Wrist pin fits

[digiex-chris]

I'm finishing up my two stroke reconstruction, seems to run great! I'll make another thread with details. My question was, I messed up the wrist pin hole in the piston a little. I drilled with a numbered drill bit the same size as my wrist pin, forgetting that I needed to go undersize and ream. So of course, like all twist drills, I got a slightly oversize hole. The wrist pin is slightly looser than the original piston. The engine sounds a touch noiser than the original piston and sleeve, though I'm not sure if that's because of the wrist pin slap or because of the change in materials.

How much damage am I going to do by leaving it? I'd estimate I'm only oversize by 0.002 or so. Doesn't seem to affect running at all. It's a cast iron piston if that makes any difference. The interior of the piston is aluminum, for weight reasons, so the wrist pin is supported by both CI and aluminum.

 

[cfellows]

I'm finishing up my two stroke reconstruction, seems to run great! I'll make another thread with details. My question was, I messed up the wrist pin hole in the piston a little. I drilled with a numbered drill bit the same size as my wrist pin, forgetting that I needed to go undersize and ream. So of course, like all twist drills, I got a slightly oversize hole. The wrist pin is slightly looser than the original piston. The engine sounds a touch noiser than the original piston and sleeve, though I'm not sure if that's because of the wrist pin slap or because of the change in materials.

How much damage am I going to do by leaving it? I'd estimate I'm only oversize by 0.002 or so. Doesn't seem to affect running at all. It's a cast iron piston if that makes any difference. The interior of the piston is aluminum, for weight reasons, so the wrist pin is supported by both CI and aluminum.

I'm no expert, but an educated guess would be that a high revving, 2 stroke engine is going to want a pretty close fit on the wrist pin. I'm guessing that engine will get a whole lot noisier pretty quickly.

Chuck

 

[Ken I]

I'm with Chuck on this one. 0.002" would be waaaayyy too much on a full sized engine - it will go from bad to worse.

The maximum accelleration occurs at TDC & BDC - with no clearance (or enough for the oil film only) the load is sinusoidal - but with (excessive) clearance there is going to be an impacting load - which is obviously higher than normal (a lot higher) and to add insult to injury, from a strength of materials calculation point of view, this is then doubled by the shock loading.

For this reason any knocking in an engine generally goes from bad to worse in a hurry.

Wrist pins are typically a very tight fit.

Regards
Ken

 

[dalem9]

you are running on borrowed time running your engine . A loose rod will do a number of things to an engine .Broken rod , bent crank if the rod was to break , cracked block , the list goes on and on . Dale

 

[Niels Abildgaard]

Two strokes running not to fast has always download on the wrist pin and therefore Your big hole does not do that much harm.If You care to give us piston diameter,mass and stroke I can easily calculate at what rpm it starts to lift

 

[digiex-chris]

Excellent, I'll go weighing when I get home. Could you share the calculation?

 

[digiex-chris]

Ok, it's as follows:

Piston diameter: .914"
Stroke: .724"
piston weight: between 0.5 and 0.6oz (my scale has a resolution of 0.1oz). Could you calculate both so I had a best and worst case?

The original ran about 9000 rpm on the ground and about 14000rpm in the air unloaded. I could prop it for a somewhat lower rpm pretty easy, the torque curve on these guys seems to support it.

 

[Niels Abildgaard]

Hello Digiex

I assume 15 bar as compression pressure at top dead centre.
On Your piston this gives about 650 Newton down force.

The opposing piston mass acceleration force is to a good approximation

1.25 times Omega squared times half stroke times piston mass in sensible units.

The 1.25 factor stems from connecting rod not being infinite long.

Formula becomes

Omega squared shall equal to 650 divided by (1.25 times0.0092times0.015)

If I have not hit wrong buttons Omega comes to 1941.

1941 divided by 2 phi gives 308 which is rounds per second or 18536 rpm.

Below that speed your connecting rod bearings will not unload

 

[Ken I]

I'm not going to argue your calculations but at some point the pressure under the piston is higher than on top - otherwise the fuel mix wouldn't go past the piston - at that point the forces change sign - for the pressure reaction - admittedly inertia is still on your side but its not the whole equation. The pressure side of the equation is therefore going to cause knocking at low revs.

