What to know before machining a numbers-matching engine block

The stamping on this Corvair engine is safe from any machining, but not every Chevy engine is the same. Kyle Smth

For many classic car buyers, originality is of utmost importance.

Lots of the value of a rare muscle car, for example, comes from its just-right combination of options that sets it apart from its similar but less exciting brethren. There are dozens of ways to help determine if a car that appears to be a rare, potent version of a muscle car is what it claims to be, and the focus of many of those tests comes down to establishing whether or not the engine is the original one installed by the factory.

A rare car without its factory-installed engine can lose a significant chunk of its value, so plenty of collectors prefer that any factory evidence of the vehicle’s authenticity remains intact.

If you do have the correct, original engine in your car but mileage or a part failure means that a rebuild is necessary, keeping the original engine and its identifying marks are likely a big priority. Many manufacturers stamped identification numbers on the oil pan rail, or on a separate boss machined specifically for the purpose, but Chevrolet often used the deck surface as it was readily available.

The problem with that is that besides boring the cylinders for new pistons and align-boring the crank journals, milling the deck surfaces to be parallel to the crankshaft centerline is one of the major processes a machine shop will take to prep for a rebuild.

Factory tolerances weren’t perfect in the ’60s and ’70s, and even when they were solid, decades of heat cycling can cause blocks to get a little wonky. Milling the deck ensures the heads have a flat surface to mount to and helps ensure every cylinder operates with the same compression ratio and valvetrain clearance. However, removing even a few thousandths of an inch off the deck surface could mean erasing any evidence of those valuable identifying marks.

This video from Jim’s Automotive Machine Shop Inc. goes over the intricacies of decking a numbers-matching block to preserve the stampings, starting by cutting the shorter of the two decks as little as possible to achieve a flat and true surface and then cutting the other deck to match. When machining a deck without stampings, the mill can be left to feed across the entire surface, but to preserve the numbers, the process is stopped short and the block is traversed so that the large cutting head passes over the entire gasket surface. That ensures an even clamping surface for the gasket while leaving the numbers intact.

“This is a process that not everyone needs to worry about, but when it matters, it matters,” said Hagerty’s master engine builder Davin Reckow. “Since keeping that stamping is extra work, be sure to discuss this with your machine shop before handing over the block for machining to ensure that it doesn’t accidentally get cut off before you can say something.”

So, as Davin noted, before you have any machine work done, know where the important stampings are on your engine and make sure your machine shop does too. Odds are your machinist can keep everything intact with just a bit of extra care and effort.

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    This obsession of “numbers matching” engine, transmission, etc. comes across as nothing more than one-upsmenship between car “investors” and another excuse for increased auction prices. Originality to that extent is for museums and preservationist to have an example of an original as reference for history and for a guide in restoration. I mean, my god, even the manufacturers knew things wore out, blew-up, had recalls (anyone ever hear about Boss 302 factory replacement engines – originals had faulty piston skirts), and just plain got swapped out some time in the car’s history. Unless your car is a trailer-queen trying to grab trophies for most original, or you are looking for excuses for charging an extra few grand $ for your flip or investment, as long as your car has the correct engine size, type, year, and era appropriate equipment. . . who cares if the numbers match? Certainly not those of us who actually drive and enjoy using our cars the way they were meant to be. That may not be “original”, but it’s pretty authentic if you ask me. Now, about that damn “barn find” term . . .

    100% agree. Add to this the large number of “matching numbers” cars that were faked early on using leftover factory stamps etc. and probably passed judging back in the day. This is especially true where factory records are mssing.

