Engine Knock

[markwill]

If you want to hear what sever piston slap sounds like, the check out this video:

http://chmafc.org/model-a-engine-knock/

Mark

[Kohnke Rebabbitting]

99.9 percent it is a out of alignment rod, nobody checks them.

Herm.

[George Miller]

Originally Posted by Kohnke Rebabbitting

99.9 percent it is a out of alignment rod, nobody checks them.

Herm.

Herm I do. I also check it while I.m turning the engine at assembly to make sure they stay in the middle of the piston.

[Bob Johnson]

Herm,

Are you referring to the babbitt not cut correctly or a bent rod or either ?

Bob

[Big hammer]

Thanks for the video now we know what that knock would sound like

[Kohnke Rebabbitting]

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Here is the problem, Bob. The rod can be machined perfectly for use, BUT there is not a rod machine that will make it a 100%, in Alignment.

Even a brand new rod machine, will not, only by chance.

You have 3 things to check,

1.twist.

2. Bend.

3. Off set.

Any body that tells you that the rod machine cuts them straight, does not know what they are talking about, and probable has never checked one, or even has a rod alinger, or knows how they work.

Modern Rods are normally done on a Sunnen Hone, the metal is removed with out Alignment, just on one end at a time. Sunnen also makes a Sunnen Rod Aligner to check alignment afterwards.

I try not to rebuild Model T, A, or B, rods any more, as there is no money in them, to compete with what is available.

Arnold Motor in Fort Dodge, Iowa builds a lot of old engines, and they send the T, A, Rods here that are rebuilt by other companies, they are all out of alignment, some worse then others, but none usable.

A piston with to much clearance can rattle until it warms up.

A knock will be heard from a piston hitting a head, or gasket.

But a noise like that should be a bent rod, and like a bent Model T Pan, unless you put them on the pan jig, you can't tell, or which way to bend them.

Ok, the 4 bolt rods, we rebuilt here, they are Marmons, The Model A rods are from another shop, that a lot of guys use.

1. Picture shows the rod press twisting the rod.

2., & 6. Picture shows the press bending Rods.

3.,4., 7., Pictures show checking offset. The gap between the little rod, and the thrust on the rod is how far it would be off in your cylinder.

5., Picture shows checking twist and bend.

The Rod aligner at that time cost about 2,800.00 dollars, and the Press was about 1,800.00. They were made for a reason!

Thanks Bob,


Herm.

[Kohnke Rebabbitting]

Originally Posted by George Miller

Herm I do. I also check it while I.m turning the engine at assembly to make sure they stay in the middle of the piston.

The only thing George is if the off set is close, it will stay in the middle, but if the rod is bent, or twisted a little you may not see that, but it will Prematurely wear the cylinder, and you wouldn't even know it.


Thanks,

Herm.

[CAPE A]

So are you saying not to reuse old con. rods on a re-build ??

[Kohnke Rebabbitting]

Originally Posted by CAPE A

So are you saying not to reuse old con. rods on a re-build ??

Not at all, just make sure they are straight..

Herm.

[edmondclinton]

Originally Posted by Kohnke Rebabbitting

99.9 percent it is a out of alignment rod, nobody checks them.

Herm.

Herm, do you see a lot of Model A engines come to your place with problems, that were rebuilt by other Model A engine rebuilders, that you determine were caused by bent or twisted connecting rods?

For those seeking someone to rebuild their engine, it would sure be good to know which of the rebuilders besides yourself have sense enough to check the rods.

From my experience, slightly twisted rods will often not create an easily identifiable wear pattern, but badly twisted rods will disturb the action of the entire piston, rings and the rod assembly and may be the cause of excessive oil consumption.

[Kohnke Rebabbitting]

Like I said in post #7, A bad twisted, bent, or offset rod, you can hear.

Just because you don't hear them, as in loud, dosn't mean they aren't bent.

The only way to know is to check, and the odd's are against you.

In 48 years, I have only found 1 rod that was perfect, and that was just luck, and I haven't found one since in the ones I have Aligned

Just about all the engines, and do over bearings have bad babbitt problems. I think if they were as bad as that video, it would have been taken back to the original bulider.

I know J & M machine always check alignment, and Vern the builder in Fort Dodge, Iowa, at Aronald Motors, has a sunnen Aligner, and the rods that won't fit his machine that are to small, he sends to me.

From my experience, slightly twisted rods will often not create an easily identifiable wear pattern, but badly twisted rods will disturb the action of the entire piston, rings and the rod assembly and may be the cause of excessive oil consumption. "END QUOTE"

The only bad thing is any wear is no good, it just takes a little longer to show, and all that cast iron it going somewhere.


Thanks Ed.

Herm.

[edmondclinton]

Thanks, Herm.

What about center to center distance in rod length as per original specs?