Also when you close the throttle on a reving engine (may not be the case here) its going to shake rattle and roll on that loose wrist pin.

Many years of racing two stroke motorcycles tells me a loose wrist pin will lead to big trouble.

I might be wrong - but I wouldn't risk it.

If the engine in "noisier" I'm betting that's not a good sign.

Ken

 

[Herbiev]

Its a whole lot easier to fix the problem now than to rebuild the engine in a couple of running hours time. 0.002 is a fair bit and as mentioned, you are running on borrowed time :-\

 

[digiex-chris]

Cool math, thanks a bunch! This engine won't turn over 16000 rpm unloaded. But like the others say, it's pretty easy to fix now.

Related question: The original was a floating wrist pin. Is there any benifit to making a light press fit, or should I just match the original?

 

[Maryak]

Cool math, thanks a bunch! This engine won't turn over 16000 rpm unloaded. But like the others say, it's pretty easy to fix now.

Related question: The original was a floating wrist pin. Is there any benifit to making a light press fit, or should I just match the original?

Not trying to teach you something you probably already know but there are 3 types of wrist/gudgeon pin.

1. Fully floating i.e. both the conrod small end and the pin are running fits. the pin is usually held in position with circlips and the small end relies on the clearances of the pin bosses inside the piston.

2. Floating pin i.e. the pin is locked to the small end and a running fit in the piston.

3. Locked pin i.e. the pin is a press fit in the piston and the small end is a running fit.

A "normal" running fit is 0.002" per inch of shaft diameter so it's a pretty big model which has a 0.002" running clearance on which ever part.

Of the 3 ways, IMHO the easiest for model work is to lock the pin to the small end and have a running fit in the piston.

Just my 2 Bob's worth and I reckon I've overcharged..

Best Regards
Bob

 

[digiex-chris]

The original was interesting. It used a floating pin, but had plastic pucks on the end to keep from scuffing up the cylinder. If I lock it to the rod, I won't be able to get the connecting rod off of the crank pin to remove it. A press fit on this size seems fiddly enough to be easy to screw up and destroy the piston. I think you've helped me make up my mind, thanks. Also, 0.002" per inch is great, I didn't know that. That means I need....0.0004" clearance. If I can see it move, it's not good enough!

Thanks all, the theory and design is a big help. I'll post the results when I've got em.

 

[digiex-chris]

One more question: does high speed steel make an ok wrist pin? I found a drill bit that's the exact size I need. The shank of it is in the softer state that I can cut with a file. I imagine it would have the wear resistance, but does it have the impact resistance?

 

[lensman57]

One more question: does high speed steel make an ok wrist pin? I found a drill bit that's the exact size I need. The shank of it is in the softer state that I can cut with a file. I imagine it would have the wear resistance, but does it have the impact resistance?

I suggest that you use silver steel ( drill rod in the US ) to get the exact size that you need then harden it by the usual methods. With an HSS drill you are tied to whatever diameter is at hand and turning those little hard bits is a real royal Pain, with silver steel you can adjust the pin ad-infinitum or till the cows come home.

Regards,

A.G

 

[Mosey]

How about drill or reamer blanks? They come in very fine increments of size, and they are quite precise in their finish and diameter.

 

[Tin Falcon]

hmm wrist pins need to be a press fit in the piston or in larger engines have retaining rings. I had a "69 Toyota that was in an accident the collision poped the retaining rings they broke loose scored the cylinder and the the thing ended up smoking like the proverbial chimney.
In a model I would suspect the wrist pin is drill rod if it can move and run along thcylinder wall I would expect scoring . as others have siid fix it NOW.
Tin

 

[digiex-chris]

I accidentally made a wrist pin that's fixed to the connecting rod. Turns out, the bronze bushing in the connecting rod didn't end up the same size as the hole in the piston when I ran the same reamer through it, so I gave it a shot pressing the pin through it. I'm ok with that, I'm still able to get the piston installed in the engine. No perceivable play now. Much happier, no question about the result now. Thanks all!