BBC Spark plug help. Not MR43t
 

Hi everyone. Wondering if someone can help me find the right marine spark plug. Just had my engine rebuilt. It’s a gen iv Mercruiser but the head castings come back as late 60’s and the typical MR43t plugs are too small. Wondering if anyone would have a part # for a correct marine spark plug. If more info is needed let me know. Basically the spark plug holes in the heads are too big for the MR43t plugs.
thanks in advance

H
 

And the casting # is ?

“Chevy V8's had 14mm-1.25 pitch threads x .375" reach with a flat gasket from the beginning in 1955. These had the 13/16" hex.

In the 70s they started using the 14mm tapered seat plugs (no gasket) on many engines. These have the 5/8" hex. “

If the actual threads are too big then something is seriously wrong or the plugs you have are not made correctly. Like SB said every Chevy engine that I know of uses a 14MM plug thread size. The early ones had a larger hex for the socket but the threads have always been the same.

Let us know the casting numbers.

Also you say you had the engine rebuilt ? I take that to mean you have had it for a while ? Are these the same heads that have always been on it ? What plugs did you use before the rebuild ?

I will get casting number when I get home from work.
the problem is I haven’t had engine that long. I bought it last year as a low hour rebuilt engine. Lasted 10 hours and had major issues. Sent engine to machine shop with plugs in it and came back with no plugs so I can’t look at part # that it had. This is the last piece of the puzzle before installing back in the boat

Casting number is 3872702. I bought a plug that has the 13/16” hex and it threads into the head but it’s not a marine plug. I just want to make sure I put the right plugs in. Any help would be appreciated.
thanks in advance

Think the early ones where 13/16 hex gasket 3/4 reach. Call ngk and make sure you don’t use something to long.

Wow,
Those are old heads. 1967-1972 If you get plugs for a 1970 Chevy truck 402 they should be ok. Although I have built plenty of marine engines I have never really used a "Marine" spark plug. Typically use an NGK plug that fits the engine. Looks like NGK YR5 is the oem replacement ? Or an AC Delco R44T should work fine too. Those are gasketed short reach plugs probably like the one you bought to try. There might be a marine equivalent.

The other thing is you probably should get with your engine builder and find out what pistons he used. Those heads are closed chamber heads and typically result in a pretty high compression ratio on a 454. You will probably have to run at least premium fuel. Make certain so you dont run into detonation issues.

Acdelco standard plugs have steel threads with a black coating that cortodes easy.

Marine acdelco with same #’s and etc is same plug but silver appearing non corrosive plating on the steel. Ngk’s come this way so don’t have a marine designation per say.

Bill K - the r44t’s are tapered seat.No ? I thought the xls’s were flat washered seat.

Ill have to look in the morning. The AC Delco site showed a washer type.

They are tapered. I use the 43t’s and 43ts’s alot andvthey are tapered. 5/8 hex. Same with the r41t and r42t used in the 525sc’s.

Ok I must have really been tired last night. Looks like the plugs changed between 1969 and 70 The earlier ones used a R43XLS which is definitely a gasketed plug. Then it switched to the R44T And if I remember correctly some of those heads could use either one. The only problem I see and its hard to tell for sure without having the plug and the head in front of me. The R43XLS looks like it is a long reach plug and I dont think that is what those heads use.

I think you need to look real close at the plug holes and see how deep the threads are. Also see if you can tell if there is a spot for the taper seat plug to seat ? A real good close up picture of the spark plug hole might tell for sure.

Too bad you dont have the original plugs :(

Hi BillK

yea having the old plugs would definitely make this easier. I can take a pic of the spark plug hole tonight and post. The plug I bought to test is a t44xls I believe and it threads in. Not sure if it’s long reach or not. I will have to measure and will also figure out a way to measure the length of threads in the spark plug hole. Hope to figure this out soon as I’m planning to install engine tomorrow and as you all know much easier to do the plugs out of the boat. Lol

Maybe XL’s and not XLS ? I’ve made mistakes like that before. Lol.

As far as bummed plugs in head are gone, i disagree. I see wrong plugs in engines a lot. Ecsmple: People install wrong length spark plugs in the sbc vortec heads all the time. Uggghhh.

Heads off engine much easier to see if length is correct. :)

Not sure if you have the valve covers still off the heads but there will also be a date code on them which would help narrow it down. Didnt think to ask that before.

If you have a small right angle "pick" you can probably use it to count how many threads there are in the head.

Here you go. Great info in this thread:
https://www.corvetteforum.com/forums...g-for-l36.html

https://cimg3.ibsrv.net/gimg/www.off...0f8fc6851.jpeg
1st photo of spark plug hole
https://cimg7.ibsrv.net/gimg/www.off...5b96115dc.jpeg
2nd pic of spark plug hole
https://cimg1.ibsrv.net/gimg/www.off...fe0a5a33a.jpeg
Plug that screws into head. Gasket type
https://cimg5.ibsrv.net/gimg/www.off...36b36c0be.jpeg
Same as 3rd pic

Not sure if pics will help or not. I can probably use the small pic idea to get a measurement of threads if needed. Also can look for date code with inspection camera. That’s how I got the casting #’s. Pretty frustrating how difficult it can be to find correct plugs. Too many options. I really appreciate everyone’s help and ideas

Info is on that thread I linked. So you have proper plug, although most of us use heat range 43 for the marine bbc’s.
So, R43XL or NGK equivalent will be gine. I’d guess NGK heat range 6 is equivalent heat range.

This might be the most important paragraph in this thread. What was the failure after 10 hours? Has the issue been resolved? Usually if an engine fails from "builder error" it doesn't last 10 hours, it's pretty quick. Did this motor slowly detonate itself to failure?

The photos of the holes in the head LOOK like taper seat. Not gasket seat.

i talked to my engine builder and the pistons they used with those heads they calculated the compression ratio to be 9.56 to 1. Fired it up for the first time yesterday and is running good. Only weird issue is the starboard side exhaust got pretty hot. Not the manifold or riser but the rubber exhaust from riser to transom. Very strange. Stainless Marine exhaustPort side was fine. Plenty of water flow. Engine temp at 130 running on muffs. Gonna take it for a trial run on Saturday and see how the exhaust is in the water if there’s any difference