Noisy CS 8/1

[38ac]

That it continues to knock when you close the rack dismisses all possibilities pertaining to fuel and timing. You have a mechanical issue that you have overlooked. Timing gear issues generally show up as a 2 clicks per revolution. For them to cause a knock would mean backlash to the extreme and would be apparant while cranking over with the handle.

[38ac]

Also,  if your determination that the noise occurs just as the exhaust valve opens, (which is just before bottom dead center) is indeed correct that too eliminates ignition related knocks and valve clearance, and valve recession.  The aluminum pistons in the 8/1 run with more clearance and also seem to magnify noise. That you have experience with these engines is telling as you are used to the noises they make. What cannot be told is if you are simply nit picking the small increase in noise that the aluminum piston engines have or if you have a real issue. Over the years similar mysterious knocks that occur at or near BDC can be traced to excessive skirt clearance, tight or loose wrist pin fits or alignment issues due to bent connectting rods.

[silvertop]

Thanks for your advice 38ac. This is what I was hoping for.
This is the only aluminum piston CS engine I have. I had not considered the aluminum piston as a noise source. My other CS engines have cast iron pistons, so I may well be unfairly comparing the noise of different engines. The noise occurs just before the exhaust valve opens and sounds similar to my air-cooled engines that do not have a noise-dampening water jacket, but they do all have aluminum pistons.

I replaced the gudgeon pin (wrist pin) and bushes and they work as intended with no tight spots. Lubrication holes are clean. Early on, I also wondered if the con rod could be bent, but I couldn't detect any bend and there is no obvious un-even wear to the piston skirt. I suppose it could be piston wear and I'll check again next time I have the engine apart.

I don't have a measurement for the piston - cylinder gap on the CS 8/1. Do you have any guidance on this?
Are aluminum pistons on the CS engines thought to wear faster than on other engines?

[BruceM]

A most impressive analysis and outstanding explanation, 38AC. You are an excellent instructor and writer.  I learned a lot, thank you!  Thanks to silvertop too.

[cobbadog]

X2 to what BruceM said.

[dkmc]

X3 regarding 38ac

Also, I'd like to hear more as to piston clearance.....as below.

I don't have a measurement for the piston - cylinder gap on the CS 8/1. Do you have any guidance on this?
Are aluminum pistons on the CS engines thought to wear faster than on other engines?

[38ac]

Below are the sizes and clearances as per the technical data sheets. It is important to know that standard practice at Lister was to machine clearances into O.D. of fitted parts not the I.D of a bore. Example the CS cylinder bore is 4 1/2". To provide the needed operating clearance the piston is made smaller than 4 1/2" not the bore bigger, get it?
lister CS bore is 4 1/2" machining   tolorances are -000 to +.002
Cast iron piston is a 4 1/2" with a machining tolorances of -.005 to -.0065
Aluminum piston is also 4 1/2" with machining tolorance of -.007 to .008.
 Maximum allowable cylinder wear is .004
Maximum allowable  piston wear is .005
What that means is if the wear is less than stated the parts are good to out back in service.

So,,do the math an aluminum piston 8/1 can have anywhere from .007 to .010 clearance and be in new engine specs. When you add in maximum allowable cylinder and piston wear. The maximum allowable clearance is .019" and that is pretty doggone loose and will make noise.

[MachineNLectricMan]

While this topic has aged a little, I'll add my 2 cents. Piston "slap" is a common annoyance in many types of aluminum piston engines. I have an 8/1 and the slap go's away after it warms up. The extra clearance is to allow for more skirt expansion as the engine gets hot, but is annoying in a cold engine. Also the engine may have excessive piston clearance even for aluminum. The noise can be unnerving and sometimes will cause a person to mistakenly look for a serious engine problem that doesn't actually exist. Better to be safe than sorry though.

You can actually knurl piston skirts, or hand stipple them to decrease the clearance. This actually causes more oil to be retained on the piston skirt too. This was an old race engine building trick in the older days. There is a proper way to do this though, so let a machine shop do this.

[mihit]

In place of a stethoscope, the handle of a screwdriver can be held up to your ear or temple, with the blade pushed against parts... obviously this only works when you don't have to put your head in the flywheels!

[cobbadog]

I also use a short lngth of garden hose if the long screw driver isn't handy.

[mikenash]

In place of a stethoscope, the handle of a screwdriver can be held up to your ear or temple, with the blade pushed against parts... obviously this only works when you don't have to put your head in the flywheels!

The long screwdriver works well if you wear earmuffs and hold the end of the handle of the screwdriver hard against the plastic cup of the earmuff