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Hi Chris, looks like you've done a fine job there, they look very good. Now comes the testy bit - getting a good fit on the piston ;).

It's difficult if nigh on impossible to measure the bore to get the size you need and you don't have much piston material to play with but if you turn up a piece of brass that will just enter as a tight fit in the bore then measure that you will get some idea of where to start on the piston leaving no more that a couple of thou for lapping. The brass will also give you an idea of whether you have lapped that clearance under the exhausts.

If you are unlucky enough to go undersize with a piston don't bin as it it may be reclaimable but shan't go into that now.

Yep you're right it is a messy business - hope you covered your lathe ways over :eek:

Regards - Ramon

Edit - When I said 'two thou for lapping' that's referring to lapping the piston on the lathe using a piston lap to size it so it will just fit into the bore. Then lap the piston to the bore by hand to a final fit - use very fine grit and an absolute minimal amount with plenty of lubricant. (I use 1000 grit silicon carbide - I don't know how that compares to diamond paste grit size)

Good luck Thm:
 
Hi Ramon, the brass plug is a good idea. I will be starting on the pistons tomorrow. Hopefully I'll have some good news to report tomorrow night.

Chris
 
I have roughed out the contra pistons and I am about to start boring the innards of the pistons. I want to clarify something before I do. If I bore the piston internals will I then be able to drill through accurately? Won't a drill bit skate across a concave surface like the inside of a piston? In the Sketchup diagram below, the small white cylinder represents a drill bit (imagine it has a pointed tip). I have no indexing capability so I don't think drilling from both sides is an option for me. Should I drill the wrist pin hole before I bore the inside of the piston, or am I worrying for no reason?
Chris

Screen Shot 2013-03-07 at 1.22.33 PM.png
 
Hi Chris, I don't think it will matter either way you decide.
I usually drill mine as they are turned using a cross slide drilling attachment, the inside having been previously bored out. I just go through with a drill but touch on the opposite face very gently to allow the drill to centre before putting any pressure on. 'Touch wood', I've experienced no deviation so far (except when I hadn't aligned the attachment properly :rolleyes:). If you are that concerned you could grind a 1/8 centre drill down to just under 3mm. I did one down to about 2mm on the off hand grinder using a round bar holder with a grub screw holding the CD in the end.

Can't wait to hear how you get on with the fitting
Regards - Ramon
 
Ok, thanks Ramon. I'll stick with Plan A, which is to drill & ream the wrist pin holes in the lathe milling attachment after I've bored them.

I turned up a brass plug for each liner as you suggested. I was very happy to find that they are slightly tighter in the top of their respective liner than the bottom. If I can just finish these pistons without any mishaps it should be plain sailing from there.

Chris
 
Hi Chris, it's all gone a bit quiet ;) - I hope it's all going okay for you. Have you lapped your pistons in yet?

My cases are finished, just taking a few days out before tackling the rest.

Hope to hear from you soon

Regards - Ramon
 
Hi Ramon,

No action this week. I have successfully drilled & reamed the wrist pin holes. I have made a piston holding mandrel like yours and I hope to lap the pistons this weekend. I plan to take it VERY slowly.

One of the liners has a slightly larger bore than the other, so I will lap the piston for that one first. That way if I go undersize on the piston I can continue lapping it to fit the smaller liner (hopefully). Then I'll get a second chance at lapping to fit the larger liner.

I'm still following your thread on MEM. Those crankcases are very complex. Nice work.

Chris
 
Well, the weekend was a write-off thanks to kids' sport, but today I managed to get a small amount done. I finished the piston mandrel and the piston lap. I hope to lap the pistons on Thursday.

piston lap & mandrel.jpg
 
Nice one Chris - best laid plans and all that eh? Looking forward to hearing more in due course.

Ramon
 
Success!

I pretty much followed Ramon's technique from the Tigre thread with a couple of changes (shortcuts actually:D).

