There's an often repeated warning that running a new ABC rich enough to 4 stroke will damage or break a rod or hog out the bushes because, on the non firing stroke, the piston will get jammed into the pinch and then the rod has to pull it back out putting great cyclic compression/tension strain on the rod. But is it true and how much pinch can be tolerated without doing any damage?

I had an opportunity to find out recently with an engine that would really put it to the test. It's a .42, an odd size but Russian so hey . But it had the tightest pinch I've ever felt on anything with a .05mm taper on the liner and there was no way I could even get it to flick over TDC, it simply locked up. To add to the fun, it has an iron piston in a steel liner so a hair dryer or blowtorch wasn't going to loosen it up either so we can forget about "get it up to running temps quickly" .

I considered lapping the piston at first then decided to run it as it was and just see what happened, after all I had another similar engine so in the worst case I could always steal the rod from that one. Starting it was a real problem but after a bit of trying I found that by turning the crank back to put the piston a little into the pinch then slamming the starter at full speed onto the prop it gave a bit of a running start to get the piston over TDC and finally it started. Being an iron piston I had it set for very rich running and it burbled quite happily at 6000 revs (no throttle, it's a CL engine). So far so good, no bits came out the side of the crankcase. At the end of the tank it stopped almost instantly with the piston jammed into the pinch which took some effort to free it. Start again, runs lovely, jams into the pinch at the end of the tank. This continued for about the next 40 minutes until finally at the end of one tank the piston bounced back against compression but was still a bit too tight to hand start. Hand starting became possible after 60 minutes or so.

The engine now feels very nice indeed, although I've only got it to the 4-2 break point at 12000 revs, but the main thing is that there's no discernible play in the rod bushes. And obviously the rod's still in one piece . So as far as I'm concerned, this puts the story about breaking rods to rest.

But let's not finish there, let's throw in a theory as to why pinch and 4 stroking doesn't damage the rod (or the piston). First, consider the bushes in the rod. They have an oil film which keeps the bush from contacting the crankpin because the oil film develops a hydrodynamic pressure over the loaded surface which pushes the rod away from the crankpin. This pressure is enough to overcome the peak combustion pressure load on the piston acting on a very small area so it's quite high. Sounds simple and it's exactly the reason why a car crankshaft floats between it's bearings. Now consider the piston/liner. It's also got an oil film between them and as the piston starts to get into the pinch the oil film wants to stay there so it also develops a hydrodynamic pressure resisting being squeezed out. This pressure also gets extremely high but now it's acting both outwards against the liner (stretching it out) and inwards against the piston skirt and crown (compressing it). The end result is that there's now a minute clearance between the piston and liner and effectively no pinch so a piston can't be wedged into the pinch and cause any damage to the rod.

This hydrodynamic pressure is why I was fairly confident I wouldn't have a problem with this particular engine although I wasn't totally sure it'd be enough to overcome the strength of steel/cast iron. It was .

I wonder if it would have been worse if it had been ABC starting cold (no preheat) with synthetic oil. I wonder if switching to castor oil would make a differance if the synthetic oil resulted in a broken rod.

Downunder - this cannot be! Those observations can't be the truth! Theory dictates that you are wrong! That engine has destroyed itself but you don't realise it yet!

Seriously... nice post. Thanks for sharing the experience.

Even though I only know 2% of what you guys do..I learn something everyday reading a well written, clear, concise and informative post like above. Even I could understand it. Rich

i don't understand.............what did this have to do with a true ABC setup and running such setup in an overly rich condition which has been known to cause harm not only to the Piston/sleeve setup, but the Rod as well.
when K&B made their notoriously tight squeaker ABC aka "elephant"....they replaced many a rod because people were running them too rich. even Dub Jett likes to run his ABC setups first before shipping as he knows how easy it is to break a rod in an "ABC" setup.
to me this iron/steel sleeve setup is nothing like an ABC setup.
the ABC setup relies on the engine to come up to speed almost instantly while an iron/steel sleeve setup can be tight when cold, but will loosen up quite quickly even in a rich condition.

