Penton piston design

[t20sl]

Any engineers out there?  I have never heard why for instance Jackpiner Penton/KTM pistons had such a low mounted wrist pin hole  compared to almost any other manufacturer of the time.  Any ideas?

Ted

[Jason Rowland]

Hi, Ted - I put your Q to a fellow by the name of David Dunbar on a John Deere garden tractor site I frequent due to owning a mid-70's era worker with similar lack of parts and knowledge that Penton owners enjoy. David is pushing 90, and has forgotten more about engines than I'll ever know. High performance vintage 4-stroke multi-cylinder automotive engines are his specialty. Although it doesn't necessarily answer your question, here's what he has to say:

Jason, First, I'm no engineer, have no formal training or education. Piston pin location ( often called compression height) is not that comparative upon casual observation. When I design or alter engines, that dimension usually comes last in the plan. Much goes into a piston design. Rod center to center (c/c) length, crank stroke, deck height and skirt strength are varied for desired torque and/or HP RPM ranges. The pin position may likely have to do w/ port location, crown thickness heat dispersal & strength of pin bosses. Then there's alloy choice, combined w/ skirt cam (ovality), skirt taper, ring pack location passing over ports and ring type depending on usage. Just some thoughts. David

[Toolsurfer]

Wrist pin location has to do with acceptable rocking motion of pistion at the bottom of stroke. This position the most extreme angle between piston, crank throw and piston skirt versus cylinder wall. That's why you get wear taper in cylinders front and rear. Also cylinder and piston are never exactly round in design they are oval be it very minimal. Hope this helps some . Retired big three engineer.

[Jason Rowland]

I'm guessing neither answer really helps that much.  If you could say that the engine had higher compression or softer alloy than the competition that would point to removing the wrist from the heat source. More, fatter, or lower rings would be easy enough to understand, as would a rod also used in another engine that they already had a bazillion of.

[Toolsurfer]

I guess you would have to ask the engine designer for only he knows all the parameters he is given to work to. My answer is only part of the design criteria, not all.

[Toolsurfer]

quote:Originally posted by Toolsurfer

I guess you would have to ask the engine designer for only he knows all the parameters he is given to work to. My answer is only part of the design criteria, not all. As the guy said, port locations in stroke,deck height,rod length,all equate to reliability as well as power at given piston speed. You can look this up on Wikipedia for more thought on WHY!

[JP Morgen]

The KTM engines starting with the 1972 175 were probably designed as a one size fits all piece, in other words, the basic motor design and dimensions could be used for the 175, 250, and 400(350) engines. I think this may have something to do with the 175 piston design, but a bigger reason may be that since the 175 is a short stroke/large bore design, in order to not have too high of a crank case compression ration, the piston was made tall to increase the crank case area.(?)

[t20sl]

Interesting.  Never understood crankcase compression and how it relates to performance.

[wfopete]

Keep in mind, this was likely not a design/end product voted on by just engineers.  Stuff like mass production, parts availability, tooling available and cost considerations probably played a much larger role in the final product.  If we had bikes designed strictly by engineers they might look similar to factory bikes and cost $500,000 in today's dollars.

Pete Petrick
175 Jackpiner
Slow but Good