Hi, just found out this block had the wrong plugs ac delco number .CR43TS suppose to be MR43T according to the good book/manual what would be the difference between the two ? heat , depth ? should I stay with the CR . engine is a 454 bravo. thank you.

Are they ngk. Possibly copper core resistor and ts would be tapered seat with 5/8 hex and extended seat. If it's not an ac delco plug I'm not a 100 % on the 43 being heat range.

Should be able to do a cross reference. I've always used nothing but the ac mr43t on marine NA but a number of other plugs are used also.

Heat range can be your biggest enemy and depth off threads. Mr43t's are close to 5/8" from top of seat to the bend of tip

Is it just a std carbed 454 mag

yes both ac delco

Their pretty much the same plug, M is always spec for Marine for nickel outer jacket, differences

CR - Hex 5/8, gap .044, nickel chrome alloy housing, Cost 1.99
MR - Hex 13/16, gap .040, nickel alloy housing, Cost 2.69

There u go. I kinda messed up on some wording as I was driving down the road but you got what u needed. Although strange I've never used that plug. Provably the less expensive plug looked enticing even though it's probably the least expensive thing we do.

On the flip side of things is there a said rule of thumb on heat range go down as you go up in power. I've used the ac43 from stock 454 - 330/365/480/675 hp without issues. What's recommended for a sc 900 hp plus build?

I Dont mean to dirty up the OT however while in the topic....

NGK BR6FS gapped at .40 and on some apps depending which 454 --- BPR6EFS gapped at .45

just saying as the OP never said which 454 he had but to prove not all 454 took the same plugs nor parts cause I know doing this everyday. OP side shows a 1998 Crownline 266 so a guess which 454 he has. Working on boats one has to take the guess work out of the mix completely. Posting plugs numbers not knowing exactly what one has can be an failure in the making --- a huge mistake of advice.

Keep in mind it is important to post exactly what one has and engine serial number. Why because it is a dart board for proper parts. I can not stress that enough. Never assume what was in there to begin with being correct in the FIRST PLACE, My self can care less what the last guy whom work on the boat did because it can be wrong with used boats. Actually not uncommon

BPR6EFS is a projected nose tip and extended reach plug. All spark plugs are recommended per app only not across the aboard.

number 29 in the parts script below from the link and even pic links of the plug. One can see the difference of a projected tip and long reach vs non

MerCruiser 7.4L Bravo (MPI) (Gen. VI) GM 454 V-8 1998-2000 Distributor & Ignition Components Parts

c=commercial
m=marine

43 is the heat range

ts is an extended tip vs the t

there are no mR43ts's for some reason, so for those like me that like to have the slightly longer tip we get the cr43ts's.

hope that makes sense.