40mopar Posted January 27, 2022 Share Posted January 27, 2022 I recently acquired a 1938 Buick Series 40 Special Business Coupe. It took some research, but based on postings on AACA and some other sources, I’m reasonably sure that this is truly a ’38 Special Business Coupe. The firewall plate info and the frame serial numbers all seem to agree. The vehicle has coil springs in the rear, the only firm difference I could find between a ’37 and a ’38. My problem comes with the engine. An original ’38 Buick shop manual says the plugs should be 14mm and require a 13/16th socket. My plugs take a ¾” socket and are 17mm. After a pretty lengthy search, I finally found the engine serial number. It is for a 1937 engine. OK, no big deal. I know that using parts from the previous year was common place, but further research indicated that the original 1937 plugs were 18mm, not 17mm. So, I wonder what I have? You would think that if in fact GM used ’37 Buick engines in ’38 vehicles, there would be some reference in the shop manual. If a previous owner swapped engines, why the difference in plug thread size? Guess it could be both a different engine and a different head, but how could I determine that? As long as I find the correct plugs and get this thing running, it’s no big deal, but I will always be wondering what I have when the need comes to replace other parts. Comments appreciated. Thanks.+ Link to comment Share on other sites More sharing options...
pont35cpe Posted January 27, 2022 Share Posted January 27, 2022 `37 248 has flat top pistons , `38 248 has domed pistons I`m not sure about the spark plugs. My `36 233 engine has the large threaded spark plugs(18mm), takes a 7/8" thin wall socket. I think `37 248 uses the same plugs as my `36. Link to comment Share on other sites More sharing options...
EmTee Posted January 28, 2022 Share Posted January 28, 2022 What sparkplug is currently in the engine? Is there any evidence that the head may have been removed? Link to comment Share on other sites More sharing options...
1939_Buick Posted January 28, 2022 Share Posted January 28, 2022 (edited) Welcome to the world of straight 8 Buick 6 hours ago, 40mopar said: My problem comes with the engine. An original ’38 Buick shop manual says the plugs should be 14mm and require a 13/16th socket. My plugs take a ¾” socket and are 17mm. After a pretty lengthy search, I finally found the engine serial number. It is for a 1937 engine. OK, no big deal. I know that using parts from the previous year was common place, but further research indicated that the original 1937 plugs were 18mm, not 17mm. So, I wonder what I have? You would think that if in fact GM used ’37 Buick engines in ’38 vehicles, there would be some reference in the shop manual. If a previous owner swapped engines, why the difference in plug thread size? Guess it could be both a different engine and a different head, but how could I determine that? Probably an engine swap some time in the last 84 years. At one time would have been a 25~30 year old used car with low value. Casting number on the head and/or manifolds? Info-->https://www.buickheritagealliance.org/index.php/restoration/engines/straight_8 We like photos! Edited January 28, 2022 by 1939_Buick (see edit history) Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted January 28, 2022 Share Posted January 28, 2022 Pictures please. The grill is different between 37 and 38. The 37 grill has much thinner bars and gaps. Link to comment Share on other sites More sharing options...
EmTee Posted January 28, 2022 Share Posted January 28, 2022 The car apparently has rear coil springs, so at least the chassis appears to be a '38. As stated above, however, the engine (or just the head) could have been swapped at some point over the last 80 years. Pictures would definitely help... Link to comment Share on other sites More sharing options...
40mopar Posted January 28, 2022 Author Share Posted January 28, 2022 8 hours ago, 37_Roadmaster_C said: Pictures please. The grill is different between 37 and 38. The 37 grill has much thinner bars and gaps. Here's the front end. 2 Link to comment Share on other sites More sharing options...
40mopar Posted January 28, 2022 Author Share Posted January 28, 2022 13 hours ago, EmTee said: What sparkplug is currently in the engine? Is there any evidence that the head may have been removed? The current plugs are Champion D 16. It's the 17mm thread that has me puzzled. It's a 1937 engine number, but 37's were supposed to have 18mm plugs. Here's a photo. Am I measuring wrong? Link to comment Share on other sites More sharing options...
Bloo Posted January 28, 2022 Share Posted January 28, 2022 (edited) I suspect you are. My guess is 18mm is the size before thread cutting, and the very tip tops of the threads would be close to 18mm, yet still a bit less. I have never heard of a 17mm plug. Brillman, who should know because they deal in electrical stuff for tractors (and cars) of this era, think a Champion D-16 is an 18mm plug. https://brillman.com/product/d16-champion-spark-plug-18mm/ Nice car! Edited January 28, 2022 by Bloo (see edit history) 1 Link to comment Share on other sites More sharing options...
MCHinson Posted January 28, 2022 Share Posted January 28, 2022 That is an 18 mm plug. It also cross references with AC Delco 86 which is what Bob's Automobilia sells as correct for 1929 to 1937 Buick engines. 2 Link to comment Share on other sites More sharing options...
DonMicheletti Posted January 28, 2022 Share Posted January 28, 2022 That front end is definitely a '38 Link to comment Share on other sites More sharing options...
EmTee Posted January 28, 2022 Share Posted January 28, 2022 Seems like it's a '38 with a '37 engine - or a '37 head? Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted January 28, 2022 Share Posted January 28, 2022 Yep, 38 grille and an 18mm plug. Your measurement is fooling you. When standard 60 degree threads are cut there is relief cut on the thread crests so when you measure as you have shown the measurement will be less than the actual specified thread diameter. This is why you are seeing 17.08mm. .0326 inch is the crest relief which equals .92mm. Link to comment Share on other sites More sharing options...
40mopar Posted January 29, 2022 Author Share Posted January 29, 2022 17 hours ago, 37_Roadmaster_C said: Yep, 38 grille and an 18mm plug. Your measurement is fooling you. When standard 60 degree threads are cut there is relief cut on the thread crests so when you measure as you have shown the measurement will be less than the actual specified thread diameter. This is why you are seeing 17.08mm. .0326 inch is the crest relief which equals .92mm. No, my measurement wasn't fooling me: I was fooling myself. Guess who failed to zero his electronic caliper before measuring?🥵 Plugs measure about 17.8mm which is close enough to 18 for me. So, I'm satisfied I have a '38 Buick with a '37 engine. Just wish I knew the history. Why would you install an engine from an older car? Guess that's part of the fun having these old relics around. You never know what secrets they are keeping. Link to comment Share on other sites More sharing options...
Mark Shaw Posted January 29, 2022 Share Posted January 29, 2022 I have been told that Buick dealers offered black replacement engines after WWII. If your engine is painted black, you probably have a replacement engine. Immediately after the war, there were no new cars available due to conversion of the car factories for the war effort. So, dealers did what they could to extend the life of cars built before the war. 1 Link to comment Share on other sites More sharing options...
DonMicheletti Posted January 29, 2022 Share Posted January 29, 2022 The '38 Special coupe that was my parts car had one of those post war replacement engines. It was black and had the later style intake manifold with the "dog leg" at the rear runner. You couldnt use the original air cleaner, it would hit the hood 2 Link to comment Share on other sites More sharing options...
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