I am working on a T engine that has a knock. We think we have it narrowed down to No. 2 cylinder. We looked for anything unusual. The rods, mains, and camshaft all seem tight. There are only a couple of things we see unusual. The first is the No. 2 rod on the crank journal has quite a bit of axial movement.,,like 0.060 to 0.065". The rest seem to be at about 0.030 to 0.035". We are wondering if it might be a twisted rod. The second thing I noticed was perhaps some wear on the bronze, large timing gear teeth. What we are experiencing is a big "thud" on No. 2 cylinder when it is running. If the rod is bent, could it cause that noise? When the journal is at the bottom of the stroke, the rod seems loose (that is when we get the easiest axial movement of 0.060 to 0.065"). As the rod continues up at about 3/4 the way to the top, it doesn't want to slide back and forth axially on the rod journal near as easy as it did at the bottom. How much is considered "bent" for a rod? I have seen photos of the fixture to check the straightness of the rod but have never seen one used. We can't see that the rod or piston is twisting as it is going up or down, but if it is only twisted a few thousandths, we may not. I don't know that I really have in my mind how the bent rod acts on the piston (or vise-versa) as the crank rotates. Do you think this is the thud we hear?
Ideal end play on connecting rods is .005"-.007". Serviceable is .016", repair at .018".
.030"-.035" is excessive. .060"-.065" seams unreal.
I'm sorry, "seems" unreal. Auto-correction is, at best, a shot in the dark.
They should call it "auto-mistake"!
Thanks for the response. I guess I am having a hard time understanding why the excessive end play on the large end of the connecting rod can cause a big "thud" when the crank rotates. If it oscillates freely back and forth on the large end why would that cause a thud? The large end of the rod slides freely back and forth when the rod/crank is at the bottom. About 3/4 the way up, we notice that it seems to have migrated to the back side of the crank journal and it does not necessarily oscillate freely at that point. After some grabbing it and moving it with our hands, it did free up some but it wasn't as free as it was at the bottom of the stroke.
Just thinking about it, the rod could be bent in one or any combination of three planes. Do the jigs that check for a bent rod check in three directions? What would it hurt to have a large axial clearance at the bottom end of the rod? If it is too much can it cause the wrist pin to extend past the piston and score the cylinder wall? I don't have any information about the machine work that was done on this engine or who did it. The crank rod journal areas may have been ground too long by someone and the rods poured/machined to a standard length on the large end.
"Do the jigs that check for a bent rod check in three directions?" No, usually just 2, and those deformities are referred to as "bend" & "twist".
"What would it hurt to have a large axial clearance..." Because, as you're finding out, the rod will move back & forth in that clearance and the clearance will only increase.
"If it is too much can it cause the wrist pin to extend past the piston and score the cylinder wall?" Absolutely.
The rod is slamming back & forth and making the noise because it's twisted. At this point, since you have excessive side play already, you most likely need to have those rods rebabbitted and checked for bend & twist. You could probably also buy a set of rebabbitted rods and fit them to your crank. But in that case, be sure they've also been checked for bend & twist.
With the rod off, measure the crank shaft journal all around with a micrometer, the journal might be worn oval - and if the diameter differs more than a thousand, your new rod will also wear much too soon - the crank needs to be reground.
If needed, don't let just any machine shop grind your crank - better with someone who has a good reputation of being knowledgeable about T engines (they need a larger radius than most cranks at the ends of the journal and they need some extra care when setting them up, to find the true center)
Rods have to be checked for twist, bend and OFFSET.
They are all equally important!
With that much side play it sounds like either your Babbitt thrust is gone, or the rod has been machined to narrow.
The rod width should be 1.500, if not, it is wrong.
As far as making noise, they can make a lot, if out of alignment.
I assembled the engine years ago from parts that I assumed had been machined to fit each other. The crank had been ground, the mains were poured and line bored. The rods were rebabbitted, new pistons, new valves, tappets, timing gear crank gear, etc. I had no idea how well any of the work was done. I just assembled it. Since that time, it probably has less than 500 miles on it. I can't see that there is any end play on the crank. I have not pulled a rod to see if the journals are out of round. The cylinders look nice to me. No scoring and they are smooth. No oil burning. It started and ran OK, just knocked. I think the next thing I'm going to check is the length of the rod journal to see if it has been ground way over the 1.500" size that Herm mentions. Thank you all for the consultation and verification of what I am suspecting. I believe I will be looking at another set of rods...straight ones and ones that have the right width on the bottom end.
I don't know that it really does any good but here is a photo of the engine from below. The cell phone really doesn't take a very nice photo. It was coated with Glyptal paint when it was assembled. It appears to be pretty clean...of course it doesn't' have many miles on it either!
Verne, your rods can be straightened and the babbit repoured. They simply need a little more thickness on the thrust surfaces to better fit the width of the journals. It's just a guess on my part but it sounds like your rod might be at 1.500" and the width of the journal up around 1.562". If you got a little bend or twist, even a few thousands, it can be kicking that rod back and forth on the journal. It's probably kicking the wrist pin too but not far enough to hit the sides of the cylinders. At any rate new rods may put you in the same position you're in now whereas straightening the rod you've got and repouring the Babbitt and remachining it to .008 clearance will correct the knock. I'm obviously speculating regarding the problem but in my twisted mind with my limited knowledge it seems to make sense.
Your problem might be the center main has too much clearance. Why don't you check the clearance on the main and 2 and 3 rods? You should also pull your timing cover and see why at 500 miles you see wear on the teeth. Do you set the journal clearances on the crankshaft when you put it together? It's unlikely but have you looked to see if something on your no. 2 rod is clipping the camshaft?
A twisted rod will sound like a loose rod bearing