AC Delco MR43T Spark Plug question

[khe]

I thought I'd replace the spark plugs in my 1997 5.7 EFI since they have 556 hours on them. The Mercruiser parts manual stated AC Delco MR43T so I bought eight at the parts store.

I started to replace them and when I pulled the first one out, the existing plug was a half-inch shorter than the replacements yet both have the MR43T part no on them? The porcelain is the same length - it is the lower part that is different. The thread length is the same - it is just the portion that is between the threads and the porcelain that is longer.

Why would there be such a drastic change in the physical characteristics of the spark plugs? It is like they are two different spark plugs with the same part no.

 

[alldodge]

You sure the ones with the longer electrode are not MR43LTS ?

What is your engine serial number?
Is this an inboard or ski boat?

 

[khe]

This is an I.O. Engine serial number is 0K007751.

The length of the threaded portion is the same - it is just the portion that is between the threaded portion and the porcelain that is longer than the plugs that are in the engine. Both the plug I removed and the ones I purchased have MR43T printed on the porcelain.

It seems like a change was made to the plug but one would think the part number/designation would be different.

 

[alldodge]

With the threaded piece being the same, then they are the same heat range and that is the most important item. The length in porcelain is not an issue, probably a change in design to better meet the requirements.

IMO plugs never need to be changed unless performance has changed, but that's just me

You are correct Merc says MR43T for that serial number

 

[khe]

It is just the metal portion of the plug that is between the tapered seat and the underside of the hex portion that is about twice as long as the MR43T plugs that are in there for nine years/556 hours. I agree it is most likely not an issue but I'd like to know why they would make a change like that.

I plan to contact AC Delco to see what they have to say but I'm not holding my breath.

 

[khe]

I found an archived post on another forum on the MR43T plug. Apparently, about nine years ago, the MR43T plug underwent a design change "to provide consistent resistor performance and provide fantastic flashover protection".

At least the mystery is solved.

 

[khe]

I just finished installing the new plugs - funny thing was the #8 cylinder was the only one that had the old style short MR43T plug. That was the one I pulled out first when I started the job the other night. All others had the newer version like I purchased.

That must have been a leftover when the previous owner had the plugs replaced nine years ago.