I started by zeroing the piston in the 4-jaw chuck and turned it down to about 0.08mm (3 thou) larger than the brass plug I had previously made to a snug fit in the liner. Then I lapped the piston with 600 grit diamond paste and oil until the liner felt like it was just about ready to slide onto the piston. I cleaned piston (still mounted on the mandrel) and lap in the ultrasonic cleaner and continued with 800 grit until the liner would just slide onto the piston by about a third of the piston's length. Another clean in the ultrasonic cleaner and then I finished the job by hand-lapping the piston to the liner with 1200 grit and lots of oil. I 'wrung' the piston into the liner until it popped through, then called it quits.

A final clean in the U/S cleaner and I checked the fit with a good film of oil. Feels nice and snug. Now onto the CPs and the conrods.

Edit- the shortcut was that I didn't make a special T-handle for wringing the piston into the liner. I just left the piston on it's mandrel (last pic).

IMG_20130321_113834.jpg


IMG_20130321_113943.jpg


IMG_20130321_104614.jpg


final lap 1200 grit.jpg
 
Nice work Chris, wont be long now.

Just a thought - don't know if you've made the wrist pins yet but fitting them with brass pads each end will help prevent the potential of scoring the liner if the pin floats. As they are quite small diameter that might not be quite viable so failing that a nice well polished radius on the ends will do and if you can make them so they are a tightish fit in one side of the piston even better

I'm back on mine as of today :)

Looking forward to seeing them finished

Regards - Ramon
 
Thanks Ramon. I've been giving the wrist pins a lot of thought. The plans call for a 9mm long pin with radiused ends. Given that the bore of the piston is 6.5mm where the pin goes through, it doesn't leave a lot of cast iron for the pin to engage in (<1.25mm each end). Brass pads would reduce that even more. I think I will go with polished ends and a tight fit as you suggest.

A couple of questions if I may:
1. Should I make the pins with a very slight taper by setting the topslide over (say 0.5 degrees)? That way I could turn them over-length then slip the piston on to the point where it's a tight fit, mark the pin and part off.
2. I assume I am supposed to heat treat the drill rod after turning and polishing the wrist pins?

I hope you had an enjoyable day in the workshop.

Chris

Screen Shot 2013-03-23 at 9.42.55 AM.png
 
I don't think I would turn them on the taper Chris, as that would mean the little end is wearing in one spot. If you are going to turn them make them very slightly up on size overall then 'polish' across (240 grit emery cloth, 320 wet and dry or a fine grade stone) until the pin will slip with a push fit into the piston from one side coming tight on the trailing end as it starts to engage on the other side of the piston. You can still make it over length as you suggest and then cut and finish turn to suit. Polish the ends before and after heat treatment. I don't know if you are familiar with H&T - I know silver steel is usually water quenched but I always use oil to lessen the potential for brittleness. Don't heat them too much either, just a nice uniform dull red and get them in the oil quickly before they begin to cool. Also temper them in hot sand for much better control. I have a small tobacco tin full that I preheat and bury the pins in it but constantly turning and checking them - at the size yours are they will soon heat up and you could take them too far - a nice blue going on brown is about right.

Hope that helps

Regards - Ramon

PS yes I did ;)
 
I finished the conrods today. There's a lot of work in these and I found it difficult to get the sides of the rods parallel.

I started by drilling and reaming 2.5mm and 3mm holes at 20mm centres in 3mm thick aluminium plate. I made a pair of filing buttons from silver steel (drill rod). These are 5mm/3mm for the bottom end and 4mm/2.5mm for the top. 2mm screws hold the buttons and rod to a steel plate with M2 holes tapped at 20mm centres.

Note the Aussie 10 cent coin is the same size as a US quarter.

Chris

Screen Shot 2013-03-30 at 4.38.48 PM.png


filing buttons.jpg


filing conrods.jpg


finished rods.jpg
 
Nice work Chris - I'm looking forward to the big moment :)

Ramon
 
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