New Limits Overhaul

cessna170bdriver · Post by **cessna170bdriver** » Wed Nov 09, 2005 10:14 pm

Is there an "official" definition of what constitutes a "new limits overhaul". The article at a Bellanca site http://www.russellw.com/planes/cruisema ... ptions.htm give a pretty good description, but alludes to the fact that an undersize crankshaft may be incuded in a "new limits overhaul".

The overhaul manual for our stock 6-cyl. Continentals gives new and service limits for the crankshaft main and crankpin journal diameters, but with with a footnote: "If crankshaft is worn beyond these limits, regrind journals and crankpins to .010 undersize and re-nitride." There is not a separate set of new and service limits for .010 undersize crankshafts. (This info DOES exist for oversize cylinder bores.) There is however, a set of limit specs on the main and rod bearing CLEARANCES, with standard or undersize journals not specified.

I very much agree with an earlier post that a crankshaft freshly reground to .010 under, with new .010 undersize bearings is a better situation than having a crank at or near service limits with new standard bearings. The clearances are obviously tighter in the former case, but can it be called a "new limits" overhaul with undersize journals?

Miles

lowNslow · Post by **lowNslow** » Wed Nov 09, 2005 11:20 pm

I don't know official answer to this, but I agree with you - it is the clearance that is the critical element here. As the long as the clearance is "new" or "standard" that's all that matters.

Tom Downey · Post by **Tom Downey** » Thu Nov 10, 2005 7:05 am

Read FAR 43.2

then read the overhaul manual for any engine, there is a portion of the manual that gives tolerances, one column will give the new limits and one will give service limits.

all machines are built to tolerances, such as a 2" measurment will have a new limit tolerance of 1.999" to 2.001" an in service limit may be 1.995"

SO, when an engine overhaul is described as a service limit it is under sized beyond new limit, but with in service limits.

When the manual says it may be fitted with a .002" undersize bearing that is not a new limit. that is a authorized undersize allowed without an STC for resizing.

Tom Downey · Post by **Tom Downey** » Thu Nov 10, 2005 7:13 am

""I very much agree with an earlier post that a crankshaft freshly reground to .010 under, with new .010 undersize bearings is a better situation than having a crank at or near service limits with new standard bearings. The clearances are obviously tighter in the former case, but can it be called a "new limits" overhaul with undersize journals? """

This is a bit misleading,

I would much rather have a .002 undersize crank running, than to re-grind one that does not require it. That limit will give you the proper oil pressure and allow one more run before the crank must be reground.

You re-grind a crank to .010" and some thing happens to it, you can throw it away.

cessna170bdriver · Post by **cessna170bdriver** » Thu Nov 10, 2005 4:52 pm

All this talk about new limits vs. service limits, and standard vs. undrsize just became a moot discussion, I think. I did a few quick measurements last night and all the journals are at or near service limits EXCEPT the #5 and #6 crankpins seem to be .010 to .012 OUT OF ROUND.

I'm on my way to the machine shop with my box-o-parts and let them take a more professional look at it, but I don't hold much hope.

Where can I start looking for another O-300 crankshaft?

Miles

c170b53 · Post by **c170b53** » Thu Nov 10, 2005 11:13 pm

What do you attribute the wear findings of your crank to? Was it topped a couple of times before the bottom end was taken care of? My last engine had cylinders replaced prior to my ownership and upon my teardown of the engine I found ridges in the piston pins adjacent to each side of the small end. Can't help but think that that's a short coming in trying to extend the life of an old engine where the bottom end bushings wear and the rods pound the crank.

lowNslow · Post by **lowNslow** » Fri Nov 11, 2005 1:05 am

cessna170bdriver wrote:Where can I start looking for another O-300 crankshaft?

Miles

Maybe time to look for good IO-360.

cessna170bdriver · Post by **cessna170bdriver** » Fri Nov 11, 2005 3:23 am

c170b53 wrote:What do you attribute the wear findings of your crank to? Was it topped a couple of times before the bottom end was taken care of? My last engine had cylinders replaced prior to my ownership and upon my teardown of the engine I found ridges in the piston pins adjacent to each side of the small end. Can't help but think that that's a short coming in trying to extend the life of an old engine where the bottom end bushings wear and the rods pound the crank.

I attribute the wear most likely to having my head up my @$$ not quite 5 years ago. One day after arriving in the runup area, I noticed the oil pressure at zero.

Any sane person would have shut down immediately and procured help to tow the airplane back to the hangar. But noooooo, not me.

I reasoned that since there were no unusual noises coming from the engine and the tappets weren't noisy, that it was just a problem with the gauge, and I proceded to taxi to my AI's shop. The first thing he did was put external pressure on oil pressure line from the engine. The gauge came right up. Then we removed the fitting and checked it. Clear. The next thing was the oil pressure relief valve. It looked OK too, so we put it all back together, started it up, instant oil pressure in the normal range. We suspect a piece of carbon or other trash that we didn't catch when we pulled the relief valve.

