Thrust Bore Face Clearance Off during assembly?
Cylinder #5 Photo
- Thread starter Heartworm
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I think you may be onto something with the machining mistake. Looking at the piston, there is more than the deeper vertical scraps that are concerning. The shiny area of the piston is where it was making consistent contact with the bore. In my opinion, this area is way too large. The bottom of the piston or piston skirt area should be the only part of the piston that's been polished by the bore. The fact that the piston was making that much contact means the pistons were not sized correctly to the bore or visa versa.
In rare cases, a run of pistons could be mismarked, or made from the incorrect compounds allowing them to expand beyod normal tolerance.
Watch out because unless it is an obvious mistake then the machine shop is likely to blame your break in procedure, driving habits, or overheating of the engine.
Good luck.
Oh, I would pull all six pistons. My guess is that they will also have large scuff areas just like this one.
In rare cases, a run of pistons could be mismarked, or made from the incorrect compounds allowing them to expand beyod normal tolerance.
Watch out because unless it is an obvious mistake then the machine shop is likely to blame your break in procedure, driving habits, or overheating of the engine.
Good luck.
Oh, I would pull all six pistons. My guess is that they will also have large scuff areas just like this one.
Most likely..
Additionally, once the piston starts to gall slightly, it is downhill slope rapidly..
It is allways a good idea to check piston clearance on a new motor.. Simple things can matter, a lot..
- Thread starter
- #24
Stiff Wrist Pin
Two more bits of information:
1. I have only pulled piston #5 but the piston is very stiff on the rod. You can move it but it is not smooth; rather binding actually.
2. I had the NAPA man put the caliper on the piston. He got 93.89 mm at the top and 94.2mm at the bottom. The FSM says stock piston diameter should be 93.96mm to 94.01mm. How true those measurements of my piston are is unknown. My cylinders are supposed to be bored 0.020 over which I assume was in inches. I will check the top of the piston as suggested.
Two more bits of information:
1. I have only pulled piston #5 but the piston is very stiff on the rod. You can move it but it is not smooth; rather binding actually.
2. I had the NAPA man put the caliper on the piston. He got 93.89 mm at the top and 94.2mm at the bottom. The FSM says stock piston diameter should be 93.96mm to 94.01mm. How true those measurements of my piston are is unknown. My cylinders are supposed to be bored 0.020 over which I assume was in inches. I will check the top of the piston as suggested.
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.020" is approx .5mm.
So a .5mm O/S piston would be approx 94.5mm
The caliper at NAPA has been dropped on the floor a few too many times.
The pictured piston has been scuffing the bore, probably because the bore to piston clearance was too small.
Probably a machining error when the block was re-bored.
The other possibility is the piston over expanded and began to scuff/stick in the bore. It would expand too much if it got too hot. The usual cause of high piston temp is late ignition timing.
FWIW, lean mixture can cause burning of piston crown, but doesn't actually produce excessive heat.
The wristpin/piston bore fit opens up at operating temperature. It should be tight at room temp.
Lehi makes a good point. These engines squirt oil on the thrust side of the cylinder through a squirt hole drilled in the shoulder of the rod, bleeding oil off the rod bearing. AFAIK, there is no reason the rod could not be installed backwards, squirting oil away from the camshaft and thrust side. Or the rod bearings could be installed wrong, blocking the squirt hole.
Hate to say this, but you've got another suspect rebuild on your hands. As others have said, at this point all the pistons should come out to be inspected, check rod orientation, etc.
So a .5mm O/S piston would be approx 94.5mm
The caliper at NAPA has been dropped on the floor a few too many times.
The pictured piston has been scuffing the bore, probably because the bore to piston clearance was too small.
Probably a machining error when the block was re-bored.
The other possibility is the piston over expanded and began to scuff/stick in the bore. It would expand too much if it got too hot. The usual cause of high piston temp is late ignition timing.
FWIW, lean mixture can cause burning of piston crown, but doesn't actually produce excessive heat.
The wristpin/piston bore fit opens up at operating temperature. It should be tight at room temp.
Lehi makes a good point. These engines squirt oil on the thrust side of the cylinder through a squirt hole drilled in the shoulder of the rod, bleeding oil off the rod bearing. AFAIK, there is no reason the rod could not be installed backwards, squirting oil away from the camshaft and thrust side. Or the rod bearings could be installed wrong, blocking the squirt hole.
Hate to say this, but you've got another suspect rebuild on your hands. As others have said, at this point all the pistons should come out to be inspected, check rod orientation, etc.
iron_giant
SILVER Star
Couldn't a rich fuel condition where unburned fuel on the cylinder walls effectively destroys the oil film layer leave similar, streaked scoring on one side of the piston? I've only seen it a few times on completely different motors but I thought I'd ask while were on the topic.
