willclark77

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What kind of life do yaw normally get out of a connecting rod and how do you know when to replace it, besides breakage, discoloration, sloppy bushings, etc? Just curious. I'm about to put a new piston/sleeve in two of my mach 427s and my back up mach 454 before long. The bushings still fit snug on the pins and each engine has 4 to 6 gallons on it and one 427 has new front and rear bearings. Thanks!

Chris_RC

If there isnt any slop between the rod/crankshaft then your fine.

Lille-bror

Should you only look at the bushings? How about metal fatigue?

downunder

It's generally accepted that up to about .006" ovality is ok for aircraft engines. Rod bushes will only wear in one direction, upwards on the lower bush and downwards on the upper bush. The upper bush can't really be checked without removing it from the piston but there shouldn't be too much movement when twisting the rod sideways. When rod bushes wear excessively the piston then sits lower in the liner while running and this changes the port timings and lowers the compression slightly.

As for life, it's hard to say because there's so many variables but my son's Mach28 has 40 hours of racing and the rod has only .0005" ovality. But that's using a lot of castor .

willclark77

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Thanks for the replies gents. I was mostly referring to metal fatigue. I've built quite a few 1:1 engines, mostly small and big block Chevies and one LS1, and aluminum rods aren't typically used in street applications because of their relatively short lifespan (and the fact you usually have to grind the crap out of the blocks and run small base circle cams to clear em). That was what I was curious about.

I reused the rod in one. Haven't pulled the others apart yet. There was no measurable play in either end. I broke her in yesterday evening. It went much easier than any engine I've done ever, which leads me to the following:

DownUnder, you mentioned using a lot of castor. The only fuel I have available, besides ordering and paying $$$ shipping, is Blue Thunder which contains no castor. The cylinders and bottom ends are quite "dry" and clean inside, even ran rich. Cleaner/dryer than even my race tuned two stroke MX engines ever were. So, before breaking this engine in I mixed in 2% pure castor. The break in went EXTREMELY well and so easy I was getting worried. Didn't even foul the USED plug after 5 tanks of heat cycling and one playing but rich enough to hold the temps at 200 even. Anyway, I'll continue adding castor until a better fuel is readily available. How much is a good amount? 2% castor has been clean enough thus far that I may go to 3% or more. Thoughts?

downunder

I've never played around with small amounts of castor but it's generally accepted that 3% castor (mixed into the normal synthetic) gives reasonable protection. Apart from being an excellent lubricant it's also just about the best ARO you can get. Personally I use only castor, no synthetics at all, and my son also used 20% all castor in his car engine. What I'd suggest though is trying out some plane fuel which has castor in it, the extra oil won't hurt the power and the engines will last longer.

As for metal fatigue, this doesn't seem to be a problem with model engines because plane engines can run for maybe 1000 hours (although at much lower revs) with no rod breakages. Usually rods break when the lower bush seizes on the crankpin and this twists the bottom end off. Castor will help avoid this.

willclark77

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Thanks again for replying. I was adding 2% and just tried a few tanks at 2.5%. They sure seem to like it. On one I've ran 3 to 4 tanks plus 5 heat cycling to break in on the same glow plug and things have went eerily smoothe.

rcnitrohead

I've never replaced a rod in any of the 6 engines I've run. I got up to 9 gallons with two before they lost compression, never had a rod go on me. It depends a lot as well if its a sport or race engine, for a race engine I would just do it for insurance since it is an investment and a high performance engine.

Colt4g63

Just for safety and re assurance i recommend changing the rod after the 1st 1-2 gallons of fuel. Depending on engine the pinch can be VERY tight which is hard on the con rod. Its just good maintenance and cheap insurance IMO.

SLAYERDUDE

I'm just a moron and when I can tell there is some play in the top end by when the piston is at TDC I can "feel" some play it's time.
There is a definite "loose spot" for lack of better words.
At TDC I can feel a spot where the piston stays and I can wiggle the crank and I'm pretty sure the rod is moving.
This is a K5.9 HPI Savage XL engine with 7 gallons.

j_blaze

that's the best way to tell, you can get away with a little slop but too much will take away idle, or worse. thats why its important to tune to smoke and not temps. if the rod is properly lubed it should last 5+ gallons easy. also if properly lubed the rest of the motor will also. best thing to do is every race day, in practice, richen the hsn about 1/4 turn right from the start, then run. lean it 1 hour at a time until it has enough performance for th track and layout. something like a big triple is a good gauge, lean it until you have good smoke and can clear the triple with proper run up. then adjust your lsn a bit if needed.

willclark77

My Feedback: (2)

Part of the piston not moving at bdc and tdc while the crank rotates is the piston dwell time. It's just from the crank pin moving in an arc while the piston moves in a straight line. The piston moving while the crank doesn't is slack/wear though. Pretty sure you knew that. Just wanted to clarify for anyone else reading that may not. Luckily my rods in question are showing no measurable wear.

Anyhoo, thanks again gents for the replies. I'm still fairly green to itty bitty nitros. I still haven't ASSembled the other Mach 427 or 454 yet. I'm gonna do some whittlin on the 427 sleeve and compare it to the one I just put together when I finally get some time away from work. Not that my subpar driving ability needs more power, but lower temps would be great. We'll see. I may have a new keychain.

Zerodefect

In engines with that horrible new engine pinch like the Os vspec and Go engines I'd get a rod to use just for breakins. The stressed rod will usually wait to snap or explode its busing long after break in on a race tune.

That way you can pop a fresh rod fter a gallon or so. Keep the old rod to use during breakin of your next Vspec. Put the rod that Vpec came with back in after a gallon.

With RB and the high end Novarosi's I wouldn't worry about it. They loosen up alot with a good heatgun.