Swapping piston and rings??
 

I have two older K&B 40's which are very similar, but not exact. One runs like a top but is well worn, and uses a strap on muffler, the other one is mint, with a nice muffler, but has cracked piston rings. I want to average the the parts on the two engines to make one very nice one. Specifically, i want to take the piston and rings out of the running engine and put it in the broken one. can this be done without damage or bad performance? The carbs have already been swapped, and I have a new head to go on either one, So i want to make a nice engine out of my two older ones. Am i asking for trouble? Both motors have already been broken in.

RE: Swapping piston and rings??
 

Buy two new rings from Frank Bowman and fix both engines.


Frank Bowman
1211 N. Allen
Farmington, NM 87401

[email protected]

(505) 327-0696


Rings are like $8 each

RE: Swapping piston and rings??
 

As Jim said, the simplest thing is to get a new ring, either from MECoA or Frank Bowman. That way you will have two engines in running condition.

If you insist on making one from the two you have, the best way to move the good ring into the newer engine is to slide the liner and piston out of the older engine as a unit - do not pull the piston out - and slip the whole thing into the newer engine. You may not get the sleeve exactly aligned, but by leaving the piston in the ring will turn with the cylinder and remain in the same relative position, you will still have a piston ring that has been broken in. If you pull it out, either to put it in the liner that is in the new engine or to make it easier to change the sleeves, it will never be in the same position, it will have to wear to a new seal in either liner.

While you're at it, don't just get one ring, get two and a new crank case. Put the new ring in the newer engine, and replace the crank case of the older one. That way you'll be able to use the bolt-on muffler on both of them. Yes, all the 0.40 crank cases will interchange, cost is about $25 from MECoA.

Bill.

RE: Swapping piston and rings??
 

How do you slip the liner out? Is it just as easy as "slipping"? I don't want to bother with ordering new parts as they are old engines, not at the top of their game anymore.

RE: Swapping piston and rings??
 

If you need the liners out you need to heat the case on most K&B 40s.

RE: Swapping piston and rings??
 

Ur:

The cylinder liner in a K&B 40 engine is a light push fit when all the parts are clean, but they tend to become locked in place after a few hours running time.

Practice with the engine that has the broken ring. Pull the head and back plate off, use a hardwood stick and try to push the liner up and out - you can push against the bottom of the cylinder liner on the side away from the exhaust, under the bypass ports, inside the crank case.

If you can't move the liner then as Paul said, heat will be needed. After taking the front cover off also put the crank case with the liner, piston and rod still inside, into your oven at 275-300F for 30 minutes and try again. If either liner will not come out just stop, switch the good ring and piston only. Reassemble the engine and enjoy it. The ring will have to reseat, but if you force the liners out you're likely to do more damage.

Bill.

RE: Swapping piston and rings??
 

Thanks William, and everyone else...

I did what Will said before i read the post (about just swapping pistons), and am pretty pleased. I took my time and cleaned everything as i went along, and the whole process went smoothly. Once reassembled i put her back in the plane, put teh electric starter on it and it fired up after 2 rotations. Starts just as good if not better than before, and the new muffler and crankcase look good and sound better. Thanks all.