    When the Mustangs arrived in our Dealership in late64 all the young studs from the Nearby Highschool arrived and filled our Premises at Lunchtime and ohed and ahed how they would love to own one and asa few short years went buy these Mustangs became available and were bought by these still young fellows looking for ” like newand all Original Examples and a large Number of them which were purchesd under those dreams were wrecked within weeks or even Days cept one that I remember a maroon coulered65 Mustang purchased by the Daughter of the owner of the Local Music Store she drove it and kept ( like new and original all her live to the end of her driving Days and yesTroy I don’t think she ever cared if it had Matching Numbers but did she enjoy that Car and weare enjoying our 48 Dodge Coupe with a flat 6 in it and I have never even looked for the Numbers and I wont because if I should find the Numbers are not Matchin I might Grieve to the end of my days

    Troy, in the auction world, auction houses and sellers need to create any way to differentiate product for higher values, warranted or not. This is similar to the “1 of 27” in this color and options combination and other silly ways of making something sold in the tens or hundreds of thousands sound rare.

    As a buyer, I’d rather have a nice driver at a good price than a pristine numbers-matching example if the difference comes to thousands of dollars for no other benefit. As a seller, I’d rather have whatever features, options or other pointless items that create perceived value in a buyer’s mind. 😁

    So refreshing to hear this point of view. My friend drives a 1966 427 Corvette and it is not numbers matching. When our Corvette Club goes out for a cruise his car is just as breathtaking as all the other generations. I always try to work it so I’m driving behind him They gotta be driven.

    “who cares if the numbers match? Certainly not those of us who actually drive and enjoy using our cars the way they were meant to be”

    Well clearly lots of people care and of course if they’re willing to pay up for matching, more power to them. As long as people “severely discount” the value of an in service replacement engine like the 1970 LT-1 spec counter replacement engine in my 71 LT-1, (yes, when it blew up in 1972, the owner backdated a year to 70 specs) that means I can get all the performance, if not the cachet for about a 25% discount. yeah me!

    The difference in price for ‘matching numbers’ versus not can be THOUSANDS of dollars. Numbers can be faked on engines, transmissions and VIN’s these days. I personally like everything as original as possible, but I’m not paying premium price for it. For what I buy, it doesn’t matter because they are 6 cylinder or base V8 usually. There are more Chevelle SS’s out there than GM ever produced.

    it is an obsession, as prior commentator says… I go off the deep end when a guy says he has triumph TR6 with a chevy V8 engine, well, my mind, it aint a TR6 anymore. My TR6 fortunately is original…but so what if I drop another TR6 engine in it if my original engine melts? Originality is wonderful, but then there is real life…

    Agree, I feel the same way about LS swaps. Can’t beat ’em for drivability but it makes them harder to sell since there is not much way to tell if they’ve done a good job. I’ve got a ’65 Impala SS with a rebuilt ’66 396. They don’t match but it doesn’t spoil my fun and a potential buyer can tell right away if there are issues…

    Too many people are obsessed with numbers matching garbage anymore. Some cars like the first generation Chevy Vegas are almost impossible to find with a numbers matching engine because all had the engine replaced at least once and many multiple times under the original warranty because the blocks and heads warped. VW beetles are the same way as the engine block would wear out because of the thrust bearing pressure around 100K and the only choice when rebuilding was to replace it.

    Engines get replaced not infrequently. In one of my restorations, the original block was long gone. I was given a later manufactured good block. The advantage was the newer block offered full pressure lubrication as opposed to a bypass system. So, it offered a better, more durable setup to rebuild. Big deal. I built the engine for me, not some investor or resale cowboy. I’m keeping it for life. If it brings 25% less at sale after I’m gone, so what. My kid’s basis in it is zero. After all, it’s a hobby. You spend money, not make money.

    Has anyone had success with acid etching in an attempt to restore identifying markings or numbers that got shaved off? If so, any ideas as to who does this type of work? This is done with firearms that have a serial number obliterated. Fortunately in my case I took photos prior to sending block to machine shop but they shaved off the “HP” designator on the pad from a 383 engine. I did not think about the possibility of numbers getting shaved off nor did machine shop mention it so it was not discussed. The engine number is still there however. I will not attempt to restamp the missing designation as that looks deceptive.

    I was under the impression that only Corvettes and maybe some other rarities had “numbers matching” components. Aren’t all the others just period correct with correct casting numbers and date codes?

