GARY F Posted November 21, 2018 Share Posted November 21, 2018 I am old and all my cars are 1989 and back to 1937 Olds. I fully restored two 63 Pontiacs so I am not in the dark.. I am looking to buy a compression tester and every one I look at says the threads on the tester are metric. Ex. m-10 m-20- m-30 etc. Are they metric on my 89 Mercury, 89 F-150, my two 63 Pontiacs and my 37 Olds. This might be a stupid question to you, but I am confused which does not take much. Thanks Link to comment Share on other sites More sharing options...
Spinneyhill Posted November 21, 2018 Share Posted November 21, 2018 If you look at your spark plug designations (written on them) then ask the www about them, you will probably find the threads. And yes, probably all of them are metric, 14 mm IIRC. 2 Link to comment Share on other sites More sharing options...
GARY F Posted November 21, 2018 Author Share Posted November 21, 2018 9 minutes ago, Spinneyhill said: If you look at your spark plug designations (written on them) then ask the www about them, you will probably find the threads. And yes, probably all of them are metric, 14 mm IIRC. OK. Example AC-44 Link to comment Share on other sites More sharing options...
Spinneyhill Posted November 21, 2018 Share Posted November 21, 2018 (edited) I think that is an AC plug of thread size 4 (14 mm) heat range 4. AC's code chart. https://toms427impala.info/CT/ACPlugInfo.pdf Pull a plug and measure the thread. Edited November 21, 2018 by Spinneyhill (see edit history) Link to comment Share on other sites More sharing options...
Spinneyhill Posted November 21, 2018 Share Posted November 21, 2018 Here is a cross reference website. From there you can ask for modern code systems. http://www.sparkplug-crossreference.com/convert/AC_DELCO/44 https://www.mrcycles.com/sparkplugspecguide AC-44 == NGK B5S for example. B = 14 mm. Just ask the www! Five or ten minutes should be enough. Link to comment Share on other sites More sharing options...
GARY F Posted November 21, 2018 Author Share Posted November 21, 2018 I want to thank you guys for the info. Now I know they all are metric. Link to comment Share on other sites More sharing options...
Rusty_OToole Posted November 21, 2018 Share Posted November 21, 2018 Spark plugs in American cars have been metric since the Model A days. Very early cars had tapered pipe threads like plumbing. But since the twenties they have been 18mm, 14mm or 10mm in almost all cars. In fact I can't think of an exception. Link to comment Share on other sites More sharing options...
Bloo Posted November 21, 2018 Share Posted November 21, 2018 (edited) Most spark plugs are metric. Metric threads have been the norm on many American cars back into the 1930s, and existed on some American cars long before that. Most compression testers (and leakdown testers) come with one hose that has stepped 18mm and 14mm threads. That works for most cars. Many older Fords have 18mm and the 14mm fits almost everything else. The Pontiacs are metric. Your 37 olds is probably metric, but I would check. Look at the plugs for the 2 1989 Fords. Some plugs from that period have an unthreaded area between the threads and the tapered seat. If that is the case, a typical tester won't work because the threads on the tester wont reach the threads in the head. Snap-On testers (expensive) may have 3 adapters. Mine has a short 14mm, a long 14mm, and a short 18mm. In this case you use the same thread length as the existing speak plug, and partially threaded spark plugs are no problem. Whatever you buy, get extra Schrader valves if you can. The ones for tires look the same, and fit, but don't work worth sour owl manure. Really old cars that have pipe threads or American threads of some sort, or 10mm metric will probably require an adapter. Edited November 21, 2018 by Bloo (see edit history) Link to comment Share on other sites More sharing options...
dibarlaw Posted November 22, 2018 Share Posted November 22, 2018 To fit on my 1925 Buicks that take a 7/8" threaded plug, I took apart a old 2 piece 7/8" thread plug. I had the 18mm adapter to thread into the smaller end. A bit awkward but it works. 1 Link to comment Share on other sites More sharing options...
Real Steel Posted November 22, 2018 Share Posted November 22, 2018 I've solved this by buying excellent used compression testers on eBay. I buy only vintage American-made quality testers. Yes, American tools...you know...they were made for American cars. The price of a used tester is frequently so low, that even if it doesn't work out, its no big deal. Concerned about accuracy? You should be because there can be significant variation from one compression tester to another. This is just as true with new gauges and well as older gauges. Its best to check your gauge with a known source if you can; a NIST-traceable calibrated source would be even better. The biggest failure of the older compression testers is the check valves, most notably, the seals on the check valves. It's best to buy a gauge that requires that you only need to change the Schrader valves in order to fix the problem, then done. Not all gauges can be fixed this way...watch what you buy. Link to comment Share on other sites More sharing options...
Frank DuVal Posted November 22, 2018 Share Posted November 22, 2018 Yep, spark plugs are metric (except the pipe thread (NPT) types). Remember Albert Champion was making them in the beginning of time (well, 1905 or so) and he is French, so metric it was. Like Mr. Olds, Albert has his name on two spark plug Companies. Unlike Ransome, Albert’s named companies are still in production. AC Delco and Champion. 1 Link to comment Share on other sites More sharing options...
Crazyfamily Posted March 17, 2019 Share Posted March 17, 2019 On 11/21/2018 at 8:07 PM, dibarlaw said: To fit on my 1925 Buicks that take a 7/8" threaded plug, I took apart a old 2 piece 7/8" thread plug. I had the 18mm adapter to thread into the smaller end. A bit awkward but it works. Ok so I’m down at the ‘27 Buick and thinking I’ll check the compression, well just as stated above no go! My compression tester just like most others only came w 14mm long and short adaptors as well as 18mm! So I’m not understanding your fix for this @dibarlow. id be interested in seeing or hearing how I can adapt what I have to make an adaptor to fit the ‘27? As always, it is GREATLY appreciated! Link to comment Share on other sites More sharing options...
PFitz Posted March 17, 2019 Share Posted March 17, 2019 One way is to knock the porcelain insulator out of an old spark plug. Drill and thread it for 1/4 inch pipe thread and screw in a male quick disconnect fitting. There's your adapter. Or, for hard to reach plug holes, make a longer adapter by welding a 6 inch long 1/4 inch pipe nipple to the spark plug body and thread a female quick disconnect to the 1/4 inch pipe. Put an o-ring on the spark plug threads to seal without over tightening. Paul Link to comment Share on other sites More sharing options...
Rusty_OToole Posted March 17, 2019 Share Posted March 17, 2019 Or get the kind with the rubber bung on the end and stop worrying. 1 Link to comment Share on other sites More sharing options...
pont35cpe Posted March 17, 2019 Share Posted March 17, 2019 35 minutes ago, PFitz said: One way is to knock the porcelain insulator out of an old spark plug. Drill and thread it for 1/4 inch pipe thread and screw in a male quick disconnect fitting. There's your adapter. Or, for hard to reach plug holes, make a longer adapter by welding a 6 inch long 1/4 inch pipe nipple to the spark plug body and thread a female quick disconnect to the 1/4 inch pipe. Put an o-ring on the spark plug threads to seal without over tightening. Paul I`ve also used the spark-plug with the quick disconnect to pressurize the cylinder to replace valve springs or seals.. Link to comment Share on other sites More sharing options...
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