MTFCA Forum

Hello again, This is another try at hoping to get some other ideas of the origin of a %$#@ rattle in my 1923 Touring. The rattle occurs when at idle or coasting. The engine was rebuilt some years ago with aluminum pistons, compression is good. Here is what I've checked: shorting out #3 the rattle stops, babbitt is mint, the rod cap is adjusted to proper clearance (on all 4), the rod cap bolt head(s) is/are not hitting the cam, the center main bearing nor the center cam bearing have any play, the rods do not move side to side in the pistons, the cam has no end play, i have replaced the timing gear as well as the generator gear. I'm truly stumped. Any ideas you folks would have would be greatly appreciated. Thank you.

Camshaft end play?

Thanks Les. I've checked it and that's not it.

Loose fan belt pulley?

I have had problems on 2 engines with piston pins seizing in alloy pistons after a couple of thousand miles. That causes the piston to cock over and clatter against the bore. Pull #3 and check the piston pin.

I also have a noisy cam gear on 1 engine, even though it's new. It has 7thou backlash, the main bearings have been bored wrongly so the crank is too far away from the cam. I have an oversize cam gear on order.

Try more polydent....I discovered it was just my loose dentures making the rattling noise!

Jem- I've about concluded you may have hit upon the cause. Actually, there is nothing left!! So what catastrophic event is looming? In other words, what is the danger in driving it until winter when I can pull the plate and the head? Many thanks for your response.

So Jem, what's the remedy??

A tight wrist pin will typically continue to knock even if the plug is shorted out, and most often shows up in relatively little time after a rebuild. It will tend to knock at all speeds and is certainly not most promenent when idling or coasting unless other noises are masking it under other conditions. An engine built "some years ago" would have to have precious few miles on it to now just start knocking due to tight piston. It will typically start as a cold knock and go away when warm and then possibly start to seize and knock all the time. If it is tight, it will become a very bad thing soon enough. I personally don't think that's what it is, but am certainly always able to admit to being wrong, and learning something new.

A LOOSE wrist pin MAY stop knocking if the plug is shorted out. A loose pin is not a big deal unless you like running at 50MPH all day long, and may not be even then...

Re: Tight wrist pin - If you can move the rod along the crank pin, the pin's not tight. If not enough slop to move on crank, drop rod cap, rotate crank away and move rod around. If it swings and slides in the piston, it's fine. You've already adjusted the rods, so, I'm assuming you did not find #3 to be tight in the piston.

An odd but slightly possible issue would be a wrist pin clamp bolt working loose if it was not cottered correctly

Under the belief that this is a new noise, I'd rule out a bent or twisted rod.

Scott, Thank you for your help AGAIN!! The rod swings freely and the wrist pin bolt is in place with a cotter key. So how can the pin become loose??

Loose is measured in thousandths of an inch and is a function of wear. In an aluminum piston, it is steel against aluminum, with an oil film supporting the union and running silently...a little wear in the bore of the piston allows movement beyond what the ideal film thickness is of the oil and you get a little knock. If only slightly worn, and running too thin oil, the knock might be heard where a slightly thicker oil, and the oil film will continue to support/cushion the bearing. I like 15/40 in a well run engine and am OK with 10-30 in a new tight engine. Am not trying to start an oil war, just giving you something to think about or try.

This info is for speculation purposes on my end, and perhaps some thinking points for you on your end.

Thank you Scott. Point well taken.