    Hi Gman. Mopar nuts watch numbers pretty closely from what I’ve seen. Chrysler provided that ability a long time ago by stamping the last 8 characters of the VIN on the engine, trans, and many body parts. The last 8 of the Chrysler VIN on vintage cars includes the last digit of the year, the factory code, and the 6 digit build sequence number of the car. Mopar folks have also been tracking what casting numbers for parts like heads and intakes go with which year and type of engine. Its actually pretty interesting to see the level of detail some folks go to checking whether their cars are numbers matching. Other items like ECUs, voltage regulators and more are also date coded if you really want to get picky!

    When the Mustangs arrived in our Dealership in late64 all the young studs from the Nearby Highschool arrived and filled our Premises at Lunchtime and ohed and ahed how they would love to own one and asa few short years went buy these Mustangs became available and were bought by these still young fellows looking for ” like newand all Original Examples and a large Number of them which were purchesd under those dreams were wrecked within weeks or even Days cept one that I remember a maroon coulered65 Mustang purchased by the Daughter of the owner of the Local Music Store she drove it and kept ( like new and original all her live to the end of her driving Days and yesTroy I don’t think she ever cared if it had Matching Numbers but did she enjoy that Car and weare enjoying our 48 Dodge Coupe with a flat 6 in it and I have never even looked for the Numbers and I wont because if I should find the Numbers are not Matchin I might Grieve to the end of my days

    I love reading all the comments, and hate to be critical, but if you made sentences by throwing in a few periods here and there, your comments would be much more readable.

    All the comments I’ve read here make sense.

    However, it’s simply the economics and market dynamics. If someone is willing to pay a lot more money for a “wonkey” original…then good for them. Their need for wonkiness is just as important as our need to use our treasures. I get it…I’ve been stuck in the middle of both arguments. The good news is, we set the expectations as participants in this market/hobby. And if BIG MONEY rewards weird then it’s something we can laugh about with our fellow car-folks. Or laugh about it all the way to the bank!

    Matching numbers typically means that the original and subsquient owners took care of their car. They were careful with engine maintenance and the rest of the car. It never fell into the hands of bubba who never changed the oil or left it outside with the heads off saying I’m going to get to it one of these days. It is a sign that the owner cared about the car and that’s why it brings more money.

    Living in the Los Angeles area, it was easy to put on over 100,000 miles in 5 years! So, when my 1968 SS 427 Impala needed a new engine after those 100,000 miles and needing a car for work, I bought a rebuilt 427 from a speed shop in Pomona. Still a 427, but in todays world, it is about a $20,000 difference if I had kept the original block for the value of the car, which I still have and is on its 3rd motor. Still driving it and not planning on selling it, and yes I am over 75 years old and rebuilt the engine myself this time in a friend’s 6 car garage.

    Cars are for carrying humans across the planet in some sort of style. They are NOT investments like a warehouse full of refrigerators. Anyone that views cars as simple investments gets what he deserves, in my estimation.

    To Wes Gray, I have a ’74 MGB GT into which I swapped a ’95 Firebird 3.4 V6. I still call it an MGB GT
    cause in spite of what anyone says it is, North Carolina DMV and I agree on that…anyone who would like
    to “restore” it to original, I have the “numbers matching” block under my bench. All it will take is my
    price (write for details) and it’s all yours. In the meantime, see you in the twisties!

    My 69 GTO convertible has a service replacement engine, all date block, codes, matchup, carburetor, distributor numbers, are correct. It has number 62 heads which are usually on big car 400s. I am her caretaker since 1996 & staying with me till I’m gone. I drive Her regularly and I’m not afraid to push it for all it’s got… I don’t hate on trailer queens, I just prefer to drive my cars regularly

    60’s Mustangs came with 12 month or a 12 thousand mile warrenty. But if you ponied up for a HI-PO engine you were given a piece of paper to sign that said your new warrenty is 90 days or 3,000 miles. Ford was not stupid. They also stamped the VIN on the engine block. Ford knew that these cars were going to be rode hard and put away wet. Ford didn’t want anyone playing games with replacement engines. Which is why Shelbys with original engines are worth much more.

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