Pistons shot, what else to replace?

[standish]

Alrighty.

I opened up the head on my force 90 (1990) to check it out. I figured since I'm replacing the engine it'd be fun to see if I could find and fix the problem... and if I just make it worse no big deal.

I found my issue, it's two roasted pistons as seen below.

Now, my question is can I replace those pistons? What else would be wise to replace while I'm at it?

I the cylinders around the pistons seem fine to the naked eye, and the pistons slide very easily on a hand crank.

Any tips or things to think about? I've not worked on an engine since college, and even then it was more experimental than practical... though my stuff ran - mostly
http://www.flickcabin.com/sessions/93a13a958b54dd205e9a9e1684d0591195f79b9b/DSC_0073.jpg
http://www.flickcabin.com/sessions/93a13a958b54dd205e9a9e1684d0591195f79b9b/DSC_0074.jpg

 

[Frank Acampora]

Re: Pistons shot, what else to replace?

First off, those are 3.375 (3 3/8) bore pistons if the engine is stock and has not been previously overbored. They usually have a semi-keystone shaped (tapered) top ring which uses combustion pressure to force it tighter to the cylinder wall while the second ring is a flat cross-section.

It is difficult to see the damage in those photos but it looks like the pistons have a little melted damage at the exhaust side. This could also indicate broken rings. Rings can and do catch on the exhaust ports, so no matter what you do, while the pistons are out, chamfer the edges of the ports slightly. But I can not see the typical pitting damage from detonation on the piston crown and baffle so I can not hazard a guess as to cause of the damage. But typically, with that small amount of damage, the bearings and crankshaft will be ok so it is basically just a piston replacement.

You indicate the cylinders look fine but you need to run your nail around the bore to check for scoring (scratches). Light scoring can be tolerated but deep or large amounts of scoring absolutely requires overboring and this involves disassembling the entire engine.

While boring the cylinders is always the best option because they give you basically a brand new engine, there are other options.

IF--and that is a BIG if--the cylinders are not scored It is possible to simply replace the pistons without dismantling the block. The rod big end bolts can be accessed from the front by removing the manifold. Unbolt the two 12 point screws and simply push out the piston. retrieve all 16 rollers and the two cages. The difficult part is pressing out the wrist pin. You need a 6 ton press and a special tool to avoid damaging the small end rollers. This is not a problem if you buy WISECO pistons. They come with a wrist pin and you can/must buy a special bearing kit for the piston. Wrist pins are slip fit held with circlips.

When you install the new piston and rod, you first coat the big end with grease to hold the rollers. Insert the rollers and cages being sure all 16 are accounted for. Bolt on the cap and align it carefully before torqueing the 12 point bolts. This is a critical step and you must work carefully and take your time.

New pistons will get you running again, but understand: If the cylinder is out of round or tapered by more than .002, you will not get full performance. At this point, the factory recommends overboring the cylinder(s). So, it would be wise to "mike" the bores before making any decision. Also, It is recommended that you lightly hone the affected bores to "break the glaze."
Cylinder liners get "work hardened" and sometimes can get as hard as glass. Light honing will help the new rings seat against the walls. This can be done with the engine assembled if you are very careful and remove all traces of grit and oil from the honing.

So, there you go!