70hp merc triple 49cu in pistons

[Arkansas river rat]

I was going through what I need to rebuild my 70hp merc and came across that there is two different style pistons for the 49cu in merc. One is a low dome and the other is a high dome. I believe the difference in the dome height is .10 not for sure tho. I'm going to buy wiseco pistons. I had good luck with them on dirt bikes so I trust them but they have the high dome and low dome piston for 1980 to 1983 70hp merc. Can anyone shead some light on this for me. Raising the compression would give it a little more bottom end but i'll have to run prem fuel and watch the timing on the motor. I'd like to know if any 70hp merc can run either piston basically the do's and don't on these pistons.

[Mark75H]

Premium fuel and watching the piston dome and spark plug for signs of aluminum.

If you can't consistently get good premium or don't want to spend the extra money, go with the lows.

Your measurement is accurate.

[Fastjeff57]

Sam:

I'm very interested (concerned, actually) that I may have inadvertently installed both high and low dome pistons in a triple of mine--maybe even in both of them!

1. Seeing that the in-line sixes use the same pistons (larger motors), do they also use these two types? And, if so, which motors?

2. Is there some definite way to determine the pistons in place? Not sure that measuring from the spark plug threads would work, but...

The 70 triple has its exhaust ports raised at least 3/16 " over the 65 hp triple. I hope Merc didn't get the compression ratio back up with high dome pistons for I MUST have used a 65 slug in at least one of my 70 hp rebuilds.

Thanks as always for your input,

Jeff

PS: Thanks to Mike for determining all this.

[Mark75H]

Merc used the high domes on the early 150's and switched to low domes after a few years and sold low domes as the only replacement.

There is no compression change in the blocks that I know of ... if there was there would be all this big concern over having the "right" block and being able to make a hot rod if you had the right block.

I doubt you can measure them in the motor and would not worry as long as I had a top vent on the triple (NOT NEEDED ON THE 6's)

[Fastjeff57]

Thanks, good buddy. Came to the same conclusion (ie: leave it alone).

Jeff