Deck height deviations from Tonawanda and Flint factories in the '60s

[Samuel J.]

I've read several informative threads on deck height fluctuations, mostly relating to the small blocks from the Flint factory, but relatively little about the big blocks coming out of Tonawanda. The blueprint standard, nominal deck height for the small block is 9.025, and apparently it's not uncommon to find them as high as 9.040", but surely there must be shorter than 9.025 as well. By comparison, the nominal deck height for a big block is 9.8" (not including the 10.2" tall deck for truck applications), but I haven't read much about the specific highs and lows for the big blocks.

It seems reasonable to me that due to the reality of acceptable manufacturing tolerances in a mass production environment, some blocks were taller than the nominal spec, and some were shorter, and I imagine the same is true for rods and pistons, albeit to a lesser degree.


Question #1: In your experience, what's the highest and lowest deck height you've aware of for an original, unmolested big block? For a small block?



Question #2: Obviously, taller-than-spec blocks, while reducing the quench and CR, did not pose the risk of piston-to-valve collisions that shorter-than-spec blocks pose. So, did the factory automatically trash those shorter-than-spec blocks?

Or, did they just double up the standard gasket, or use a thicker gasket? I believe I saw a thread where somebody (Duke?) noted that Flint used double shim gaskets in the early '60s. Did they do the same in the later 60s, and with big blocks?

Or, and this is pure speculation on my part, were the short-than-spec blocks set aside to be paired up with taller-than-spec rods and pistons, in order to get the piston-to-valve spec back into the "comfort zone."

I'd love to hear your thoughts!

Sam

[Gene M.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

I don't think double gaskets was used for short out of spec blocks. My guess it was for reducing compression.

Perhaps somebody has copies of GM prints that state the tolerances on the dimensions you seek. In the day keep in mind tolerances of machined surfaces were not process control inline process. Everything was prior to CNC machining we have today.

But I would not discount selective assembly back then.

[Duke W.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

I've seen SB decks anywhere from right on the 9.025" spec to up to .015" high, but .005-.010" high is more typical and I've seen up to .010" side to side variation. I've never seen a Flint-machined block that was low. My theory is that the high decks are caused by broach wear and that at some point, maybe .015" wear, the tools were replaced. I don't have enough BB data to know, but given the same production philosophy was used I expect BB variation would be similar. The only time double gaskets were used that I'm aware of is mid-'62 through '63 340/360 HP engines to address customer detonation complaints. The variation in compression ratio is primarily from the variation in deck height. Piston compression height, rod length, and crank throw radius were controlled to one to two thou, so they are not a big factor. The deck height/clearance variation (.025" nominal deck clearance for SBs) is why deck clearance should always be measured on all eight cylinders prior to block tear down so you can get a handle on compression ratio management.

[Samuel J.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

Besides the variance from tooling wear, I was thinking there would also be some measurable deck height deviation stemming from how they positioned the block in the broaching machine.

It boggles my mind that they were able to keep as close to the nominal spec as they did!

[Duke W.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

Over the years there have been stories about the decks being "sloped". This could happen due to chips on the pads that the block was tired to during the broaching operation, but I've never run across a block that had any significant slope within the accuracy of my measurements nor have I run across any significant deck warp. Probably 99 percent of the time there is no reason to "deck" the block, but most machine shops do it anyway. They don't want to spend even the few minutes it takes to measure deck clearance or warp, which has cost a lot of guys lost stampings on the OE block.

Some years ago John Hinckley posted a photo of the Flint broaching machine. It's probably about the size of a typical single car garage, and the cycle time was probably no more than about 30 seconds. Equally impressive is the boring machine that bores all eight cylinders simultaneously in about the same time.

I never saw the Flint transfer line, but I did see the Pontiac transfer line when I was a production engineer with Pontiac in the late sixties. To a kid with a freshly minted BSME it was a mighty impressive sight!

A precision engine restoration includes a high target CR with maximum variation of no more than 0.1, preferably .05. If the left side of the block is high it can be machined down to equal the right side, because once the left side head is installed there's no visible evidence of the machining. If the right side is high you can usually make up most of the variation by using different head gasket thicknesses, which are available from .015" to .045" in increments of about .005".

