VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Engine knock at 25mph

I have posted this in mechanical but thougt some chev 4 guys might have the answer.

Hello from down under,
I have a 1928 Chev Roadster which has an engine knock at 25mph that I have been chasing for about a year. I have now rebuilt the engine with new pistons, valves, valve springs and new whitemetal bottom end with all relevent machining including crank grind. Just fitted engine and the knock is still there at the same speed and same sound. Its quite a deep sounding knock very simmilar to a big end. When I stripped the engine I found the centre main had excessive play but now thats all been rectified.

Any help on this subject would be greatly appreciated.
Thanks Neil

Wow!
Neil, hope you find a quick solution. If it does it (knocks) under a load (accelerating) I am not sure (main) but take your foot off gas and if it knocks its a rod, thats my understanding. Perhaps a stupid proposition would be to ge it going at 25 foot off gas and throw it in neutral see what it does. You sure its not in the drivetrain? Someone here knows.
best of luck!
Ken

Thanks for the reply ken
Have tried what you sugested, knock is there when accellerating,cruising or de accellerating. Have thought of the drivetrain but if you put clutch in noise goes away. You can also get the knock on quick acceleration in neutral. So if it is the drivetrain it would have to be the input in the gearbox.
Thanks Neil

See my reply to your other post. It is the brake cross shafts vibrating. At least it is on my '28 Coupe.

Thanks Chipper for your reply.

Checked brake cross shafts and can't feel any play, so drove car and applied brakes thinking this would load the cross shafts but no change. Also you can get the knock when you rev motor sharply in neutral. Found if I retard ignition knock lessens severely, rechecked timing and that is correct.

Thanks Neil

Update on the problem.

Still havn't found the problem. Knock comes in at 25mph but is worse under load, Lessens if I retard the spark. Have now replaced the camshaft to no avail. The only things still original is the lifters, push rods, oil pump and distributor, but i can't see any of these making the noise. Everything else has been either replaced or repaired.

DESPERATE FOR ANY IDEAS

Thanks Neil

Neil
I do not think it could be push rod, lifter or oil pump as these would do same in lower gear. However the distributor would require a look see, it is auto advance that kicks in and advances the timing perhaps to far. It seems it has to be a timing issue as your retarding the distributior changes the situation. However this should also be a problem at lower gear. Neil the 25 speed factor suggests that it is not in the engine but in the other moving part area. The retarding suggests its a timing issue. However a timing issue would be there in any gear. Why does it fail to appear in first gear at similar rpm. Does it change on an incline?
Ken

I have checked the advance curve with an adjustable timing light. I got Static timing 25deg BTDC, Reved motor to get 40deg BTDC, Retard ignition TDC. On tuesday I took the car to a freinds place to put over his pit. We jacked up the rear end and ran the car to 25mph and listened for the knock with a stethoscope. Found the noise was loudest just behind the sump plug at about number 3 piston. Shorted plug leads to try and identify knock but there was no change on any cylinder. We are 99.9% shure the noise is in the motor. One thought we came up with was engine ballance. I put a tacho on the other day and found the knock was at about 1300 to 1500 RPM, ran the car in second to 1500 RPM and knock was there but didn't seem to be as bad.

Thanks Neil

Hi Neil,

From what you are finding, if you rev the engine rapidly and the knocking is pronounced, and less pronounced with slightly retarded timing, its sounding like a rod big end with too much clearance...

But, these symptoms also sound like a ill-fitted piston pin in the small end...

Would you want to use some plasti-gauge to check big end clearances first, then if you had to, remove the pistons? If you have already checked clearances, hmm, I would be removing the pistons and carefully check the piston pin fittment.

Jay

Jay thanks for your reply
When I assembled the motor, I checked all crankshaft clearances with plastiguage, all were between .0015 and .002 thou clearance. When I fitted the piston pins I checked the pin fittment carefully. I dont think it is any of these as the knock hasn't changed since relacing these parts as this knock was the reason for rebuilding engine.

Thanks Neil

Neil,
Plastigage is not a reliable method to check the gap on these engines. The proper method to set the bearings is to take out shims until the bearing just locks then add one shim. On rods they should be tight enough so they can not be pushed sideways by hand but move with a gentle tap of a small hammer. Any rods with greater than 0.002" will make noise.

Chipper
Whitemetal was done by retired gentleman who did his apprenyicship on pouring whitemetal. He machined rods and line bored mains and set clearances, I only checked them. The thing that discounts the big ends for me is that after redoing the bearings the knock never changed but I maybe wrong. As I said previously is this rebuild was done to get rid of this knock.
Thanks Neil

Neil
ask the gentleman who poured the bearings for advice.
Ken

This knock smells like a main bearing. The next time you have the pan off I would remove a shim from the three bearings.

Just pulled the sump off to check bottom end. While looking at an old oil pump noticed a washer on the pump shaft wher the drive is, but it's not on mine. Should this washer be there and if it is would this maybe cause the knock in the engine.
Thanks Neil

The washer is to keep the distributor shaft plate (where the points mount) from rubbing against the distributor housing.

Found the problem!
It was the main bearings. Went on a car run today with no problems. Thanks to all that responded to my question.
Thanks Neil

Steelin_Time.
That is good news. How did you locate the problem?

Got an old mechanic that grew up with these cars to check it out. He used an old feeler guage to gheck the mains and found we had .008 thou on the centre main and .006 thou on the rear.The guy that remetaled the bottom end fixed the problem but can't work out where it went wrong.Did about 80 miles today with no problems.
Neil

Not sure where he went wrong either. If the procedure of taking out shims until the bearing just locks when tightened and then add back a shim was used to fit the mains that problem will not occur.

Word to the wise: If you use and rely on plastigage you are asking for problems. Yes is has a value to check taper on bearings and get a general idea of tolerances. Too many things can go wrong with crush of plastigage for it to be only thing relied upon.

Thank you both for the information now I just need to remember it.