DOH....too tight

[snowman]

So, I just finish turning my piston down, polishing it, now it's time to lap the two together.

Load it up with toothpaste and put the piston in the lathe spinning really slow (I wanted my valve grinding compound, but couldn't find it).

Turn on...lapping...lapping...lap...NO...NO...NO

The cylinder sucked the piston in and it is officially seized. Can barely turn it with an 8" crescent. I have it in the freezer right now hoping I can get the piston cooled enough so that when I hit the cylinder with a little heat really quick, they'll pop apart...but it's not likely.

That's a waste of a morning.

oh well

 

[BobWarfield]

You never know. The heat of lapping a tight fit may have expanded it to seize in the first place.

BW

 

[snowman]

Nope, it's done. lol

Just snapped it.

Too bad too, it was coming along pretty good....

Guess next time I follow my instinct and lap the bore PRIOR to lapping them together!

 

[gilessim]

Some toothpaste has glue like stuff in it that works with heat and friction, once I had a piece of metal stick to glass while polishing it ,try some alcohol as a solvent, it might work!

Giles

 

[Mcgyver]

sounds like it galled up, which believe it or not means you welded the pieces together.

Guess next time I follow my instinct and lap the bore PRIOR to lapping them together!

I just want to comment on lapping. I'm not sure what engine you're building, and maybe what you are doing is spot on in meeting the objectives, but lapping isn't rubbing the two mating parts together. Lapping, whether in the round or flat, is a quite different process -
you have to use a lap - not two mating parts. The lap is softer than what is being lapped so the abrasives get embedded in it and is expandable so there is control. This also avoids galling because the two metals are dissimilar. If you are putting abrasives between two mating parts you are just creating more clearance, ie slop, and have almost no control over how much clearance is created. personally I would never do that, but perhaps for some engines it might do the trick. How for prosperity, I wanted to clarify its not what's meant by lapping....might be important one day if you trying to follow instructions that call for lapping a bore etc

If done properly lapping will create a very straight and round bore with a very fine finish. It's usually easiest to lap the bore first and then piston to suit - although i rarely bother with external lapping (its the rings that are in contact) OR if a simple engine, polishing suffices. Very fine fits are possible.

Too bad the morning was wasted :-[ onwards and upwards

 

[compound driver 2]

HI
Two points that spring to mind;

Firstly why use toothpaste? Yes ok its got a mild abrasive in it but it also has a heap of muck that is more likely to cause problems rather than solve them.
If you have to use a liquid polish try a metal polish like brasso. Id put money on doubting any one would brush teath with Brasso so why polish metal with tooth paste??


Secondly if you have enough clearance for the toothpaste why do you need to take more metal out?

Bore the cylinder first to a few thou under size then hone the bore to size using either a three point stone set up or a lead plug cast to the bore. Once you have a good working finish on the bore. And by that I dont meen a super high polished finish im talking a nice fine cross hatched finish. Machine the piston to fit the bore minus a couple of thou and bobs your uncle you have a working fit piston and cylinder.

The cross hatch pattern left from the hone is important it helps oil cling to the bore.
Very important in a steam engine. Polish the bore to a mirror finish and theres a very little oil held on the cylinder wall.

The piston should be a fine finish but not super polished. As McGyver says the piston should never touch the bore thats the job of the rings.

IL try to post some macro pictures of a 1 7/8th bore traction engine cylinder tomorow.

cheers Kevin

 

[rake60]

Personally I DO use a thinned toothpaste and the piston to hone the final running
clearance in my own flame lickers.
I machine the mating parts for a stuff to stuff fit then use a variable speed hand
drill with the slip clutch at it's lowest setting to spin the piston.

I use plain Crest toothpaste and it works very well for me as a mind abrasive.
Any other type has proven to be too gummy.
Aside from the gummy thing, engines are not too concerned about fresh breath.

Rick

 

[rwells]

Any comments on a graphite piston to a brass cylinder? I have the cylinder turned to .599 (shooting for .600) and am getting ready to turn the piston. Some tips have said to use regular copy paper to polish the outside of the piston and work it down to size. Would it help to put a high polish on the cylinder walls? My cylinder can be a bit over .600 and I'll just size the piston to match. Thanks

Rick

 

[shred]

Graphite in brass works great in my Sterling Fan. There you really want a polished and true bore.