Duke

[Samuel J.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

I've seen that broaching machine photo from John Hinkley on this discussion board--too cool! Now, if only someone would upload a video of it in operation!

I spoke with a professional restorer and NCRS member this afternoon who has seen countless unmolested C2 small and big blocks during his career. Based on his experience, he confirmed that deck heights are +/- .015 of nominal, adding that he has not only seen unmolested big blocks with double .021 steel shim gaskets on each head, but also 2 steel shim gaskets on one head and one on the other, presumably addressing uneven deck heights (i.e. between passenger and driver side). He has seen similar "creative gasketing strategies" on small blocks, including '63 Z06s. He has also seen an original unmolested block that had actually been welded! Looks like they went to great lengths to "salvage" less than ideal blocks.

[Gary C.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

Don't know about deck height differences. But on a '57 283 block on the left rear a section on a Flint block was milled and a Tonawanda block was rough cast. Never really measured the areas to see what the actual differences were.

Gary
....

[Duke W.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

I spoke with a professional restorer and NCRS member this afternoon who has seen countless unmolested C2 small and big blocks during his career. Based on his experience, he confirmed that deck heights are +/- .015 of nominal, adding that he has not only seen unmolested big blocks with double .021 steel shim gaskets on each head, but also 2 steel shim gaskets on one head and one on the other, presumably addressing uneven deck heights (i.e. between passenger and driver side). He has seen similar "creative gasketing strategies" on small blocks, including '63 Z06s. He has also seen an original unmolested block that had actually been welded! Looks like they went to great lengths to "salvage" less than ideal blocks.

I think people tend to interpret more into what they see and read than is really there, and very few people understand high volume auto production unless they were or are in the biz. Back in the sixties Chevrolet sold about a million cars/trucks per year, about half of GM total production. Let's say half had small block V-8s. That's about half a million a year or over 2000 per day. If a raw casting failed inspection due to a serious flaw, do you really think they would spend even an hour of UAW time to try to repair it? Not likely. The cost of the raw casting at that point was likely less than an hour of high skill repair labor. A "bad" casting would simply be recycled back into the pot, so to speak, and recast. Any evidence of welding is most likely a field repair. As I said previously ALL '63 340/360 HP engines received double head gaskets at Flint inlcudng the 360 HP engines installed as part of the Z-06 option. (There is a TSB on the subject.) When built, Flint didn't know if a 360 HP FI enigne would go into a Z-06 or a non-Z-06. The only thing "creative" about this was that it eliminated the numerous customer detonation complaints that began with '62 340/360 HP engines, and the double head gasketing started about mid-'62 production.

[Steven S.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

I think all these small blocks were poured in Saginaw Mi. I used to call on Nodular and Grey iron foundries that made these blocks. They were pouring 8000 blocks per shift, 3 shifts a day, 7 days a week.

[Samuel J.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

Actually, you didn't say previously say "ALL '63 340/360 engines received double head gaskets" but thanks for clarifying. I like learning new things. Did you like learning that double gaskets were used on more than just '63 340/360 engines? Did you also like learning that shorter-than-nominal deck heights left the factory too?

I see your point about the welding, and I'll see if I can get some more detail for you on that welded block.

[Joe L.]

Re: Deck height deviations from Tonawanda and Flint factories in the '60s

I think all these small blocks were poured in Saginaw Mi. I used to call on Nodular and Grey iron foundries that made these blocks. They were pouring 8000 blocks per shift, 3 shifts a day, 7 days a week.

Steven------


Small blocks were cast at both Saginaw Gray Iron Foundry and Tonawanda Gray Iron Foundry as well as St. Catherines, Ontario, Canada. Saginaw cast blocks were machined at Flint Engine. Tonawanda cast blocks were usually machined at Tonawanda Engine. St. Catherines cast blocks were usually machined at St. Catherines Engine. GM also has a gray iron foundry at Defiance, OH. Among other engines, big block heads and blocks were cast there after the Tonawanda Foundry shut down in 1984.

I am not aware that any small block heads or blocks were cast at the nodular iron foundries. GM used to have several nodular iron foundries in Saginaw and one in Defiance, OH. They also had a malleable/ductile iron foundry at Danville, IL. Except for Defiance, OH, all the nodular and malleable iron foundries are now shut down.