To make a long story not too much longer, the next oil change a couple of months later revealed a pickup screen and oil filter packed full of aluminum shavings. Suspecting a piston, we pulled the clylinders and found piston pin plug totally destroyed, and a couple more on the way. Complete teardown revealed mild scoring on a couple of journals on the crankshaft, but my AI's measurements showed that they should polish out. I wasn't in a position to do an overhaul at the time, so to stay in the air, I bought an engine with about 1500 SMOH for the airplane and stored the original in my garage until yesterday.

Not having anything better to do before packing it up to take to the shop, I thought I'd take some measurements. My unchecked, uncalibrated dial caliper showed at least service limits on everything until I got to #5 and #6 crankpins. There doesn't seem to be any scoring or heat discoloration on the journals, but a check of the #5 and #6 rods showed bearing material completely gone, just a bare shell!

As I said before, I took the crank to the shop anyway, just in case there is any hope at all.

Miles

Post by **Bruce Fenstermacher** » Fri Nov 11, 2005 3:49 am

So if this is your origianl engine remind us what is wrong with the crank in the engine you most recently had in the plane.

cessna170bdriver · Post by **cessna170bdriver** » Fri Nov 11, 2005 8:11 pm

N9149A wrote:So if this is your origianl engine remind us what is wrong with the crank in the engine you most recently had in the plane.

I thought I might get this question. Here's my reasoning:

The engine that I replaced a cylinder on this summer is still on the airplane (in a rented hangar in Mojave), and is still in running condition. I am in the process of bidding on buying a hangar in Tehachapi, and if I get it, I want to be able to fly the airplane to it's new home. The crankshaft in that engine has more total time on it than the original (3000 vs 2500) and is really an unknown at this point. I'd hate to tear that engine down and find an unserviceable crank and then not be able to move the airplane, and have to buy another crank anyway.

If the crank currently in the airplane is good, I should be able to sell it later to recover the cost of buying one now. If it's unserviceable, then nothing lost.

Just trying to keep my options open.

Miles

Tom Downey · Post by **Tom Downey** » Sat Nov 12, 2005 3:23 am

Talk to Superior in Dallas Tx, they may have a .010 or smaller STC for re-grinding your crank.

cessna170bdriver · Post by **cessna170bdriver** » Thu Nov 17, 2005 12:14 am

Well, good news from the machine shop!

Heinz Hubbert from Nicksons just called and said that my O-300A's crank is just under servicable limits and CAN be ground undersize. I was hoping that MY measurements were bogus, and that turns out to be the case.

He also said the crankcase just needs clean-up and line boring, and all of the other castings are good to go. The only negative is that the camshaft is unservicable, but after hearing that the crank is grindable, that news doesn't seem so bad.

Looks like I'll be continuing on with the O-300A for the forseeable future. I was getting a bit excited about the possibility of a bigger engine, but I also think it's cool that '98C can continue the coming years with the same engine issued to her at the factory.

... not to mention the fact that my bank account just dodged a bigger bullet.

Time to go order some parts!

Miles

Dave Clark · Post by **Dave Clark** » Thu Nov 17, 2005 1:25 pm

Miles it might be good news that the cam is bad. I subscribe to the new cam new followers belief.

Sounds like you're well on the way now. I really like old Heinz

cessna170bdriver · Post by **cessna170bdriver** » Thu Nov 17, 2005 4:15 pm

Dave Clark wrote:Miles it might be good news that the cam is bad. I subscribe to the new cam new followers belief.

About 5 years before I had to temporarily retire the O-300 off the airplane, I found a piece of a lifter body face in the pickup screen during an oil change. (I presume the result of a past stuck valve.) At the time the engine only had 5-600 hours since overhaul, so while we had the case split, I just put all new lifter bodies in at that time. I had reused the lifter bodies at the last overhaul, so they had about 2000-2100 hours total time on them. Also at that time we mic'd the cam journals and eyeballed the lobes with a jewlers loop and decided the cam was good to go.

At the most recent disassembly after finding LOTS of metal in the screen and filter, the cam lobes still LOOKED good, but there was some visible scoring on the journals, so I'm not surprised the cam failed inspection. (Heinz didn't say what was actually wrong with it.) With only 4-500 total hours on the lifter bodies, they still look practically new. I think I'll just send them off for NDT and inspection. I will however install all new hydraulic units.

Dave Clark wrote:Sounds like you're well on the way now. I really like old Heinz

I liked Heinz also. He seemed very knowledgable of the O-300, and was well aware of the rarity of the crankshafts. Even though Nickson's isn't set up to NDT lifter bodies, Heinz picked one up, looked at it, and said "Oh, you've got the later style with the snap ring groove." I was suitably impressed! I'd love to be able to follow him around for a week and see how the machine work gets done.

Miles

S2D · Post by **S2D** » Tue Jan 03, 2006 5:43 am

cessna170bdriver wrote:
With only 4-500 total hours on the lifter bodies, they still look practically new. I think I'll just send them off for NDT and inspection. I will however install all new hydraulic units.

Miles

Not a good idea in my opinion. At the very least, send them to AEA in Dallas TX. Putting used lifters with new Camshaft is a recipe for disaster.