An ABx type would actually have given the rod an easier time I think because the alloy piston is more easily compressed. A brass liner would expand more easily than a steel liner too. As for using synthetic oil in a similar test, I'd only try that if someone gave me an engine . Synthetics are thinner than castor and combined with the fact that castor likes to stick to metal I'd suspect that a synthetic would be squeezed out of the pinch area much faster which would remove the oil film and the hydrodynamic pressure would be lost.

Maybe I should add one thing here. You don't get something for nothing so having the piston cause that very high oil pressure takes work which means added load on the rod and crank. This engine was a truly extreme case and yet it showed no sign of having to work harder to keep running, in fact it's temperature was the lowest I've ever seen on any engine at 125F .

summerwind
You posted while I was typing . What it has to do with an ABC set up is that a pinch is a pinch and a rod is a rod regardless of the type of engine. The difference with mine is that there's no chance of engine heat reducing the pinch so as far as the rod's concerned it's like running a tight ABx stone cold all the time.

Proven? In all my years I have never seen this happen, not saying it hasn't. I think it goes back to the first ones with very high tapers that is simpy not used anymore.

downunder,

you are comparing a iron/steel sleeve setup to an ABC setup........it's not the same.

the pinch is there, but changes drastically between the 2.

a tight iron/steel sleeve setup loses the tightness with just a fraction of heat. the ABC setup needs to be hot to lose the tightness, whereas if it is run too cool in an overly rich condition, the pinch is notoriously hard on a rod.

I'm not sure how you've arrived at that idea but, instead of me going on about differential co-efficients of expansion, I just tried a practical experiment. I took my other similar engine and felt the pinch when cold then heated the engine to 155F and the pinch didn't vary.

Interesting....

What if the "brass" in the ABC setup was made of S.A.E. "Standard No. 70" Commercial Brass?

This is a Commercial off the shelf brass that comes in Grades A,B & C. Grades A&B are used for deep drawing. As this brass is used for many purposes requiring properties not indicated by ordinary physical test data, it is often advisable to obtain from the manufacturer brass having an anneal or temper adapted to actual requirements.

This brass comes in various tempers: Quarter Hard, Half Hard, Three Quater Hard, Hard, Extra Hard, Spring and Extra Spring.....

On the Rockwell "B" Scale they can reach from 40 to 95 Rockwell "B" .

A Melling Cast iron cylinder sleeve. http://www.mellingcylindersleeves.com/ registers 241-293 Brinell. When converted to Rockwell "B" it comes to about 106 Rockwell "B" which is just shy of the 95 Rockwell "B" brass...

Brass can be pretty hard.

When Bob Brassel wrote:

Something of note. 'Taper bore' engines and AAC/ABC engines can be different. Some have a 'fixed' taper that does not change or changes unevenly. A well designed AAC/ABC engine has a taper and piston/sleeve fit that DOES change. Both the piston and sleeve change with temperature. When the engine is cold (at start) there is an interference that you can feel and hear turning over the engine... with a Jett, the audible 'squeek' is a good thing to hear! As soon as the engine starts (for sport engines, at idle) the engine begins to warm up, creating the appropriate fit between piston and sleeve. As you take the engine to full power and it reaches operating temperature, the fit becomes optimal, and you will hear the engine rpm increase to its full potential. If anything, you want it on the 'tight' side.

Read more: http://www.rcuniverse.com/forum/m_1890371/tm.htm

Perhaps the "Not so well designed engines" that use a brass liner that's real hard "less exspensive material" may in fact NEED the rich break-in proceedures outlined in thier instruction manuals.

Just because it says ABC doesn't mean it's "Dub Jett" designed ABC.....

An engine with a meehanite piston and leaded steel liner ave very close expansion coefficients. They both expand almost exactly the same amount. (according to Duke Fox) I believe this is why he used this combination in his lapped engines.

Yep, its good stuff...http://www.meehanitemetal.com/