Ohrndorf 5 Cylinder Radial

[petertha]

The pistons were made from 7075 aluminum. I mentioned earlier in the build post that I decided from the onset to use commercial piston rings for the O5 as opposed to making my own. I wasn’t sure I would be up to the task of rings on a first engine build. Maybe not so much machining the rings & fixtures, but doing the proper treating for the gap because I don’t have an oven & wasn’t quite sure about using a torch. I wanted to provide the engine the best chance to run, so this seemed logical at the time given all the other variables of engine building. As mentioned during liner making, buying the rings might be kind of false logic, or at least on a multi-cylinder engine, because it thereby requires you to make each cylinder bore identical to one another, to a correct dimension within tenths and to a correct finish, 5 times plus spare(s). Collectively, I think is more work & more challenging.

The O5 bore is essentially the same as an OS 56 4-stroke (0.56 CI). So along with the rings I also I purchased a single piston to replicate the ring grove dimensions and slight diameter reduction around the crown. Picture shows the commercial die cast? piston alongside my tester blank. Also of interest, the OS piston ring dimensions & open gap with is pretty much bang on what Trimble method works out to. The pistons are 0.0025" undersized to liner so just requires careful finishing & measuring using the same micrometer.

[petertha]

The ring groove was cut using an undersized Nikcole grooving tool. I set up a tenth’s indicator on the carriage to better monitor carriage displacement. The width was verified with a feeler gauge stack matched to the O.S. piston. The inner diameter (depth of ring groove) was measured with blade micrometer.

[petertha]

The piston blank was parted off oversize, flipped & trimmed to length. I used tape to protect the finish from the collet. 

[petertha]

The 5mm wristpin holes were drilled & reamed while the blank was still solid. On my initial tester, I drilled the hole after internal material was removed, but I didn’t like the feel of the drilling through the wall, breaking out in the middle & continuing the hole through to the other side. I couldn't actually measure out-of-square but just preferred this solid method, basically pecking 0.050", clear chips & repeat.

[petertha]

Re-chuck the pistons back in the lathe to drill a 0.375” pilot hole to remove material & counterbore the skirt ID.

[petertha]

I made a mill clamp fixture to hold the piston orientated to a dummy wrist pin dowel. Then the rod clearance slot was milled away.

[petertha]

The non critical edges were lightly deburred with rubber abrasive, part cleaned & dimensions confirmed. I installed a ring to do a test fit in a lightly oiled liner. Insertion & movement felt about right and the ring gap seemed the same as the commercial assembly.

[petertha]

The wrist pins were made from O1 drill rod, brought to diameter using the sandpaper lapping block method previously described. I reamed the piston and rod holes the same diameter so the piston fit was a snug sliding fit. And I lapped the rod hole using super fine compound just slightly to what I thought felt like a close running fit. The rods always seemed to require a bit touch up after drilling the oil bleed holes (internal burr?). The wristpins are solid but each end is drilled slightly to accept aluminum pads on either end intended to wear vs scratch the liner bore should the wristpin drift. I have seen Teflon pads used in commercial engines but I didn’t have the right material & was less confident of how to retain them.

[petertha]

I hardened the wristpins using torch method. I’m not sure hardening is required because they run inside aluminum.

[petertha]

The aluminum end pads had a little bleed hole drilled through which I learned the hard way is required, otherwise they can ‘hydraulic’ when the Loctite is applied & can set up in an incorrect position. I left the diameters slightly oversize so that once bonded, they get blended to the pin diameter, chamfered & finish length in one lathe operation.

[petertha]

Assembly pics

[Kim]

Wow, very tidy work, Petertha! 

Kim

[jcge]

Nice work petertha!
Looks like the 7075 machines very crisply (particularly the milled pockets), without pickup on the tool.
Consistent tiny chamfers and your deburring methods leave a wonderful level of finish.
John

[steamer]

The aluminum end pads had a little bleed hole drilled through which I learned the hard way is required, otherwise they can ‘hydraulic’ when the Loctite is applied & can set up in an incorrect position. I left the diameters slightly oversize so that once bonded, they get blended to the pin diameter, chamfered & finish length in one lathe operation.

Oh that little trick is getting filed away for the 917..   Nice work Pethetha!

Dave