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Well, we are all speculating about what caused this only because you need to remove the cause of it or it will happen again. Was the rod in backwards? Was the bearing in wrong, blocking the oil hole? Is there some debris from the machining process left in the oil passage that may have blocked oil from that particular rod journal? You have nothing to lose by presenting this to the shop that did the work. The least they can do is put a decent micrometer in the cylinder and on the piston and perhaps hone your cylinder and help you get another piston and ring set. They might be able to help find the cause. More eyes on the project will yield more information.
- Thread starter
- #28
I'm with you Lehiguy. I want to be certain that I get a diagnosis of what caused the problem. Will get measurements and will try to figure out if the rod and bearings were in right. There are marks on the bearing. I doubt that they will photograph well but will give it a try tomorrow and post if decent. Fortunately I have other vehicles so this is only a crisis because I miss my rig dearly.
Some have suggested a vacuum leak but the vehicle has idled great and the brakes have worked well too. We are thinking improper fitment right now, stay tuned.
I greatly appreciate the help from you'all.
My current plan A is to pull the block and get a rebuild on the lower end of everything that needs it. Certainly some of it is still good. Will even look into a MAF Hi-Pro short block if it is compatible with my new Jim C. engine head.
Some have suggested a vacuum leak but the vehicle has idled great and the brakes have worked well too. We are thinking improper fitment right now, stay tuned.
I greatly appreciate the help from you'all.
My current plan A is to pull the block and get a rebuild on the lower end of everything that needs it. Certainly some of it is still good. Will even look into a MAF Hi-Pro short block if it is compatible with my new Jim C. engine head.
- Thread starter
- #29
piston notch?
Further inspection and reading my FSM revealed that the notch on all 6 pistons is toward the FRONT of the engine. The FSM says to put the notch to the REAR. These are aftermarket pistons from the rebuild; they have 45 on the top. The oil hole in the rod is on the PASSENGER side of the engine. Don't know about the Thrust side; some folks speak in tongues, at least to the unintiated. I couldnot find the little mark that was referred to in the FSM that is supposed to be on the side of the rod. Will check on the oil hole match up next.
More drama..
Further inspection and reading my FSM revealed that the notch on all 6 pistons is toward the FRONT of the engine. The FSM says to put the notch to the REAR. These are aftermarket pistons from the rebuild; they have 45 on the top. The oil hole in the rod is on the PASSENGER side of the engine. Don't know about the Thrust side; some folks speak in tongues, at least to the unintiated. I couldnot find the little mark that was referred to in the FSM that is supposed to be on the side of the rod. Will check on the oil hole match up next.
More drama..
The thrust side, the side of the cylinder subjected to the most load on the power stroke, is the passenger side, so the oil hole should be on that side. As far as piston markings, I believe it's a universal truth that all indexes, markings, arrows, etc. point towards the front of the engine. I have never seen an exception to that rule, but I haven't rebuilt every engine in existence.
Why not send it to Jim C? If he is half as good with an engine as he is a carb you are golden!
rchalmers3
I speak Toyota
Don't rule out a lean condition yet. I'm not so sure the answer is in poor machining or fitment. In fact, it's easier to achieve proper tolerances and fitment than it is to make the thing receive the proper fuel and spark evenly across all 6 cylinders. When the motor is on a stand with all the junk stripped off, measuring important assembly tolerances is fairly academic.
Who do you know takes the time or spends the money to measure the fuel distribution and power balance of assembled, installed motors? Who invests in careful dyno tuning to maximize power, economy and reliability? Not too many, that's for sure.
For this reason, I find it hard to deduce the chicken or the egg in regards to excessive piston skirt contact. I lean towards usage as the main factor, especially with a slightly lean condition, insufficient EGR gasses or too much ignition advance affecting the operating temperatures of the aluminum pistons.
Cheers,
Rick
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Oil squirt holes point at the cam (passenger side depends on what country you're in).
Piston notches go toward the front.
Piston notches go toward the front.
- Thread starter
- #34
Engine Post-Mortem
I pulled the engine and took it to a rebuilder in Helena, MT. (Sorry Jim C., I could not afford the trip to the Cruiser Spa although that was my first choice). The machinist said that the piston got too hot and distorted. He indicated that a normal piston is actually egg-shaped, with its widest diameter from side to side on the engine. On a new piston, the places where my piston are worn are actually smaller in diameter than the lateral (passenger side/driver's side surfaces) by several thousandths. So, to see wear where it is located in my pictures means that the piston has overheated and the skirt-tails actually collapse in and the piston becomes more "square". The problem was not broken rings, it was not poor fitment. It appears to have been a intake manifold leak for cylinders 5 nd 6 from inspection of the used manifold gasket. This fits with having both 5 and 6 showing the damage. Also he suspected the 85.5 octane gas that I am running, lean fuel mixture, and ignition timing. I set my timing on the BB and have not heard any pinging. Other than burning oil, this engine ran great! It probably would not have lasted for a long time though. Will also look at my brake booster even though the braks work great.
