How do I select a replacement spark plug?

[mrcvs]

For an antique car?  In this case, it's a 1917 Maxwell, but what's the principle behind it so that I can select a replacement spark plug for any vintage automobile, and not something I look up in a stock manual of specifications of replacement parts for recently manufactured automobiles?  Right now, currently installed spark plugs are Champion.

[playswithbrass]

I like running old n.o.s. plugs form the era of the car.What I do is measure down the spark plug hole and measure what clearance I have in relation to the piston and valves in the open position and then get a plug that will put the electrode the deepest into the cylinder.I do not worry about heat ranges like in modern plugs because the compression ratio of older engines is not hot enough to cause worry.I don't even know when spark plug manufacturers started making different heat ranges for the same application.Also index the plug so the ground electrode is opposite or away from the combustion chamber--old drag race tip--but it works for more complete combustion.

Hope this helps.  Cheers Pete

[TerryB]

From a 1927 auto parts supply catalog, Maxwell is AC type A

 

 

 

[mrcvs]

Where does one obtain NOS spark plugs?  Also, if not new old stock AC Type A?

[playswithbrass]

There are a couple of sparkplug vendors at Hershey by the stadium,and ebay buy them one at a time ,may take a couple of months.

I have bought from Don McKinsey. Great guy  765-785-6284 has probably the most plugs of anybody around,tractor trailers full.

[Terry Bond]

Check your owners manual/parts catalog to see what the original brand plug might have been.   If AC is what you need then you'll need to search Ebay or the recommended vendors for them.   They should not be difficult to find.   

[PONTIAC1953]

most used spark plugs can be cleaned, check the gap, and reuse.

 

[Mark Shaw]

This is a handy guide to help identify spark plug equivalents:

 

Spark Plug List

 

7/8" spark plugs cold down to hot
New Champion	Old Champion	A--C	Auto Lite	Edison	Bowers	Globe	Blue Crown	Leonard
W-10	0 Com	73 Com		40-T	C78-C	6	73-Com	4-F
W-14	1 Com	74 Com	T-7	37-TS	C78-N	7	74-Com	4
W-18	2 Com	C 77L	T-9	35-S	78-N	8	75-Com	3
W-20	3 Com or  No 20	78 Com	T-11	31-T	78-H	9	76-Com	2-A
W-89D	44	18	3077			10	78L-Com	1-A
W-85N	901
W-95D	45

The bottom three plugs are special extension models

Recommended torque for a cast iron head is 37 lb-ft 35 lb-ft for aluminum

10 mm cold down to hot
New Champion	Old Champion	A--C	Auto Lite	Edison	Bowers	Globe	Blue Crown	Leonard
UY-6	Y-4A	104	P-4	3-S	10C	F-10	T-4	10-F
UY-6	Y-6	106		4		F-11	T-6	10-E
UY-6	Y-8	108	P-6	2-S	10N	F-11A	T-8	10-D

Recommended torque for cast iron head is 14 lb-ft 12 lb-ft for aluminum

14 mm cold down to hot
New Champion	Old Champion	A--C	Auto Lite	Edison	Bowers	Globe	Blue Crown	Leonard
J-57-R	J-2	42	A3	58-T	C14S	12A	M-2-Com	14-G
H-8	H-9	43L	A5	57-TS	14C	13	M-4-Com	14-F
CJ-8	J-8	45	A7	55-S	14N	14	M-7	14-E
UJ-12	J-12	47	A11	52	14H	15	M-11	14-C

Recommended torque is 30 lb-ft for cast iron heads 28 lb-ft for aluminum

18 mm cold down to hot
New Champion	Old Champion	A--C	Auto Lite	Edison	Bowers	Globe	Blue Crown	Leonard
D-6	4 Com	82S			C18C		R-82	6
D-9	5 Com	83-S	B5	49-TS	18C	24	83 Com	6-F
D-14	6 Com	84	B7	48-TS		25	84 Com	6-A
D-16	7	85-S		45-S	18N	26	85 Com	5-HD
D-16	8 Com	86	B9	43-TS		27	86	9
D-21	C-15	87	B11	43	18H	28	87-S	5
D-23	10 Com	88-L	388	44-HS			88	8
D-89D	49	18A		42-T

The bottom plug is a special extension model

Recommended torque for cast iron is 34 lb-ft 32 lb-ft for aluminum heads

For 1/2" pipe plugs recommended torque in cast iron is 35 lb-ft

All values are for clean lubricate threads.

Over tightening a plug with a .025 gap the gap will increase to .031.

Created from a 1947 heat range list and current Champion list.

by Chris Klossner (email address deleted from original post)

 

[mrcvs]

Dumb question, but I don't know...

 

Just noticed parts catalogue reprint states AC spark plugs.  My mind was thinking AC Delco.  Is this one and the same or two different spark plugs?

[TerryB]

AC spark plugs became AC Delco many years later.  The pictures I posted are from an original 1927 catalog I have.  The asterisk next to the auto brand indicates it was what the factory installed.  As stated above, old AC plugs are out there, you just have to look hard for good useable ones.

[oily rag]

Maxwell may have used Champion plugs. The last one I worked on had (Montgomery)  Wards #18 plugs in it.

[C Carl]

As you see, the replacement chart Mark has given us does not go far back enough to include AC "type A", as spec'd in Terry's data. In any case, even THOSE 1927 spec's may not apply to your 1917. Gasoline was different in 1927, from that of 1917, (and obviously RADICALLY different from todays fuel). And that is assuming everything else in your engine being equal over the intervening decade. Shaky assumption, at best. You will eventually find a perfectly acceptable modern substitution for your plugs. More important yet, is setting a significantly higher initial advance on your timing, and having your distributor re-curved for running on modern 87 gasoline, rather than the 40 octane of 1917.  -  Carl