[Bob Johnson]

When I had my B engine rebuilt I requested that the rods be checked and straightened if necessary. Before I completed the assembly of the engine I checked each rod to see if it was straight. I do not have equipment worth thousands of dollars so I used the following 2 methods.

The first method was to turn the engine upside down and with the crankshaft installed attach each rod to the crankshaft pointing up (away from the cylinder). I put in the wrist pin and was able to use a dial indicator at the end of each wrist pin to check the height of the wrist pin. I checked one side at a time. After I checked one side I would disconnect the rod and flip it 180 degrees on the crank. Then I could check the other side. I was able to find several rods that were bend in the front to back direction.

The second method was to install the crankshaft, rods and pistons. I then raised the piston to almost the top of the cylinder. I used a depth gauge to check the height of the top of the piston at the front and back of the piston. I measured the same differences that I found in the first test. I also marked the top of the piston 90 degrees from the center line of the crankshaft. With the piston at the top of the cylinder i could check for any twisting by checking that mark with a line on the block perpendicular to the crankshaft.

I took the rods back to the machinist and he rechecked and straightened them as I watched. I rechecked them and they were good to go.

These tests may not be the best way to check the rods but they worked for me.

Bob

[Kohnke Rebabbitting]

Originally Posted by edmondclinton

Thanks, Herm.

What about center to center distance in rod length as per original specs?

While that is important, Ed, after a rod is machined, you can't change that.

Herm.

[markwill]

Herm,

I understand rods bend and twist, but I'm not clear as to how a misaligned rod causes the knock heard in the video.

Also, the engine in the video was not a new rebuild. I have no idea on how many miles are on the engine, but it's well used. All I know for sure is it hadn't run in over 5 years based on info provided by it's previous owner. In fact, the previous owner never saw the engine run so it may have been sitting longer than that. The engine is in an original 28 Tudor that the previous owner was going to hot rod but never got around to it so it just sat in his garage until I rescued it. The engine has .080 over pistons according to what's stamped on them, so it was rebuilt somewhere along the way. Also, it didn't knock when I first started it, but developed shortly after a few short runs up and down my street. The all the cylinders have about the same amount of scoring/wear. It doesn't smoke and has plenty of power, at least as compared to my Dad's 28 coupe.

Thanks, Mark

[George Miller]

Re: Engine Knock

I understand rods bend and twist, but I'm not clear as to how a misaligned rod causes the knock heard in the video.
__________________________________________________ ___________________________

If the rod pivot points are not in line with a twist in the rod. When the rod tries to swing on the wrist pin it will put the piston in a bind. If you have a bend in the rod it also puts the piston in a bind from top to bottom. If it is bent enough the rod can hit the edge of the piston wrist pin bores.

Kind of hard for me to put into words. You should always check rods for straightness.

I do not build engines for a living now days, but I use to.

[edmondclinton]

Originally Posted by markwill

Herm,

I understand rods bend and twist, but I'm not clear as to how a misaligned rod causes the knock heard in the video.

Also, the engine in the video was not a new rebuild. I have no idea on how many miles are on the engine, but it's well used. All I know for sure is it hadn't run in over 5 years based on info provided by it's previous owner. In fact, the previous owner never saw the engine run so it may have been sitting longer than that. The engine is in an original 28 Tudor that the previous owner was going to hot rod but never got around to it so it just sat in his garage until I rescued it. The engine has .080 over pistons according to what's stamped on them, so it was rebuilt somewhere along the way. Also, it didn't knock when I first started it, but developed shortly after a few short runs up and down my street. The all the cylinders have about the same amount of scoring/wear. It doesn't smoke and has plenty of power, at least as compared to my Dad's 28 coupe.

Thanks, Mark

Mark, Herm rebuilds these engines for a living but that does not mean that your rod is automatically bent. Regardless, a knock that loud is no good and taking the engine apart is the only way to know what's making the noise.

But first, is the noise affected by timing changes? The noise of loose bearings, piston slap, loose piston pins will increase as the spark is advanced and will decrease as the spark is retarded.

[Kohnke Rebabbitting]

I am taking it at face value that the piston wasn't hitting gasket, or head with .080 overs in some piston brands, it can happen.

This is like estimating a wrecked car with out seeing it.

There still may be a loose rod bearing also, but you said when you started it up, it wasn't there until it warmed up, that should have been at its worst at start up.

To me I hear more then a single rod knock.

When bearings are cold, they knock the loudest, or it went out the short time you drove it, unlikely, but you never know.

The picture in post #7 shows the clearance of the piston digging into the cylinder walls, and the excess clearance on the other side of the piston, and that changes, at the end of each stroke, and when the piston reverses, the slack it takes to the other side, is what is heard.

I should be saft to say they were never checked for Alignment, because many, do not. I looked around and found some reading that may help, or not.

Thanks Herm.