Motorcraft SP435 / Autolite 5125 Plugs

I was checking the plugs on my 92 F150 EFI 4.9 a while back and noticed the plugs have much longer electrodes that stick out in the chamber further. I remember reading in the F handbook about a preference for this extended type of plug over the oem Autolite 46 and 45 type but I could never find them. Are these the plugs? I know the N/A 300 oem plugs are 45's and are a step colder, but what rating are the 5125's by comparison? Is it still the last number, so they would be a heat range 5 like the 45's?

Should I use these plugs instead of the 45's I was going to use?



VS:

Anyone?

I haven't tried to use the 5125 in a Ford small six but according to the Autolite specs both plugs have projected tips with the same reach of .468 inch into the combustion chamber. This spec assumes the plug is used in the head it was designed for, both have the same tappered seat and are same thread of 18 MM x 1.50. I don't think it would gain you much if anything over a 45. But if you happen to have a 200 / 250 head off you could sure try them both and see if there is any difference between the two plugs as to how far they protrude into the chamber. Many years ago they used to offer both regular reach and extended reach plugs and there was a real befit to using the longer reach. I haven't seen the old regular reach plugs for a few decades I wonder if any plug manufactor even makes them now. Ford also recommended them as replacements and went with them for stock specs in the late 1960's. Though it might not look like it the 45 and 46 Autolites are what was once meant as that longer reach plug spec. Good luck edited I do have an 1981 200 head already off if I had a couple plugs could test the fit to see and take some pictures.

Howdy:

The extended tip idea and development came from Smokey Yunick collaborating with Champion Spark Plug Co in the 60s for use in the hot NASCAR engines of that day. The rationale behind the expended tip plugs was two fold; Getting the spark kernal deeper into the fresh intake charge and to fill the chamber slightly for a small increase in compression. In reality, it was a giant step for a small progress, but most plug manufacturer went to a longer tip forever after. That's the origin of extended tip spark plugs. Will they help our engines? Probably some, but how much is a very large question mark?????

Let us know your progress.

Adios, David

It's strange that the spec would indicate they are the same length. I don't have a spare to look at but it sure looked longer when I had it in hand. In that picture the plug body extends below the threads further and the ground strap looks longer. Like you say though does it really make any difference.. I'll pick one up and have a look while the head is still off. Should have my cam from Schneider soon.

If you look at the Late 4.9L EFI Combustion Chamber it is shaped much different than a small six they are designed with the fast burn technology too. Yeah the plugs from seat appear to be much longer though they have the same listed spec but that spec really only tells you how far it extends below the combustion chamber roof when it's installed in the head it was made for. One other thing that helps is to index all your plugs takes some time but it's almost free, if you have the head off it would be easy to see it. Also for a race motor only you can cut the ground strap back to the center of electrode though the plugs won't last you very long. Good luck edited