piston wrist pin boring.

A friend of mine asked me if I could do some machining on the pistons of
his vintage wayne air compressor. There is excess play on the wrist pins in there bores.
I am a mostly self taught hobby machinist but I do have a general idea how I can do this on the mill.
The thing I am having problems figuring out is to set up the pistons on the mil. I was planning to use my universal
mil in a horizontal way. the problem is that the bore where worn out ovally and my friend thougt he could get them round again with reaming them. He got them round for sure but the reamer found his way trough the hole and they are not in center annymore. How can I get a refernce on the round piston so I am sure the bore goes straight through the piston.

regards

Tim

timbo917 said:

A friend of mine asked me if I could do some machining on the pistons of
his vintage wayne air compressor. There is excess play on the wrist pins in there bores.
I am a mostly self taught hobby machinist but I do have a general idea how I can do this on the mill.
The thing I am having problems figuring out is to set up the pistons on the mil. I was planning to use my universal
mil in a horizontal way. the problem is that the bore where worn out ovally and my friend thougt he could get them round again with reaming them. He got them round for sure but the reamer found his way trough the hole and they are not in center annymore. How can I get a refernce on the round piston so I am sure the bore goes straight through the piston.

regards

Tim

Click to expand...

If I am understanding your situation correctly, the answer is you can't with a reamer. It is going to follow the existing hole.

The reamer would get you close to clocked correctly but then I think you want to affix the piston to your horizontal mill table and center the piston in x using the outside of the cylinder walls as your reference surfaces. Then adjust z to whatever height on center for the holes. Then the only other variable is clocking it radially so the exit hole lines up with the entrance hole. Boring head to make the new hole straight through.

Welp correct solution is to recognise you didn't charge enough and up the price to sort out his comprehensive mess up so the job gets taken away and given to some other victim. Probably not politically feasible now tho'.

Main problem is finding stable reference surfaces unaffected by wear or misaligned reamer to mount it by and pick-up the true hole line and position.

If its full skirted piston the base should still be true so you can clamp it down onto a flat plate with suitable bar across the piston crown and expect it to sit vertical. Unless you are sure the crown is dead flat best to use a stepped ring between clamp bar and piston so the holding forces go straight down the piston wall. The insides of the piston around the wrist pin bores adjacent to the con rod are unlikely to be worn. Make a plate a shake free sliding fit where the con-rod small end goes and set it vertically on the flat plate you found to support the base of the piston. Make a true bore in it at wrist pin height a little smaller than the original pin to give a smooth shake free fit on a piece of straight rod. Using the rod as a reference you should be able to get the piston aligned so you can re-bore the wrist pin holes in the correct position. Take your time. Its a total pest of a job.

Hopefully the inner surfaces of the piston adjacent to the small end were machined but on an old compressor may simply be left as cast with sufficient clearance for the small end. Even if left just cast the alignment should still be good enough for this job.

What does your friend plan to do about wrist pins? If you use larger ones the small end will need to be bored out to match. Not as easy as it sounds without proper jigs.

Clive

Buy a new piston. WTF? Yeah, it's fun having machine tools, but, come on. If your dude can't afford 50 bucks a piece, can he really afford setup time, machine time, and the fucking hard to find oversize wrist pin and bearings?

Buy the right shit.

haven't worked on engines and the like for some years, but check me if I'm wrong. The wrist pin is NOT supposed to be on centerline of the piston. At least I've never seen one that was.

Thanks for the replys everybody. Yep he realy messed it up with the reamer. Schould have left it allone.
Hey Clive,thanks for the advice. I think I got what you are saying. Why could I not figured that out.
I think he had the wrist pins ground until the wear was gone and was planning to rebush the conrod and place bushings in the
piston wrist pin bores to match the wrist pins.

Sorry but my english writing is not that good. The bore is not in the centerline of the piston. It is closer to the top of the piston.
I was heaving problems about figuring out how to get the bore straight to the piston. This with the existing hole not straight annymore.

