on the main caps with bearings. Then take the crank, block and the couple of pistons down to your machine shop to measure them the correct way...with a micrometer and a dial bore gauge. A set of calipers is not accurate enough to check bearing clearances. The only way to do it precisely is to measure the crank with a micrometer...then use the same micrometer to set up a dial bore gauge. Then use the bore gauge to measure the bearing ID. No other way is accurate...period.
I think you need to keep looking to find the real cause of the knock and oil pressure loss, I dont think it is in the clearances. I am not saying I would build it that way but I have seen them way looser than that and run just fine for a long time.
Just my opinion,

Bill:

Appreciate the quick feedback. The measurements did come from the machine shop - G&G in Tomball. They have all the right stuff and were very careful in procedure. And you are exactly right. The clearances may be suspect, but we did not prove or disprove that clearances were the source of the knocking.

Mike

Mike,

just posted over at team Camaro so bear with me..

How are the piston pins and piston pin bores b/c sometimes they knock if by chance one was started up dry or maybe even too tight??? pdq67

I was reading this and thinking the same thing, "How are the piston pins?". Also, is the knock a "once per revolution" or "eight per revolution" thing? And did they check the thrust clearance on the crank? I don't think any of these would cause an oil pressure problem, but I like to attack one thing at a time.

I will check. Thanks for the prodding. I jumping in the truck and heading to Houston in an hour or so to review the entire set of measurements.

More later tonite or tomorrow,

Mike

Another area to check is rod side clearance. If you'll be running those loose clearances on the rods, I would hope you side clearances are tight enough ( .015) to keep some oil on the rod bearings.

here this will help


http://www.babcox.com/editorial/us/uhs89734.htm

http://www.thirskauto.net/BearingPics.html


http://www.babcox.com/editorial/ar/ar20128.htm

http://www.babcox.com/editorial/cm/cm99828.htm

http://www.thirskauto.net/Engine_Thrust_Bearings.html

http://www.aera.org/tech/tb1465r.htm

http://www.dragnbreath.com/Clevite/TN%2016.html

http://www.dragnbreath.com/Clevite/TN%2022.html

http://www.dragnbreath.com/Clevite/TN%2028cambrgs.html

http://www.dragnbreath.com/Clevite/TN%2028cambrgs.html

http://www.diabolicalperformance.com/clearances.html

http://www.jimcookperformance.com/TechNotes/TN%2028cambrgs.html

http://www.babcox.com/editorial/ar/ar10180.htm

http://www.babcox.com/editorial/us/uhs69720.htm
let me point out this chart[/b]
http://www.diabolicalperformance.com/images/clearances1.jpg

http://www.diabolicalperformance.com/images/clearances2.jpg

http://www.diabolicalperformance.com/images/clearances3.jpg
http://www.diabolicalperformance.com/clearances.html
heres other info,
http://www.babcox.com/editorial/ar/ar10180.htm

http://www.thirskauto.net/BearingPics.html

http://www.waynesgarage.com/docs/oil.htm

http://www.jimcookperformance.com/TechNotes/TN%2023.html


http://www.cryoeng.com/images/EngineDurabilitySecrets.htm

http://www.melling.com/engoil.html

http://members.aol.com/carleyware/library/engine2t.htm

WOW! :D I am one stoked Cheveller. Thanks for the info.

I was just with the builder and rod side clearance was as I'd posted earlier - are you ready for this? .045 was the worst - .026 was the best - he forgot to give those to me when he gave me the bearing clearances the other day. The measurements on the wrist pin will come tomorrow. Like I say, I am very lucky that this thing didn't just come flying apart. The builder is measuring very carefully, but got pulled away from my project to run a guy's new motor around in 2 days because he it the wall @ 140+ last weekend.

Again - thanks VERY much for all the info. This site is my lifeblood in my quest to get this beast roadworthy.