Tolerances in original BBC assemblies

[Steve Antonucci]

Hi all !

I was involved in a conversation the other day and I'd like to throw a question
out to the BBC community ( possibly SB too ). I met with an old engine builder
( race applications ) that built numerous 396, 427, 454, etc. engines back when
they were new. He told me that they found that the tolerances of the bores,
piston-to-deck height clearance, combustion chambers, and intake/exhaust port
volumes were all over the place. He didn't give GM ( Tonawanda ) a good score
for precision. Is this true of all production engines from all companies?

Still another knowledgeable builder shocked me when he mentioned that he once
took a new fairly pristine L/72 apart, spec'd out everything and found that the
actual compression was 9.8:1 instead of the 11:1 advertised.

I would like to ask all of the members here that raced and built engines back
in the day about all of this. Could this situation explain why some BBC
engines clearly ran stronger than others of the same ilk? More importantly, is
this the real reason for cc-ing your engine?

Steve

[Dick W.]

Re: Tolerances in original BBC assemblies

Clem Zahrobsky could answer this with more accuracy, but I will take a stab at what little that I have observed over the years. New cylinder heads in the late '60's could vary by several cc's. The deck surface, in relation to the C/L of the crankshaft, could vary by as much as .020 in from end to end and side to side. Rod lengths could vary a few thousands and actual crankshaft throw length (stroke) could vary up to .008 in. This is from my observations of blueprinting several L/88 crate engines in the late '60's and early '70's Take an engine that all the tolerances were out of spec and you could have a very sour performing engine.

Today the manufacturing tolerances are very close and usually what is advertised is what you get.

[Joe C.]

Re: Tolerances in original BBC assemblies

Steve,

I am not surprised at what the engine builder said. I know that many engines using forged pistons burned 1 quart of oil every thousand miles, when new. Tolerances were nowhere near what they are today, what with CAD/CAM (CNC machining). Actual compression ratios were less than advertised, back in the day.
One example: I recently built a 327/365 using all stock components, and calculated actual CR using measured values for chamber volumes, piston-to-deck clearance, ring land volume, piston dome volume, and compressed head gasket thickness. The actual CR varied between 10.39 and 10.65 to one. Advertised CR for the engine was 11.0:1.
Advertised horsepower is another bone of contention. Some engines, like the L88, and L72 were underrated, but many were overrated. The reason, is that SAE gross horsepower measures the output of a BLUEPRINTED engine (at flywheel, no accessories, no mufflers).

Joe

[Clem Z.]

never saw .020 but have seen .012 on deck

to crank C/L. most took .015/.020 deck cut to get the pistons +.005 out of the bore to get the CR correct with a .045 gasket. i used special fixtures to check rods,pistons and crank throws to mix and match them up to get the correct deck heights within .002. the heads CCs were always larger than specs and varied all over the map. some were better than others so that is why some ran better than others stock.

[Steve Antonucci]

Re: Tolerances in original BBC assemblies

Dick,

If you had to take a stab at it, what degradation to HP could you effectively
see? In other words, could the tolerances change perfomance numbers by 25 or
more HP?

As a youngster, I was always amazed at how two Chevelles with 396-375hp engines
would run so differently. We always considered the driving skill the biggest
factor. Now, I think I know different.

Steve

[Warren L.]

Re: Tolerances in original BBC assemblies

My Grandfather was a tank mechanic during the war and an Engineer at GM Locomotive Division. He told me he sometimes bought cars that had been made for the president or vice president of GM and that the machine work was done to tighter tolerances, in short that they lasted many more miles and were better cars.

His last car at 65 years old was a 65 Pontiac Custom Sport. I don't think he was misleading me about the care and tolerances but that story about no food so they used to catch and eat flies during the war, seems there was a glint in his eye when he told that one.

Warren

[Joe C.]

Re: Tolerances in original BBC assemblies

Warren,

My late father had a war buddy who worked for GM, as a paint inspector. He was careful and honest in his evaluation of the paint. His boss told him to "close your eyes, and let more cars pass inspection, or else......." Get the picture?

You tell me, just what's "on the level" as long as money is to be made. Matter of course, that any car rolled out for a press release has been thoroughly massaged. Same goes for any car on loan to a magazine for evaluation.

