Packard Forum Index » Post-War (1946-54)

Compared to the great Packard mysteries (e.g. Why did Packard go out of business?), this is almost non-consequential in importance. Why did Packard go from a standard sized sparkplug in 1937 to a much smaller 10mm plug and have that last until mid 1950 production, when it went back to a 14mm? I have looked through the various Counselor and other service documents, including the Bob Aller CDs, but cannot find the justification for these changes. How prevalent were the smaller plugs throughout American car makers of the eras? Did the smaller plug fitment apply to all the Packard engines of the period, or just the 282 and 288 c.i. ?

The 10mm plug movement was initially promoted by GM's AC division with support from Buick and the use was extended to some other GM makes (Cadillac for one) and somewhat sporatically across the industry in general but with exceptions; the were promoted by AC as being more compatible with higher compression ratios. The 356 engine also used 10mm plugs though after it was discontinued Packard did produced a 356 head in 14mm.

Plenty of garages at the time made some extra money by drilling and retapping heads for 14mm plugs. That was not without it's problems however as the thickness of head at the plug threads corresponded to the threaded length of the 10 mm plug and when modifying for 14mm, you now had several turns of thread exposed in the combustion chamber.

If you scout around advertising material of the era, you'll find AC ads promoting the virtues of the 10mm plugs.

I think the 10 mms were supposed to "warm up" faster at cold start, proved to not really have much of an effect.

I thought that it had something to do with the placement of the spark plug for the best burning of the gases and the 10mm was small enough to reposition the spark plug for that.
Wes

Wesley, I've had 10mm and 14mm heads for the same engine side by side; can't say I was specifically looking for spark plug location but I sure don't recall that they were any different.

Akin to pondering one's own navel.
Asking why they used left hands threads, etc. It was the current thinking of that day and once you're a tooled up manufacturer, well, it sticks around.
There is a bit of thinking behind a smaller hole being less affected by compression forces, simply from the cross sectional area and pressure effect but is that an after or fore thought?
Spark plug technology has pretty much evolved with not much substantiative change. Screw a wire down a hole somehow. Rudy Diesel was the only one that didn't seem to see a need for those silly wires...

As small or minor it may seem, the placement of parts often have a reason. That's why the spark plug isn't over the piston but placed closer to the valves. So until I can find that article again, I'll just stop here.
Wes

Quote:

It may be that after determining the best location for the 10mm plug, the same location was used for the 14mm when it replaced the smaller one, or not.

(o[]o)

I just checked the plug location on 2 cylinder heads from 320 engines, from a 1934 Eight with 14mm plugs, and 1936 Eight with 10mm. Plug location is identical and of course, over the valves. Going back a bit, there is a nice engine cross-sectional view in the 326/333 owner's manual that shows the plug location in the same place; as I recall those plugs were 7/8 NPT thread, and later 18mm.

Would the plugs being directly over the valves (I'd assume that over the intake is more important than the exhaust) mean that you could almost check for a sticking valve with a wood dowell or plastic stick of some sort inserted through the spark plug hole as the engine is cranked. Is that a practiced shortcut used by experienced mechanics? Think of the side valve cover gasket replacements that might save.