MTFCA Forum

I've been working on my T for a while and just when I thought it was ready to be driven, I hear a knock. With a stethoscope I believe it is coming from cylinder #1. I pulled the #1 spark plug and it is oily. All other plugs were dry. Initial thought is a cracked piston ring. Took the head off and the #1 piston out and the pistons are intact. I measured the bore and is straight.

BTW, I tried to get the top ring off and broke it so going to have to replace the rings anyway.

My questions are:

1. Could improperly seated rings cause a knock and allow oil to pass through?
2. What else could it be or should I check?
3. The top of my piston, which is iron (tested using a magnet and it sticks so not aluminum), has .005 stamped on it. Do I order standard size rings or get .010 and file down?

Additional info: I recently checked to see if the rod caps are within tolerance and they are all good.

Appreciate the help! some pics of the suspect piston:

A worn crankshaft gear and camshaft gear will make a knocking noise too, sound just like a rod knock. You can hear it by hand cranking the engine over slowly.

This doesn't answer a single one of your questions, but rarely if ever does a knock require pulling a head to inspect; it may need to come off for repair, but you now have a broken ring, need to buy a head gasket, hopefully will not strip a thread in the block when assembling and are no closer to figuring out where your knock came from (in fact are farther away, because now the engine doesn't run).

Not going to answer any questions because there are lots of folks who will jump in...I'll just leave you with this: the next time you have a problem which you think requires disassembling something significant, ask for help first rather than later. It will save time, grief and $$.

Was your knock on acceleration of when going along at a constant or slightly slowing down. Main knocks or piston slap tend to knock on acceleration. A rod knock when going along at a steady pace or slightly slowing. A wrist pin knock can be a double rapping sound. Caution, if the center main is worn, do not tighten it because it could cause the crankshaft to bend leading to metal fatigue and broken crankshaft. The center main usually wears at the top in the block caused by the weight of the transmission pulling down on the back of the crankshaft. This can be caused by a very worn 4th main at the rear of the transmission or by a bent crankcase

looking at your pictures, it looks like the ends of the rings are on the same side of the piston. This will cause some oil to work it's way through the gap and into the cylinder. They should be on opposite sides of the piston so that any oil leaking by will be wiped off by the next ring above it.
Norm

Agreed Scott! I actually wasn’t sure what the knock was after checking the rod caps so I took it to a mechanic who works on old cars who told me he thought it was the ring. I guess I wanted to prove him right so I broke the ring myself! Lol.

The knock doesn’t start until the engine gets a bit warm and the rpm’s are increased. Which gave the mechanic more reason to think broken ring.

I haven’t checked the mains since I would have to take the engine out. Was hoping to avoid that.

Eli, Did you do a compression test comparing the oily number 1 cylinder with the not oily cylinders 2, 3, 4, before you pulled the number 1 piston ? That would have given you a base line in order to proceed. I can see what looks like 005 on the piston top, but on my little phone it is hard to read, it could be 035. What do the other pistons have stamped on their top ? Because you had a way to measure cylinder 1 bore, measure cylinder 2, 3, 4, with out removing their pistons, then compare your numbers. From engines patched together in days gone by it is not uncommon to find missed matching Ford cast iron pistons, and if position 1 is 005 OS and the other cylinders are even a bigger OS this could be your oiling problem, of course having the ring gaps all lined up could also be your problem, but I wouldn’t pull pistions 2, 3, 4, until I checked their bore size. If your piston sizes, which you should check also, are compatible with your cylinder bore sizes you should be able to reassemble you engine and have it running ok even if somewhat worn.

kmatt2,
I did a compression test and all 4 cylinders were at 50 if I recall correctly, including #1. You are correct that there is a .005 on the top of the piston. #2 and 3 have .010 on the top and #4 has .005. I don't recall if the ring gaps were in the same place when i removed the piston.

Given that the piston is .005, which size ring set should I get?

Personally unless you are doing a full rebuild I would do a quick hone on the cylinder(s) and use the rings you have. If you go that route, maybe someone can send you a standard size (maybe .010 os if they have one) and reassemble. If your head gasket looks ok, clean it and use copper coat. These are low compression/rpm engines and think some go overboard changing things.

Those don't look like Ford rings to me. If they aren't, you may have trouble matching them. 005 is an odd oversize. You may have non-Ford pistons, also.
.010, .020, .030, etc are much more common.


Assume nothing. Before buying anything, measure everything and determine any corrective action, such as honing or boring, that may be needed before installing any new parts.

A new set of aluminum pistons in a common oversize, with a matching set of modern style rings is probably the best path forward, assuming the crankshaft and other critical parts prove to be good condition.

I doubt if a piston ring caused your engine to knock. The carbon in # 1 cylinder could be due to a number of things, including something as simple as a shorting plug wire or a dirty spark plug, too much oil in the crankcase, a timer or timer wiring issue, a problem with the coil or coil box, or simply not having been run down the road in a while. A sticking valve is another possibility that copuld explain the carbon/oily deposit and the knock. As far as that goes, the carbon/oily deposit itself could have caused a knock.

Regardless of the universe of possibilities, tearing into the engine is not the first thing to do.

I would like to avoid a full rebuild unless it’s the only path forward. Since I broke a ring and have an odd size piston, .005, I’m guessing my best bet is to purchase a .010 and file the gap?

Hi Eli,
Check the cam for end play and colliding with the bearings. I don't remember a oily plug ever knocking? I have not seen .005 rings lately so .010
will be your best bet, if your bore will not accept .010 pistons. Look at every possible issue to be sure.
Craig.

This is a pic of what it looked like when I first pulled the head off. Not sure if this provides any more clues to the problem.

at first blush, it appears that you may have had a bad intake manifold gasket, causing front two cylinders run a very poorly combusting mixture...not intuative, but it was likely your rear intake running lean, causing you to run a very rich mixture which fouled #1&2.

as far as knocking it would be one of a myriad of issues, but finding it will be very very difficult on an engine which is not running. Too bad, as I've found most T's still running cast iron pistons are very sweet running cars.

If I was to ever design a head for a Model T, I'd have the phrase "NO, IT'S NOT UNDER HERE!!" cast into it to help prevent people from pulling the head at the first sign of trouble.

A misfiring plug can lead to excess soot and carbon deposition, which can lead to a piece of carbon getting loose and mashed between the piston and head, which can cause a knock.

Scott's manifold leak fits.
If only one cylinder was black I might test the plug under pressure to make sure its firing consistently.

Eli, Your compression of 50 lbs in all four cylinders is good for a Model T. I checked the main Model T part suppliers and only Chaffin’s lists 0.010 OS cast iron piston rings. For your mismatched pistons I would buy a standard and a 0.010 OS ring set. I would do a light hone of the cylinders and then hand fit each cylinder’s rings and end gap using what works best between the two ring sizes. I agree with Scott that it looks like you had a intake manifold gasket leak, check your manifolds for straightness and use gland rings when you reassemble the manifolds. Make sure that you clean and check your timer to rule out any spark timing issues. The good thing about re-ringing the pistons is that you can check and fit the rod bearings at the same time.

It did it again, a double post. Good luck with your Model T Eli.