Unless you're an insider, you'll get all of the leftovers when buying a car. Thank God that today's cars have a much tighter spectrum of acceptability!

Joe

[Michael H.]

Re: Tolerances in original BBC assemblies

Warren,

Grampa was right. It was not uncommon to have "walk through's" in just about every division and level of all GM mfg's. A walk through was a car, or engine/component for a car, that was scheduled for someone important in or out of the organization. One or more supervisors would follow the assembly down the line and that would automatically tell the worker that "something was up" with this one. As the level of importance became higher, the level of craftsmanship also became higher. If the level/importance of the purchasr was high enough, this inspection would carry over to engine/drive assembly also. Bet John Hinckley has a few good stories on this.

I dunno about the flies story tho.....

[Terry M.]

Re: Tolerances in original BBC assemblies

Sounds like the "8 cylinder conditioning" notation seen on some build sheets

[Michael H.]

Re: Tolerances in original BBC assemblies

Terry,

Yes, that's another interesting subject. Some cars received the "8 Cyl Conditioning" service at the St Louis plant as part of the original order. This basically consisted of a new car prep for engine specs and some touch up work under the hood. Two cars that I had years ago listed the "8 Cyl Cond" on some of the original paperwork that came with the car. One of those cars was the blue 396 that Burroughs used for the book "Corvette Restoration, State of the Art". That car was used for some GM advertising. The 8 Cyl Cond" was in the list of options on the original shipper copy. I believe some, maybe all, of the L88 cars also had this operation performed at the plant.

[John H.]

Re: Tolerances in original BBC assemblies

"8 Cylinder Conditioning" simply meant that the car would get its pre-delivery prep done in a GM Company Car Garage somewhere at the car's destination rather than at a dealership; most cars ordered for company service were shipped directly to the using location and got their "dealer prep" done at that location. It was an accounting notation that paid the receiving location for dealer prep instead of paying a dealer for it.

[Michael H.]

Re: Tolerances in original BBC assemblies

Thanks John. The paperwork that came with the 396 car indeed also stated, "Ship Company Car Garage". There were a few items on the engine of that car that were slightly orange over sprayed that shouldn't have been. I suppose some engine color touchup after a valve adj?

The L88's that I know about that had 8 Cyl Conditioning were definitely not shipped to Michigan. Of interest, one particular 67 L88 was a plant delivery to the new owner. He drove the car almost all the way home before the L88 fizzled.

[John H.]

Re: Tolerances in original BBC assemblies

Most "show jobs", "lobby jobs", and "VIP jobs" in those days were closely monitored or accompanied down the line by a supervisor or reliability engineer, and got closer on-line inspection, followed by selection for the daily off-line Quality Audit and an outside drive evaluation prior to shipment, and were signed-off by the Chief Inspector.

The only "special engines" I ever saw were the first ten Cosworth-Vega DOHC fuel-injected engines, which were hand-built at Tonawanda and each had a tag on it signed by their Chief Inspector verifying that they had been run on the dyno and met specifications. When we had the press event (TV crews, etc.) for the first Cosworth-Vega off the line, we went to start the engine and there was a loud "bang", a sheet of flame, and the air cleaner was blown right off the car, over the fender, and just missed one of the reporters. I was the Launch Manager, and saw my career going down the tubes before my eyes.

All ten of those engines had a sensor installed out of phase which caused them to fire with an intake valve open - it was obvious that none of them had ever been run. We gave the press a hastily-organized tour of the plant in our tour-train, and my techs found the problem while they were gone; we re-did the "first-car" routine about an hour later, and all went well, and none of the press reported the "explosion and fire" episode. After they were gone, I had an extremely animated phone discussion with the Chief Inspector at Tonawanda.

[Dennis C.]

Launch Manager, indeed... *NM*

[Dick W.]

Re: Tolerances in original BBC assemblies

Steve, that would be hard for me to quantify. I have seen dyno figures vary 40 hp for the same crate engines. For example the published dyno figures for open chamber L/88 engines with open tube headers varied from 525 to 565 hp. What is unknown is the skill of the dyno operator, the accuracy of the dyno, and the correction factors used. From my limited experience with the old Clayton and GoPower dynos it would be real easy to have some widely varying readings.

Remember the old addage, "figures don't lie, but liars figure"