Hunting a knock.....can you help?

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Model T Ford Forum: Forum 2012: Hunting a knock.....can you help?
Top of pagePrevious messageNext messageBottom of page Link to this message  By John Doolittle on Friday, September 28, 2012 - 06:08 pm:

We have a slight knock- not loud, just annoying (not a hollow knock). It appears at approximately 25 mph. Standing still- it knocks at rpms equal to 25mph. Short #4 plug it goes away. Retard the spark- it goes away.

Does NOT knock going uphill but DOES knock going downhill.

The owner is confident that the clearance is correct on the rods. He has checked them with plastigage @ .001-.0015.

We DO know that the crankshaft has approx .010 end play.

Any ideas?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Rick Goelz-Knoxville,TN on Friday, September 28, 2012 - 06:12 pm:

It acts like compression knock, the Z head will do that and you usually need to run a little retarded.
Rick


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Friday, September 28, 2012 - 06:43 pm:

I find plastigage to be misleading on Model T rods. It only checks one little spot. You can have an egg shaped wear pattern on the rod.

Prussian blue can tell you if the wear pattern is even, but if you have a knock only on #4, you know it has a loose rod.

I would take a shim out of the #4 rod if it was mine.


Top of pagePrevious messageNext messageBottom of page Link to this message  By mike conrad on Friday, September 28, 2012 - 07:20 pm:

John, I agree with royce on this. But one other thing to look at is a possible piston contact with the head I have found on some of my cars running Z heads that there was a small knock. After removing the head the piston had a small polished half moon area on top of the taper of the piston dome and a simular spot on the head were the piston made very slight contact. A small amount of dremalling (grinding) took care of the problem and the knock.

Rick, you may also find this to be true. You sould not have to retard the spark on a Z head.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Doolittle on Friday, September 28, 2012 - 07:31 pm:

The head is a possibility. Supposedly the knock started about the time a shaved head was installed. When I questioned this I was told that the crank was turned over at the time of installation with the head gasket absent and there was no piston to head contact, then the gasket was installed.

Would the extra compression still create the noise?


Top of pagePrevious messageNext messageBottom of page Link to this message  By James Michael Rogers on Sunday, September 30, 2012 - 07:47 am:

Check the head gasket. I have had the gasket to be too small if the cylinders have been bored pretty far.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Rick Goelz-Knoxville,TN on Sunday, September 30, 2012 - 10:44 am:

This is from the instruction sheet for a Z head,
"When operating an engine equipped with a Z head, the spark should NOT be advanced too far. The fuel mixture in a Z head burns much faster than the conventional head, and because of this increased combustion speed, the explosion does not need to occur so soon".

Rick


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Sunday, September 30, 2012 - 10:51 am:

Still can't imagine why spark knock would only be on cylinder #4. Does not make any sense.

The possible causes of knock on a single cylinder would be excess rod clearance, loose pin clearance, or twisted rod causing piston slap. Any of these demand you take off the pan inspection door and possibly the cylinder head to fix the problem.

If the piston was hitting the head gasket the noise would be there all the time.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Charlie B actually in Toms River N.J. on Sunday, September 30, 2012 - 11:15 am:

The guy on the floor above me is right. It's confined to one cylinder. Be it loose or bent rod or piston slap or whatever.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Doolittle on Monday, October 01, 2012 - 01:36 pm:

This coming Sunday will be tear-down day. Thanks to you guys we will at least have some target areas to check.

I will report what we find.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andrew Deckman, Ogden Utah on Tuesday, October 02, 2012 - 02:14 pm:

Not as fun as hunting wabbits!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Tuesday, October 02, 2012 - 05:52 pm:

If it were a spark knock caused by too advanced spark, it would happen when pulling hard such as acceleration, or hill climbing, not when going downhill. Sounds to me like a rod knock.
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Steve McClelland on Friday, October 05, 2012 - 03:15 am:

Wrist pin knock while not under a load..... ?
Just on one piston and knock goes away under acceleration , I'm no expert but seems if a piston was contacting the head it would still knock at all speeds load or no load, even with the #4 spark plug out you should still hear the knock but only louder. I would pull #4 piston out and check wrist pin bolt as well as pin.
First T we owned had a wrist pin knock in #4 it was front to back motion on ours and it cut a heck of a grove in the cylinder wall towards the firewall side, (yep) a loose bolt and the pin had been jumping back and forth is what we found.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Doolittle on Sunday, October 07, 2012 - 06:38 pm:

Doolittle has a positive report. Engine was not removed. We went straight to #4 piston, suspecting a bent rod with the knock coming from side-to-side motion of the wrist pin.

What we found was 6 thousandths clearance at the end cap. No damage to the babbitt. There were only two .002 shims present. These were removed and the cap was put on emery paper until .001 achieved.

The knock is now history- until the next time. New rods are due soon.

Your assistance is appreciated.


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