Mercury top vs. bottom guided piston question

[TODD_TN]

I am building a '92 2.5 200 EFI fishing motor (with a bottom guided crankshaft).

I ordered bottom guided pistons. I received top guided. But, before I send these
pistons back . . .

Is the only difference between the top and bottom guided the width where the rod rides inside the piston ?

I work in a large machine shop with several CNC milling machines, so I can machine these pistons out.

Has anyone ever machined a top guided piston out to use in a bottom guided motor ?

[outasite]

Yes it's the only difference,and it's been done.Going the other way,there are spacers available.But just open them up,to the width of your old ones.

[patchesII]

I send them back. As soon as you touch those pistons with a mill it's yours. Get the tolerance wrong and you own a junk set of new pistons

[Dave Strong]

I send them back. As soon as you touch those pistons with a mill it's yours. Get the tolerance wrong and you own a junk set of new pistons

Well said.

Dave

[TODD_TN]

Thanks Outasite, I measured the (4) old pistons I have and took an average. I'm at 1.065". Will get them machined today.

Thanks also to the guys warning about modifying these. I understand the risk involved.

I grenaded my 2.4 200 (think it stuck a ring) and I'm actually putting parts from (3) different motors together to build a Frankenstein Motor.

[outasite]

When your going from top to bottom guided,you do have a little "wiggle room" at the pin end,quite a bit in fact.The crank end, now becomes the critical one to watch.If I remember right,its about .125 per side.

[TODD_TN]

Turned out Fabulous !

[outasite]

There is just something about doing it yourself, that makes you feel good!! Nice job

[TODD_TN]

Yes Sir . . . I agree.

This is the 7th outboard I have built, and I really have fun digging into them.

Thanks again for your feedback.

[NZ Sidewinder]

.125 is an 1/8 inch.