Cleaning lapping compound

[Tim Wescott]

I've had a semi-completed project sitting on my bench for about six years while Life happened. Around in 2012, I took a Cox TD which (I thought!) just needed a bit of lapping, or perhaps a new piston built. It turned out that the piston and cylinder were oval, and the con-rod and crank were bent (with matching bends!). So I ended up making crank, prop driver, con-rod, piston, cylinder, and head.

The engine ran, but poorly. When I was done "breaking it in" I noticed that it has the compression of a war-weary veteran, not a brand-spanking new engine. I assumed this was either from a poor choice of metals (steel-on-steel instead of making a cast piston), or because I didn't adequately clean the lapping compound from the bits before trying to run the thing in.

I was cleaning up my shop, and when I picked the engine up from next to the lathe (a lot of "life" has happened to me in the last six years!) I noticed that the porting is way off -- the transfer ports are actually opening a bit before the exhaust ports. So, I'm thinking that since the engine is basically crap at this point anyway, I may as well move the exhaust ports up a bit and see if the thing runs. But I'm still left with the poor compression, and I don't want it to get even poorer before I see the engine working right.

I used carborundum lapping compound for the initial work, followed by Brasso. How does one make sure that one has all the lapping compound cleaned off? I understand that I could use an ultrasonic washer, but if there's some higher-labor, lower-tech way to do this I'm all ears -- I'd like to defer the purchase of a cleaner until I know that I really need it (meaning, y'all convince me, or I've done more than one half-assed job of engine building).

 

[petertha]

Short of a large magnification microscope, I mean how would you ever know for certain its entirely out? The thing is, there isn't appreciable difference between a lapping 'tool' made from aluminum which is chosen specifically to embed & hold lapping compound in order to remove material from another part like lapping a cylinder and your valuable engine part, also made from aluminum. From what little lapping I've been doing, I find WD-40 & certain hardware thinners do a great job of breaking the underlying lapping oil/grease down & the grit comes away from the part. Other solvents like acetone which I normally prefer because it evaporates so quickly, isn't quite as effective so you may have to experiment. It kind of depends on the polish itself. But abrasive that is mechanically embedded in the part is another matter. Short of an ultrasonic bath, I guess just gentle brushing with something like a soft bristle toothbrush & change the solvents often? I think diamond would be worse because its sharper & harder so would continue to lap its mated parts whereas some of the other aggregates break down more rapidly & hopefully wont wear your engine much more when re-assembled.

 

[Tim Wescott]

Aluminum laps, steel piston & cylinder. So they're at least somewhat dissimilar. I did lap the con-rod bottom end to size, but I used lead for that lap.

 

[Goldflash]

Something I always did after honing bores is to boil the part in hot soapy water
( after Solvent wash ) ,
later we bought a purpose built parts washer that hot washed the cylinders ( Some 400 Kg )
This opens up the pores in the metal and release the grit from hones or laps
Proof is wiping with a white tissue afterwards and you will be suprised how black it will get
This prevents from going to run in to run out

 

[Dr Jo]

Whenever I use any abrasive on aluminium I boil the part in washing up water in a saucepan for 5 minutes.

Jo