Author Topic: Ohrndorf V12, new challange  (Read 21130 times)

Offline Admiral_dk

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Re: Ohrndorf V12, new challange
« Reply #285 on: September 12, 2024, 04:07:58 PM »
You have made a lot of Progress since my last comment here + shared some interesting Info  :ThumbsUp:

So thank you very much  :cheers:

Per         :popcorn:

Offline petertha

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Re: Ohrndorf V12, new challange
« Reply #286 on: September 13, 2024, 05:43:12 AM »
I don't quite understand the brass weight procedure. Are you looking for consistency in travel indicating the bore is parallel? Or using this as indication of the fit itself?
Are you replicating Ohrndorf's recommended tolerances but to your own (maybe slightly different) nominal dimension? I think he had the liner bore at 21 +0.06 +0.05 and the piston OD at 21 -0.01mm

I remember checking his plans spec against an OS piston on my 5C radial because I was using the OS rings. I seem to recall it was reasonably close but I decided to match the OS piston dimensions exactly just for consistency. The OS crown diameter (portion above the piston ring groove) was reduced slightly vs the main piston body diameter which is quite common on commercial glow engines to help mitigate ring breakage, or actually worse, yet exceeding yield of the ring which usually means it breaks sometime later while in the engine.

I prepared a table of Ohrndorf dimensions because I was curious myself if he changed the dimensions over the years / engines.

Offline fumopuc

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Re: Ohrndorf V12, new challange
« Reply #287 on: September 13, 2024, 08:14:29 AM »
I don't quite understand the brass weight procedure. Are you looking for consistency in travel indicating the bore is parallel? Or using this as indication of the fit itself?
Are you replicating Ohrndorf's recommended tolerances but to your own (maybe slightly different) nominal dimension? I think he had the liner bore at 21 +0.06 +0.05 and the piston OD at 21 -0.01mm



Hi Peter, thanks for making this chart with the comparison.
Yes, I have made the liner bore to 21 mm (20,99 mm), in the hope that it will match with my purchased 21 mm piston rings.
So the max clearance, in accordance to the drawings should be max around 0,07 mm.
I have tried to make some samples with my "hobby measurement equipment and without any air conditioned surrounding", close to this value.
The brass weight was used to overcome the minor friction at this first attempt, because the gravity alone was not able to let the piston go done.
The surface of the table is not flat, there is a kind of rough structure, so the air can escape.
 
 
Kind Regards
Achim

Offline petertha

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Re: Ohrndorf V12, new challange
« Reply #288 on: September 13, 2024, 04:45:53 PM »
You are welcome. One thing I forgot to mention is that possibly Ohrndorf specifies his piston/liner annular clearance somewhat related to his own method of making compression rings, although I think those are kind of 2 separate issues. The ring is the principle sealing component, the piston is kind of the carrier. Its not that simple but hopefully you get what I mean. He doesn't use or reference 'heat set' ring making methods like Trimble, but as mentioned, aside from the reduced crown diameter, the annular gap was close to other (commercial) engines I measured. The only other variable is what aluminum alloy they use vs what is more available to us. I discovered after the fact that tensile strength degrade at different rates with elevated operating (combustion) temperature, so the strongest material at room temperature may not be the strongest at operating temp. Luckily I guessed right but was not aware of this factor.

FWIW, this year I treated myself to 2 indicating micrometers encompassing my expected work range (0-25mm & 25-50mm). You can very rapidly & accurately measure many positions of a part, either while still in-situ on a machine or on a work bench. I find the 'analog' needle range deviation display to be visually more useful than digital 0.00000, especially on operations like lapping. So, useful for direct dimensional sizing, or as a comparator to some standard like a gage block or whatever. The floating anvil is what makes this distinctly different than a regular micrometer. Brand new they are very expensive, but many nice used ones can be had on Ebay & such if you are patient. I'm kind of a Mitutoyo fan boy myself, but I came close to buying German Suhl, particularly from this seller. Fortunately for you, seems like majority are metric & closer to your home.

 

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