420 motor pistons

[lowboy]

Hello everybody
I have a 420 c 3 point gas motor
We (I) are Redoing the motor for a FFF project for my boy
when we tore the engine apatr somebody had been in there before.
the crank is .010 under on the mains and rods but they put new standard pistons and rings I think
The top ring on both pistons were broken in lots of pieces
Dad (Grandpa) says hone it out really good and put over sized rings in it
The wear is so excessive that I think I might get away with honeing and putting a .045 piston in it.
With the piston still in the block I was able to measure an 1/8 of an inch slop so I guess what im getting to is what does the .045 piston measure?
The tractor will only see "light"duty "show" duty.
We have a cat D2 for the heavy work thanks
Lowboy and Panther76

[Lavoy]

You need to take the block to a machine shop and have it measured and bored. You can't just hone it a guess on what piston to put in. An .045 piston is only .045 larger in diameter than a standard piston, 1/8" is .125 so you have some serious wear someplace. Pistons are the same price regardless of size, so you will be much better off having it measured and bored for the right oversize. I have pistons in .045, .090. and .125. I do not recommend the .125 piston in a crawler that is working for a living. If you have to go that far, sleeve it back to standard and start over.
Lavoy

[ggfossen]

Lavoy,

Whats the cost on a set of pistons and rings for a 420? I may have to explore the over-bore approach. And overall size of standard pistons?

Gary

[Lavoy]

Gary,
It will vary somewhat on size of piston, and if you need bearings and gaskets, the kit basically gets you the gasket set for free.
Pistons are roughly $275/pair with rings and wristpins. Kit is about $425 I think, can check further when you get to that point.
IF you decide to bore the engine, go through the whole thing, it is not that much more money, and then you are done forever. Nothing I like better then the sound of a fresh engine, and the power.
Lavoy

[ggfossen]

Lavoy,

What's the maximum bore diameter that standard piston and rings will accomodate? I'm honing slowly. If I go over whatever the magic number is, I will have to rebore.

gary

[Lavoy]

I don't remember, but it is very little. Standard end gap on the rings should be only about .012-.015, I would be that with new rings you will be well past that. Piston wall clearance is not much either. Keep in mind, that whatever you are feeling at the top of the bore for wear, there is probably .020-.050 wear in the middle of the bore. I have had several that were so tapered, that they would not clean up at a .045 overbore, we had to go to .090. By an large, that is why my personal opinion is that honing and ringing these engines is a waste of time. I have never had one sqare enough to trust a re-ring job, and in addition to the moving in and out that a tapered bore will cause on the rings, usually the ring grooves on the piston are worn out as well. I have heard of guys maching the grooves out and putting in spacers under the rings, but I highly doubt that is any cheaper than new pistons.
Are you using a true hone, or just a regular spring loaded one?
lavoy

[ggfossen]

I'm using a drill run spring loaded hone. Three stones. Nothing really fancy in my shop. And I have no way of checking diameter deeper into the bore. For curiosities sake, I will lay straight edge on it, and see if there is a perceptible gap...after I fix whatever is spewing water in the well house.
Gary

[ggfossen]

Lavoy,

I think I'll make it without an overbore. I have no visible swelling, or excess wear, in the middle of the cylinders. I used a straight edge and light. Anything over a few thousands would show. The bad cylinder will be somewhere around .025 beyond the good cylinder. It appears that not much has been done in the way of honing, or overbore, to either cylinder. I may miss this bullet...but not the clutch bullet. That one I will have to bite.

I also think I can save the valves. My methods are...unothodox, but I think they will work. I chucked a valve in the drill press, and gingerly approached it with a new stone on the dremel. I am then lapping them to the seats. It seems to be working well. The stem guides are quite tight.

I'm still working towards a list of parts.

Gary

[Lavoy]

Gary,
Keep in mind that if the bore is tapered, there may not be any visible wear with a straight edge, as you are just laying it on an inclined surface.
Draw the letter "V", if you lay a straight edge on either side, they are straight, but nonetheless tapered from top to bottom. This is kind of what my all fuel was like, but to the tune of .027 or more.
Lavoy

[ggfossen]

Lavoy,

In the above, you say that a "kit" is about $425. What all comes in the kit?

I'll clean the crank, and mic the bearings, today, to get an idea if that is necessary.

Gary

[Lavoy]

Pistons, rings, wrist pins, mains, rods, and gasket set.
Lavoy

[ggfossen]

Lavoy,

From front to back: 2.3785 (.001 clearance); 2.4980 (.003); 2.4980 (.004); 2.3785 (.002).

All measurement with electronic mic. I don't have plastigauge available.
As I understand it, or the info that I can find indicates, .001 to .003 is in the ball park. The .004 might be a smidge over???

Do the crank measurements appear to be undersized? I don't have specs on standard.

Once again, I have no prior experience with any of this. I'm open to knowledgeable advice.

The block is on the truck, and I will look for a shop in the morning.

Gary