rallyebmx

iTrader: (21)

as i turn the fly wheel by hand and get to tdc, the piston
'falls into a notch'. The flywheel is not stuck and has a little movement. I can turn it right past this point and feels normal. If you turn the flywheel quickly, I cant even feel this point, but obviously something is wrong. The engine is not in the car anymore. just wondering what this is?

xlgraphicspro

iTrader: (5)

That sounds like a slightly streched rod. When your at TDC, and it gets stuck slightly, and you can feel some play when the piston is locked, then it's a streched rod!

thecman26

iTrader: (112)

Stretched rod? Really?
cause I thought it was "normal".
All my engines feel that way at TDC.
One has a brand new rod in it!

Davidka

iTrader: (86)

By stretched rod he means that the bushing at the bottom is getting worn/ovalized. It can also be that the crank pin is becoming ovalized. You'll probably have an inconsistent idle and start going through glow plugs more frequently too.

bjr48

I actually noticed this on my OS VG too! So that's what I was feeling? I already checked for any visible signs of the crank pin and conrod bushing having any slop in them last time I had it apart and could not detect any. When you check for this kind of stuff is any of it measurable or is it just by sight and feel? Like maybe you could measure the diameter several places around the pin and make sure they all match like trying to measure warpage on full size car brake rotors?

Is there usually any similar bushing wear at the top of the rod where the pin goes?

Hanging out to see what other advice develops here. I thought I already checked what I could and it was just fine.

vladconnery

iTrader: (35)

Actually thats exactly what you are supposed to do. Measure the diamater around the crank pin. Make sure it has not worn down. I don't know what the diamater of an OS is supposed to be. I know what an RB's is supposed to be. Look on the OS website they probally have it posted. This is a common check for engine wear.

bjr48

OK. So more of a diameter check than trying to detect oblong. I imagine oblong/oval wear comes long after you neglect the slop?

Seems like the OP has two responses here. So I was considering the first response. If the engine gets over reved or something will the rod STRETCH and the bushing/pin fit still be like new?

DOMIT

iTrader: (1)

Sounds like a worn out rod, as others have said. The next symptom will be trouble getting it to idle and unexpected and unexplained flame outs. You have 2 choices at this point, depending on overall condition and your budget.

1) Replace the rod. (And I suggest using new clips that retain the upper end into the piston.)

2) Replace the engine.

Davidka

iTrader: (86)

You want to measure the pin at a couple of different points, take a measurement, turn it 1/4 of a turn and measure again. You are looking for a variance to tell you if the pin is out of round. Anything more than .002" variance is going to have a little slop, even with a new rod.

The rods do not stretch per se, the bushing surfaces are wearing out causing the piston to travel innacurately.

bjr48

Like I said, I feel the same thing. Before I got the piston out I expected one of the retaining clips to be dragging or something but they are fine. I had no idea what could be causing it.
Well, just for the learning experience I checked my crank at 4 spots.
0.1945"
0.1945"
0.1945"
0.1950"
I don't have a gauge pin set at home so a #9 drill bit end ( the smooth end!) that measures about 0.195" fits in the conrod snug. Not very accurate I know. Maybe I'll take it to work but I doubt it. So without verifying with a gage pin, that seems pretty new still and certainly not out of round. They fit together by hand well and only the slightest hint of play by feel. Has to be some clearance or it would be a loose press fit and they would not rotate smooth. What's the rule of thumb on too much then if everything is round?

I e-mailed OS on specs but I'm not holding my breath for any info.

And where can we find the more intricate details of engine rebuild/inspection that the less experienced can learn from to catch up to you guys. Not many help articles I have read give much details or "pro" level instructions. I wanna know them. I can handle it! Don't judge me by using a drill bit

thecman26 - You said you feel this when new. That is with the head off or at least the glow plug out, right? And if it's new are you sure you are not confusing the little bit of pinch with worn rod? Just asking - I'm sure I know less than you.

I've learned engine stuff the hard way unfortunately. Am I right that a new engine out of the box and stripped down would still hesitate at the top until it's broken in? In that case it would seem that it's consistent and not random like this symptom of a worn rod.

Thanks all! I hope rallyebmx is thinking some of this too or benefiting from it and I'm not just running my mouth. I'm done.

Davidka

iTrader: (86)

It's your rear bearing. I would almost guarantee it. That is the other place that the slop you feel when turning the engine can develop. put the crank in and while looking in the back of the case try to wiggle the crank laterally from the other end.

bjr48

Well, for me that is 100% right. My engine bearings are on order. I'm determined to keep this stupid engine running for $10 here $20 there and keep it alive to learn so I'm ready for a real engine next time. It ran flawlessly for 3 gallons which I understand is all that a typical VG(P) is worth. An air leak started that I couldn't find. Got it pinched becuase it was pretty worn. Same problem stayed. Finally found it when the pull start bushing sheared into two pieces. The whole time I'm going through clutch bell bearings one set a day. Fixed the pull start bushing, runs better than new again with still the CB problems. So I knew it had to be the engine bearings. Probably the engine bearings all along causing both the broken bushing and several CB bearings. And now when I get the new bearings I'll test the catching feeling turning it over by hand to verify that goes away at TDC! Hope this fixes things for awhile The end of the season is right around the corner so I'm hoping to enjoy the rest of it. Last few weeks were nothing but trouble!

I kind of did the test you say and it was obvious from the front of the engine not so much the rear. I'll bet if I looked closer it would show up in the rear as well. I probably missed it!

platinumjason

iTrader: (27)

Just wondering when you say you had a air leak you could not find did you ever try sealing the carb, pinch bolts and back plate? It may sound kind of stupid but if you're fairly new to nitro it can really get frustrating to mess with chasing air leaks.

bjr48

I pressure tested the engine twice by fuel line/mouth using plumbers leak detector drenching the motor everywhere. Found one or two bubbles coming out of the front bearing and also the pull start bushing. Everything else was perfect. Thought nothing of it at the time. I thought at some point even a healthy engine will push out just a little bit from the front bearing. And I thought that the pull start bushing was probably not the snuggest fitting seal even brand new so I moved on. I'll bet that the bushing slowly failed and I should have looked at it MUCH closer the whole time. For it to completely destroy itself the next tank it had to have had some kind of stress fracture or something detectable I'd guess. It just takes some time for some things to become obvious to me I guess!

rallyebmx

iTrader: (21)

Thanks for all the responses. I guess it's the rod then, as it is sitting here with brand new bearings in it. I was keeping it as a spare, but with this gross feel, I am not gonna trust it.