To reply on woodsrider,new pistons are no longer aviable for this model of compressor.
It is a verry close friend of mine that is having fun with trying to reparing this compressor.
It is a hobby for me so the time needed does not bother me,better than sitting on the sofa watching TV.
I am not charging him for the work,its a favor for a good friend.

timbo917 said:

Sorry but my english writing is not that good. The bore is not in the centerline of the piston. It is closer to the top of the piston.
I was heaving problems about figuring out how to get the bore straight to the piston. This with the existing hole not straight annymore.

Click to expand...

Just to clarify, the distance of the pin bore center to the piston crown is referred to as the "compression height" The offset that was referred to earlier is "bore offset" and means the bore is offset relative to the piston OD. (90 degrees from compression height).

If the wrist pins are still in reasonable shape I would attempt to measure them. There are many styles of wrist pins including some that are tapered. If in fact you do have tapered pins a straight bore wouldn't be a good idea. Also if they are tapered you'll have to figure out how they were originally installed in the pistons to determine which way to bore the taper.

No tapered pins,its the old system. Th wrist pin is fixed in the conrod. so all the movement is in the wristpin bore.

Grab the piston on the top around the ring lands in a 3 or 4 jaw chuck and get it to run true then move the chuck to your mill. With out specialized equipment that's about the only accurate way to hold it. You will need to bore both pin bosses in the same setup to keep them aligned.

If it's a fun project, cool. Lot's of ways to go from there. But, you'd be surprised at the piston availability that's out there. Do you have specs.?

For an air compressor, the only REALLY important wrist pin bore feature is that the piston is not cocked "left to right".

Centerline fore and aft on the piston shell accounts for skirt load and wear "somewhat".

High or low can mean loss of compression ratio (fixed with decking the cylinder) or banging the head (fixed with a head gasket)

Rotation means nothing at all, just trim the skirt to establish symmetry if its not a full circumference skirt.

So, fixture the piston crown and skirt "square", with the bad bore as close as you can eyeball. (use a mandrel and some "kentucky windage".

Bore away!

I learned when I used to do a lot of engine work on Harley-Davidsons, that the best way to do upper end bronze connecting rod bushings was with a Sunnen Hone. A fellow that I knew was a Harley machinist and he had the proper tools for the operation. He could replace the bushings and hone them to a perfect fit on the wrist pin. I always just took the connecting rods to him and he would fit both the upper and lower end to my new bushings and rollers.

I tried a few with an after-market reaming kit that held the connecting rods tight and was supposed to keep alignment so that the job could be done with the rods still in the engine. The fit was just never good enough.

Even if the fit seems good after being done with a reamer, it will not last because it simply isn't smooth enough. JMHO.

I wil give it a go next week and try to post some pictures.

If a piston is designed with a pin offset, it's usually about .060". The offset is towards the thrust side of the cylinder, or the side of the bore that the rod essentially points to as it moves down the hole. It reduces wear on the skirt as well as reduces noise from piston slap. Even with the pin bores messed up from the reamer, you should still be able to detect if the piston indeed does have an offset.

timbo917 said:

Sorry but my english writing is not that good. The bore is not in the centerline of the piston. It is closer to the top of the piston.
I was heaving problems about figuring out how to get the bore straight to the piston. This with the existing hole not straight annymore.

Click to expand...

First, the best advice I can give you is to refuse this job. You simply do not know enough and your tooling is grossly inadequate for the task.

What was meant about the centerline issue is that the wrist pin is NEVER in the center of the piston in either height or horizontally. The purpose of the offset horizontally is to compensate for side thrust on the piston skirt.

slowmotion said:

If a piston is designed with a pin offset, it's usually about .060". The offset is towards the thrust side of the cylinder, or the side of the bore that the rod essentially points to as it moves down the hole. It reduces wear on the skirt as well as reduces noise from piston slap. Even with the pin bores messed up from the reamer, you should still be able to detect if the piston indeed does have an offset.

Click to expand...

Often the piston will have an arrow on the crown. Arrow should point to the front, when installed.

One good way to size wrist pin bushings is to ballize them. This can be done with the rods installed in the motor, after the new bushings are pressed in.