Skirby12

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

SB

iTrader: (1)

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. “

BillK

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 ?

Skirby12

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

Skirby12

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

14 apache

iTrader: (1)

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

BillK

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.

SB

iTrader: (1)

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.

SB

iTrader: (1)

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

BillK

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