How to lap engine parts, what size and type of grit used?

[frank mccune]

    Hi Folks:

    I would like to lap some engine parts, p&c cyl and contra pistons, but I need to know what material  and what size of grit to use.  What do you suggest?

                                                                                                                              Tia,

                                                                                                                              Frank Mcune

[Tim Wescott]

My one effort at engine building resulted in an engine that wore out at about the same rate that it broke in.

I suggest that you meditate a good long while on the meaning of the phrase "thorough cleaning" before you reassemble your lapped parts and run the engine!

I have seen super fine (1000 grit?) diamond lapping compounds suggested.  I've also seen Brasso suggested -- that's what I used, and it certainly lapped my cylinder and piston into a smooth fit, before I proceeded to ruin the engine through inadequate cleaning.  I'm sure the diamond lapping compound would have been better.

[Robert Zambelli]

I've used rotten stone on many engines but never on ABC, AAC or ABN.
Super fit, never an issue and it does not charge.

   Bob Z.

[Tim Wescott]

I've used rotten stone on many engines but never on ABC, AAC or ABN.
Super fit, never an issue and it does not charge.

By the pattern of his other questions, Frank's planning on lapping an iron/steel engine. 

I think that the Axx engines need lapping at the factory (and so, presumably, could come to us messed up), but it would be a pretty awesome fella who would improve such an engine by touching that part.  If you have to ask here, you probably shouldn't try (or you should plan on ruining a bunch of engines before you find joy).

[Jim Kraft]

I use Dupont #7 polishing compound mixed with a little oil. I use to use Buff-eze by Martin Senour, but they do not seem to make it anymore. It was very fine but cut extremely well. It was great for polishing dope or epoxy paint also. Somehow every time I find something that works they take it off the market.

[Bill Johnson]

I've used Clover 7A for extremely close tolerance aircraft hydraulic piston-type pumps. As noted by others, a thorough cleaning is an absolute necessity after. About 5 to 10 minutes in a small ultrasonic cleaner followed by a good rinsing in denatured alcohol.

[frank mccune]

      Hi Tim et. al.:

      I have been lapping parts for many years and what the most important thing that I learned is what an imbedding abrasive is!  I learned this by ruining one engine.  No amount of "cleaning" will remove an imbedded abrasive!  I think that this is how a lead lap works.

     I asked here in order to get an idea as to what will work and not imbed.  In the past I have used many types of compounds for various tasks but I was hoping to get that latest hot information.  Yes, this is for bedding cast iron an steel parts.

        A friend of mine has abrasive grits up to and including .000015 grit.  I do not think that diamond will imbed.  I will put the glad hand on him for a pinch of the good stuff! Lol

                                                                                                                          Stay well,

                                                                                                                          Frank McCune

     

[Lauri Malila]

Frank,

Oh, diamond does embed, I'd be especially carefull with cast iron. But it depends on lapping pressure and lap material. In general, silicon carbide is quite safe.
In general we don't lap AAC pistons but sometimes I use the one in picture, with very small pressure and fine metal polish compound, just to remove the slightest deformation that may happen after milling the piston skirt. The lap is made from Tufnol 6F-45, cotton-epoxy composite.

Lauri

[Brett Buck]

In general we don't lap AAC pistons but ...

   For experts only - and almost no one here (myself definitely included) is an expert! At least you know where to stop. The willingness to do stuff like this is inversely proportional to one's knowledge of the topic - the more you know, the less prone you are to do it!

   It is my recommendation that *no one* ever touch or in any way disturb an ABC/AAC piston/liner with home tools. Unless you are willing to destroy something and buy a new one, which is the overwhelmingly likely outcome. I have seen so many engines completely ruined by various "improvements" and "adjustments" that the evidence appears to suggest that it's beyond almost everyone's ability. Note that this includes grinding the ports for various reasons.

    For iron-liner engines, on rare occasions it is necessary to loosen them up just to be safe to run. In that case, do it just enough to be safe, and then stop and break it in normally. It works a lot better and pretty much requires that you run it to get it to fit properly, there is just no substitute.

    I am well aware that people are going to do it anyway, but at least I tried...

    Brett

[Lauri Malila]

In general we don't lap AAC pistons not because some say it's stupid, but because there is a better way without risk of contamination. All you need is a very good lathe and a good diamond cutter.
Let me add that we don't modify/f**k up existing parts but make new ones. And when I very often don't know how to do something, I call the guy who is more or less the man behind the modern AAC technology. He is very encouraging.
But I agree. For those who don't have these possibilities it's better to stay far from such work.

L

[RandySmith]

Hi Frank

I do this most every week, sometimes daily, however you have very good advise here about how easy you can destroy a piston/sleeve, and most any compound will imbed into the soft metal, and wear the hard metal, that is what happens in this process 99% of the time. If you insist on doing the lapping, go very slow, and leave the piston fit tight, clean it very very well, then clean the parts again, I found a ScotchBright pad, and stiff brush works well for cleaning, and you may just want to use Comet or AJAX instead of a diamond or lapping compound, fine pumice powder... and do not be surprised if you wind up with a useless  P/S .  The ones here steering you away from lapping are correct, I have seen  so many  P/S killed by  people doing their own polishing or lapping, you really need a real hone, with different grit stones

Regards
Randy

[frank mccune]

     Hello All:

     What I would really like to lap is a contrapiston and cylinder for a ST Diesel engine.  In the past, I have learned that it is very easy to take too much material from a part!  If I really need to remove metal from a p&c, I will go very slowly or send it out to an engine guy who has the proper tools.

   Back in my university days, I researched lapping materials and the effect that lapping materials have on different materials.  I think that the lapping materials were divided into two categories, one embedding and one non embedding.  What they were is a mystery to me now! Lol  There were lapping materials to be used on different materials and it seems to me that the hardness of the metal to be lapped determined in a large part, what material was to be used.  Some would embed and some would not.  Knowing what to use was the real question.  That was many decades ago.

     I am just too old to look up all of that information now! Lol  It was very informative to say the least.

    Perhaps some of you out there can share your knowledge with us just for the fun of learning.

                                                                                                                 Stay well my friends,

                                                                                                                 Frank McCune

[Brett Buck]

In general we don't lap AAC pistons not because some say it's stupid, but because there is a better way without risk of contamination. All you need is a very good lathe and a good diamond cutter.
Let me add that we don't modify/f**k up existing parts but make new ones. And when I very often don't know how to do something, I call the guy who is more or less the man behind the modern AAC technology. He is very encouraging.
But I agree. For those who don't have these possibilities it's better to stay far from such work.

   Right - you have to be willing and able to traverse the learning curve and live with the (sometimes) negative consequences. It's obviously not impossible to do, just a lot trickier than most people expect or realize.

   Brett

[Wade Bognuda]

I recall Bon-Ami was popular way back when (70's).

[frank mccune]

    Hi Lauri:

    You are the MAN with your machining skills! I would love to access to a machine shop once again as I really enjoyed metal working. 

    Randy: You are so correct when you say that you have seen many items destroyed by people making bad choices about metal working.  I my life, I have made a few gunstocks and when I met a chap who told me that all he used was a die grinder! I wish that I had his skills as I had to use chisels, gouges, saws, etc. Lol

    Thanks to all who replied!
                                                                                                                        Stay well my friends,

                                                                                                                        Frank