Balancing pistons.

[holmen78]

I知 rebuilding my 2002 90TLRA and i知 putting in Wiseco pistons. Should I dremel out the edges marked with red or drill small pieces out with a big drill marked with yellow. Have to take off about 1 gram from one piston. Any suggestions?

[rodbolt17]

put them in and run them.
I am not a fan of wiesco pistons but they work as long as the machine shop knows to bore for wiesco.
not enough weight difference to bother with.

[holmen78]

I hope it works. The shop bored to the bore size stated on the box, actually 0.01mm bigger.

[stoker2001]

in my exper with mercury racing 2.5 2stroke motors that i rev to 10,000,i use a cheap postal scale and get pistons too within .5 grams.I use carbide bit with die grinder at 20,000RPMs plus on bottom of dome and arounf pin boss.your lucky they are only one gram diff!!Not a big fan of the aftermarket wisecos either..i know that they can be good piston but you better get the extra bore clearence right..good luck

[stoker2001]

[panasonic]

I hope it works. The shop bored to the bore size stated on the box, actually 0.01mm bigger.

What is the specific clearance stated on the box?
.01mm equates to .00039" for those of us that use imperial mesurments.

One gram is equal to about 1/4 teaspoon of sugar for comparison ....not very much.Weight the piston an see how many grams it is total...and compare to one gram.

As Rodnut said put it in and put it back together.

[holmen78]

Do you mean I need additional clearance to the bore size stamped on box? That sounds odd, why would not wiseco mention that in any way?

I have understood wisecos are good in dirt bikes and many run them because they last. I知 confused.

[panasonic]

You said shop bored cylinders to the size on the box, that size is required for the piston clearance. What exact size is that?

Then you said they went .01mm bigger than that.....yes?

[holmen78]

Yeah. On box is bore size 3.2483 inches, and that痴 82.506mm. The shop bored it to 82.51mm what I can measure with my bore gauge.

There is nothing about any clearance on the box, just what the bore size should be, that痴 why I was confused by stokers statement to build in extra clearance.

I just measured the pistons and piston to cylinder clearance at the top is 0.25mm, down at the piston skirt it is 0.15mm.

Sounds about right?

For reference when I changed piston on my chainsaw the piston to cylinder clearance was 0.04mm (if I remember right) and it runs great without issues and high compression. But that痴 probably half the diameter of the 90TRLA piston.

[rodbolt17]

out of the thousands of 2 stroke powerheads I have put pistons in I have yet to weigh one.
no need to on a stock rebuild.
manufacturing tolerances work and work well.
the few I modified and weighed for a few of our hydroplane guys we typically took it off the bottom of the skirt.
mostly on TRW forged single ring featherweight pistons.

[stoker2001]

You should be fine if you go by Wiseco specs and not OEM.I just remember back in the 70s we learned hard way on the iron bore merc v6s.its all about diff expansion rates with diff alloys and cast versus forged.then there is the diff bore/sleeve materials iron versus alum coated with Nicasel/nicom

[holmen78]

I guess I will have my answer after a couple of months when the ice is gone

[panasonic]

Sorry I have to work in imperial.

So you got 0.0059" (0.15mm) at bottom and 0.0098" (.25mm) at the top of piston.

Quite a bit of difference in the Wiseco and OEM piston clearance, OEM is only 0.0025"

But as Rodnut and Stoker say as long as it is bored to the WISECO spec (which it is)...you should be good to go!

Can I ask what happened to have it bored out to what it looks like second oversize? Did it seize up or just worn out?

[holmen78]

I have had cold start issues so I decided to open her up. Piston rings were worn (ring end gaps beyond spec) and the lower cylinder was a little out of round probably because of a destroyed lower crank seal. Compression pressure was 90psi. I wanted a fresh rebuild, that痴 why I bored it. It is the first wiseco oversize (0.5mm), and at the same time 2nd Yamaha oversize. Wiseco piston 3131p2. Two more oversize pistons available from wiseco and none from Yamaha.

[rodbolt17]

I would have used sierra piston kits way before I used wiesco.
years back we had some issues running wiesco pistons.
usually it was an oem bore size and improper break in.
then you had the cold seizure issues that all forged piston have.
back in the day you had to idle them for 8 hours before running it up.