ML Midge piston

[jack620]

No I didn't know that, so thanks Ramon. What grit of carborundum would you recommend for a 10mm bore?

Chris

 

[Ramon]

Hi Chris - bore size is not so much relevant as is the surface finish inside before lapping.

I have four grades 240, 320, 600, and 1000. The 240 is rarely used but does come in handy if there are any tooling marks that are a little too deep.
I usually begin with 320 moving quickly to 600 which does most of the work only using the 1000 for a final 'polish' and when fitting the piston to the liner.

(If I only had one available I'd plumb for the 600 - it would be a bit slow to begin with but not being too coarse will allow a reasonably good finish as it gradually breaks down in size as lapping progresses anyway)

I don't charge the laps ie by rolling it in the powder but prefer to mix a small amount with some light oil to a thin paste and spread a small amount over the lap. When lapping I find a mixture of paraffin and oil (90:10) makes a very good lubricant

I use a three pot cleaning system - three jam jars with enough cellulose thinner in to cover the part. Drop the part in the first jar and give it a swirl. Remove, wipe clean then drop it in the second. That's usually clean enough for the initial checks. Once the first jar becomes too grotty decant it into a larger jar (if allowed to settle it can be recycled later as it's surprising how much you get through) and the second jar now becomes the first and so on - the third jar is used when fitting the piston to make sure it's really clean before trying it in the liner.

Regards - Ramon

 

[jack620]

Thanks Ramon,

I'll try to source the grits you mentioned. The bores of my cylinders look very smooth, but it's very hard to tell inside a 10mm bore. I used a very sharp HSS tool and backed the tool out under slow power feed after each cut.

I have no concept of how long it takes for each grit to do its work. Thirty seconds? Five minutes? I'll peruse YouTube for a video.
Finally, how do you judge when you are finished honing?

I used to use the 3 jar method to clean my paint brushes when I was seriously into woodworking. Thanks for the tip.

Chris

 

[Ramon]

Hi Chris,

Time depends on initial finish. Tool marks - however fine - soon show up when lapping and it is these that need eliminating. Obviously that has an effect on time.

These three pics show the Racer cylinders at three stages

You are not looking for a high polish finish but a very smooth and uniform one without any trace of marking in it. Despite what it might look like in this last image the finish should have a slightly matt appearance to it.

It pays to 'dwell' the lap at the lower end of the cylinder so that the bore below the exhaust is marginally larger - virtually immeasurable with basic kit but you can feel the difference. Making this slightly larger is a big help when sizing the piston to the bore. This latter op is the important one and is covered on the Tigre thread mentioned above. Perhaps more on this when you get there if you need it.

You also need to think about what kind of lap you are going to use - do you have anything yet? The type above would be a bit testy to make for a 10mm bore but could be done however I think the tapered mandrel type used on the Tigre build would be much more suitable and might be worth considering if you haven't made/acquired anything yet

Regards - Ramon

 

[jack620]

Thanks Ramon, the photos are a big help. I haven't made a lap yet. I've been using the Tigre thread as my reference so far. I will have a look at the tapered mandrel lap in that thread.

I hope to have the OD of the liners finished today. Then I will be back with questions on how to mill the transfer ports before I lap the bores. I'll start a new ML Midge Build thread as this thread is starting to drift from its original intent.

Chris

 

[rklopp]

Model Engine News (Ron Chernich's site) has tons of good tutorial material on laps and lapping for Diesels.

 

[jack620]

Thanks, I'll check that out too.