I pulled the engine and took it to a rebuilder in Helena, MT. (Sorry Jim C., I could not afford the trip to the Cruiser Spa although that was my first choice). The machinist said that the piston got too hot and distorted. He indicated that a normal piston is actually egg-shaped, with its widest diameter from side to side on the engine. On a new piston, the places where my piston are worn are actually smaller in diameter than the lateral (passenger side/driver's side surfaces) by several thousandths. So, to see wear where it is located in my pictures means that the piston has overheated and the skirt-tails actually collapse in and the piston becomes more "square". The problem was not broken rings, it was not poor fitment. It appears to have been a intake manifold leak for cylinders 5 nd 6 from inspection of the used manifold gasket. This fits with having both 5 and 6 showing the damage. Also he suspected the 85.5 octane gas that I am running, lean fuel mixture, and ignition timing. I set my timing on the BB and have not heard any pinging. Other than burning oil, this engine ran great! It probably would not have lasted for a long time though. Will also look at my brake booster even though the braks work great.
Regarding engine timing...
Checking the initial timing is good. You should also check total timing which is the maximum timing the engine can be getting under load, higher RPM, or WOT (Wide Open Throttle). To check total timing, you need to determine initial timing (which you did), mechanical advance timing, and vacuum advance timing and then once you have these three you add them up to determine total advance.
As an example, a Small Block Chevrolet should have no more than about 36 degrees of total advance. That could be 12 degrees each of initial, mechanical and vacuum for a total of 36 degrees.
The source of the mechanical or vacuum advance timing would be your distributor. If you have modified the distributor or are using a performance or aftermarket type distributor then it’s especially important to verify the total timing the engine is receiving. I would do this even if the distributor is “blueprinted”. I’ve purchased so called “blueprinted” distributors only to find that they were nowhere near what the spec sheet indicated.
With the experience you’ve just had with this engine, I would definitely check total timing after the rebuilt is installed and running. It is one of the first things I would do. Even if you are running a stock distributor, you want to check that it’s operating correctly and not providing too much mechanical or vacuum advance.
A great tool to have is a diagnostic timing light or one with an adjustment knob which allows you to dial in the timing you want or should have and then use only the TDC mark on the engine as the reference point. There is no guess work this way trying to figure out what marks on the flywheel or harmonic balancer are 4 degrees, 6 degrees, 8 degrees, 12 degrees etc. And, when checking for total timing, you won’t generally find timing marks that go far enough. The factory marks are generally only used for setting initial timing. On an F or 2F engine this is especially true since there is just the BB or double BB on the flywheel like you mentioned.
Checking the initial timing is good. You should also check total timing which is the maximum timing the engine can be getting under load, higher RPM, or WOT (Wide Open Throttle). To check total timing, you need to determine initial timing (which you did), mechanical advance timing, and vacuum advance timing and then once you have these three you add them up to determine total advance.
As an example, a Small Block Chevrolet should have no more than about 36 degrees of total advance. That could be 12 degrees each of initial, mechanical and vacuum for a total of 36 degrees.
The source of the mechanical or vacuum advance timing would be your distributor. If you have modified the distributor or are using a performance or aftermarket type distributor then it’s especially important to verify the total timing the engine is receiving. I would do this even if the distributor is “blueprinted”. I’ve purchased so called “blueprinted” distributors only to find that they were nowhere near what the spec sheet indicated.
With the experience you’ve just had with this engine, I would definitely check total timing after the rebuilt is installed and running. It is one of the first things I would do. Even if you are running a stock distributor, you want to check that it’s operating correctly and not providing too much mechanical or vacuum advance.
A great tool to have is a diagnostic timing light or one with an adjustment knob which allows you to dial in the timing you want or should have and then use only the TDC mark on the engine as the reference point. There is no guess work this way trying to figure out what marks on the flywheel or harmonic balancer are 4 degrees, 6 degrees, 8 degrees, 12 degrees etc. And, when checking for total timing, you won’t generally find timing marks that go far enough. The factory marks are generally only used for setting initial timing. On an F or 2F engine this is especially true since there is just the BB or double BB on the flywheel like you mentioned.
