What to know before machining a numbers-matching engine block
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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I’ve rebuilt dozens of gen I Chevy small blocks and big blocks, never had to resurface any of the blocks.
Didn’t use the factory steel shim head gaskets, always Fel Pro blue composite head gaskets. Never a problem.
One customer wanted a block resurfaced, didn’t require it but I suspect he was going to have it re-stamped.
I explained that the blocks were broached at the factory, and the machine shop will use a milling cutter.
The lack of broach marks would be a dead giveaway that his block was re-stamped. He didn’t like that.
Some engines had unique casting numbers, along with other unique parts. Sum of all the parts might be good enough, especially with ownership history to back it up.
I owned a 1967 Z/28 just out of high school. I was lucky enough to get some pictures of the car in 1969 from the original owner, along with the Trans Am race history. The block had been replaced by the second owner with a 327 block. There were plenty of other unique parts with numbers and dates to match the build of the car. Enough proof existed – even before anyone knew about the 4L code on the trim tag – to confirm it was what I sold it as.
Manual steering with quick ratio – 3 turns lock to lock, metallic brakes all around, traction bar, correct carb, distributor, intake, etc. It had been repainted once, but still had remnants of the stripes under the hood.
Yes I would have got more for it at the time had the original MO block been there, but was able to prove it was not a clone.
Matching numbers cars are for a different person than me. I am more of a sum of the parts where a well done clone is good enough. Some place such a high premium on all the numbers it takes many out of the market for a car they want.
It is interesting to see a well known authority go through a car. Looking at my GTO judge, they could tell the original bumper had never been removed – and which build plant put it together. Even though it me as a RA III, the RA IV fan I had laying around was indeed the specific one off a 1969 RA IV.
From this experience, I would say if numbers matching is something you place a premium on, by all means have a expert evaluate before you buy or sell. There are likely many hidden clues few know to look for.
Frankly, if one can preserve the numbers – why not do it!? With vintage cars it is certainly nice to know that it has all the original major parts and I appreciate the care someone might take to preserve the numbers to indicate that fact. How much I am willing to pay for matching numbers depends on the vehicle and its purpose of course – play-time or investment – I have both. I appreciated reading the article and feel it was quite well done – and I really enjoyed seeing the master machinist at work in the video! Thanks for including it! In my opinion – a good job all around!
I agree with the majority opinion here. I have owned more than a few new cars in my 75 years. I still own my factory ordered 1972 Olds 442. I know everything that has happened or been done to my Olds. I don’t give a hoot about matching numbers. I have had to replace a lovely 370 HP 455 due to improper replacement of the original timing chain and sprocket. Remember the ones with the nylon teeth. When they fail the teeth can get in the oil pump pick up and seize the pump and soon there after the engine. The proper repair requires not only replacing the sprockets and chain but pulling the oil pan and pump, cleaning the pan and replacing the pickup and pump. Every dealer did not do this. This caused a seized engine, broke pistons and rods and a horribly gouged block. Engine replacement required. The original engine had a portion of the vin stamped in a boss on the left side of the engine. The replacement engine did not. The dealer response was to grind the boss and hand stamp the numbers using 1/4 inch stamps.
So technically my Olds is not numbers matching but I don’t care. It runs great, looks good and gets applause wherever I take it. That is good enough for me b
It is truly amazing how short life really is. I have been in the hospital twice and thought I would not get out alive. During each of those times, I told myself that I would enjoy my cars and not worry about originality. But, do whatever makes you happy. You can’t take it with you.
Numbers matching is nice, but I’m not going to pay many thousands of dollars for it.
Don’t buy a Chevy. Problem solved.
Yes, you should buy a Ford or Mopar and put an LS in it. Lots of problems solved.
There aren’t many cars that I would pay more for “matching numbers” Mostly original and period correct is good enough on most cars. Nowadays half the “restored” cars out there have so much replacement sheet metal and other replacement parts on them, that you might as well slap a “Made in China” label on them.
It makes a difference if you have a Stage1 or a Ram Air IV block vs a standard block and you’re buying a car were someone is claiming it to be the higher performance package. Especially if the seller doesn’t have the paper work to proove what the car really is, which is very common.
Matching numbers mean something to some owners and buyers so might as well get over it. And if you don’t want your block numbers machined off you’d better hope your wish gets relayed from the sales counter to the man doing the work, and he’s not in a hurry that day. As insurance make note of the numbers beforehand (or the numbers you wish it had heh heh) and keep a stamp set handy.
The purpose of the article was to show what to do for people who care to keep the original number and they are willing to pay. So there are many people who are prepared to pay extra for a “numbers matching” car. Why does anyone get pleasure at criticizing there ambition??? However it makes you feel good to do so carry on!
Exactly, Mike! It makes a difference if you are the buyer and looking to purchase a higher performance version of the base model (GS Stage 1 or RAIV GTO…etc) without vehicle paperwork to prove its authenticity.
mike@daystardesigns.ca
As a metallurgist, machining a few thousands of an inch off the surface will not totally remove the stamping. There are several methods to “draw out this number, ask any forensic investigator that deals with serial numbers filed off gun barrels.
Instead of worrying, take a photo at the machine shop before and after and keep a record. If someone wants prof they can etch the area and the number will pop right out.
I love driving my cars regardless of the engine numbers……..however, a 50-60 year old muscle car with a high per cent of original components, is a treasure. It substantiates the care given to the car. In spite of everyone’s comments that they don’t care, not one would turn down an extra $10-20,000 if they could get it because their car is original. Some cars are purchased as investments. Some are purchased for driving pleasure only. I have several matching number’s cars, and because I want to drive and enjoy them, I insure them with Hagerty stated value insurance. Only certain cars are worth worrying about. In muscle cars, low production and high horsepower is the magic formula. Low HP cars make great donors for the upgrades discussed in previous posts. I marvel at both approaches.
Regarding rare royalty cars, I am the fortunate caretaker of a lovely, Marina Blue ’67 Camaro. It’s just a base model. Standard interior. No A/C. Rally wheels. Per the VIN, it was born with a straight 6 but was retrofit with a 350 V8, Edelbrock intake manifold & 4 bbl, carbon steel headers and dual exhaust. So just a base model with an engine swap – the polar opposite of a rare numbers matching auction queen. I would not trade this car for much of anything – she drives wonderfully, looks nice, sounds incredible, and my older sis owned the car before me, driving once a year for 20 years in the Woodward Dream Cruise. She’s still around, my sister, can’t drive anymore, and I love looking after her car.
I get some comments about going out of your way to own a “numbers matching” vehicle. BUT, the purpose of this article is to identify what you have and protect it if it’s numbers matching. Both my GTO and GS400 have Born-With powertrains. I didn’t look for that. Just happened to be. The GS vin is stamped on the deck and was protected during the machine work of the 400. No worry about my GTO block as it’s stamped in a different area. So, why wipe a deck clean if it’s numbers matching??? Great article here!