Engine noise

[Rammy]

I have a 51 fl that I rebuilt a few years ago , When the motor is cold I have a slight knock , after it worms up it runs perfectly quiet . I used the V twin cast pistons set to .001-.002 skirt clearance . I have put 10,000 mi. on it with no change in cold knock. Are the vtwin pistons any good?

[Cotten]

Tedd doesn't make his own pistons, of course.
Nearly all distributors repackage the same ones (from Korea, I believe.) You will see an emblem inside that looks like crescent moons inside of a full moon.
They are excellent pistons for most applications.
My wife's tired old '65 has audible "slap" when it fires up too, and doesn't quiet down until fully warmed,....but it has decades of sidecar duty on it.
Normally it takes a good .0035" of clearance to get 'loud', but that's still no big deal on a workhorse. If it has compression and doesn't use oil, it ain't broken. Take it apart, and it will be!

[flathead45]

hmmm , thats a hard one to answer . I'd have to hear the noise myself to know what it is and were its comming from

[Rammy]

Could the Knock be from piston pin alignment , things are tight when cold making piston slap more and when the pin is a little looser in the piston from heat the piston is quieter ?

[Cotten]

If you have an alignment problem, meaning the rods themselves, then wear will be accellerated of course.
But I believe most of what is commonly heard is skirt to cylinder slap.
Remember that not only does aluminum and cast iron have different thermal expansion coefficients, but the cylinder does not elongate uniformly. The spigot at the bottom not only does not see combustion heat directly, but the cases and oilspray cool them quicker. And the spigot are also the tightest part of a bore: no ring wear, and fastener stress that will often pucker the wall inward at larger overbores. It takes time for all of this to come to equilibrium as a motor warms.
It becomes obvious that all rods should be straightened and aligned as part of a lower end rebuild,... and overbored cylinders should be fitted using torque-plates.
In the mean time, you might as well run what you got; It will all come out in the wash of the next rebuild anyway.

[Plain]

Rammy:

To give you another opinion, with the caveat that it is difficult to diagnose a problem on an engine that cannot be seen, heard, or felt, I agree 100% with Cotten. A bad fit of the wrist pin/wrist pin bushing, or wrist pin play in the piston will add to the noise problem, but it typically is not the noise that you are hearing. What you describe is usually the sound of piston skirt slap.
Depending on your overbore, if the bore and finishing of the cylinder was not done using torque plates then your chances of noise or other problems increase. Thinner cast iron walls act differently. Cotten's message hits it dead on the head. If your problem was quite as a mouse on start up, then increasingly loud as the motor warms up this would be a very different problem.
There is very little difference in the cast aluminum pistons on the market. For your application, my opinion is that you are better off running a cast aluminum piston than a forged "name brand" piston. You only have so many rebuilds on any given engine. Unless something else crops up, ride it for as many miles as you can and smile. I have a good friend with a 54 pan with a bazillion miles on it. It's his daily ride. This is a very well cared for machine. It sounds like a damn hay baler when started, then purrs like a kitten when warm.

Adios-----Plain

[Rammy]

The motor is .060 over and wasn`t honed with torque plates . So that could very well be what is going on....
Thanks guys Rammy

[sleeper]

Plain-
Likely, I misunderstood your statement.
You only have so many rebuilds on any given engine.
Surely you must mean on cylinders, right ? But then you could cut & put in "liners" to save them.
How else are we to get a bazillion miles on our HD's ?

[Plain]

Rammy:
.060" over really needs to have been done with torque plates to be successful. Factory limit was 0.070". I have been associated with engines that were taken to 0.10" over, but these are extremely thin cylinder walls. If you search on old threads on this forum on torque plates you will find some excellent information on the use of torque plates when reworking cylinders, and the need thereof on serious overbores.

Billy:
No, I ment engine, but I see your point. I suspect that I consider everything to be finite, and that at some point in splitting cases or torquing heads a number of times, something bad is going to happen, even with the utmost care. That number could be high enough though that the point is moot. The point that I was trying to convey is that piston slap is not an engine killer, just an indication that somewhere down the road lies a rebuild, but not now, unless Rammy just can't live with the noise.

[Rammy]

So is the cyl flexing out of round when the head bolts are torqued to couse enough clearance to make a knock?
Are the new cylinders any good ?

[Cotten]

Rammy!
The headbolts cause minor distortion, but it is outward and only costs ringseal and compression. There is similar "pull" above the basenut flanges.
But the major distortion is at the bottom spigot, where it pulls radically both inward and outward. Here you can have friction and in the worst cases, fragmentation!
Please be aware that distortion is negligeable at stock bore and the first couple of overbores, but as the metal gets thinner, it flexes to a geometric degree.
A rough guestimate is that a Pan cylinder spigot averages a couple of thou inward and a couple outward at .060" overbore. It is not a symmetrical distortion, as the drain gallery in the casting stiffens the left side.
Since this is roughly common clearance, many get away with it,..or make their own clearance by running them! (The pistons are expected to collapse slightly as well.)
Distortions depend upon "meat" of the casting to push upon. Only a boregauge and close observation can tell you exactly what you are dealing with.
I also measure the thinnest part of a cylinder above the flange, when concerned with an extreme overbore, although I have encountered a cylinder that ran fine at nearly zero!

[Rammy]

Cotton,
By spigot are you talking about the bottom end of the cylinder? Olso can you give me your input on the new cylinders that are out there .
Thanks

[Cotten]

Yes, "bottom spigot" refers to the section below the base flange that slips inside the case bores. (The top has a small spigot as well, the lip that extends within the head.)
It has been quite some time since I had any experience with repop cylinders, but beware that last decade there were few alternatives to oriental production that had dangerously bad core float (the machining wasn't in the middle of the casting) on the rear cylinders, thus allowing only a .030" overbore before hitting daylight!
I can only suggest shopping for the brand name with the best reputation.