Cam Thrust / Plunger Spring

31 Woody

31 Woody

Senior Member
Two Questions:
1) Should the cam plunger/spring push the cam back to the block after manually pushing the cam forward?
2) Could .044” thrust clearance between the cam and block make a thud/dull rattle when running under heavy load?
BACK-STORY:
My Model A has a thud/rattle when under heavy load such as climbing a hill. I dropped the pan expecting to find a bunch of Babbit. There were a few small pieces but not enough to be alarmed. I checked each rod for play and they did not move at all. So, I decided to check the cam gear. After fighting the side cam gear cover off, I found the cam can slide forward but the plunger spring does not automatically push it back—it stays forward. There is .044” thrust clearance between the cam gear and the block with the cam pushed forward. Could that be the mystery noise? I also checked the valve lash and only one or two of them were a couple thousandths off. I adjusted them to spec so all valves are set properly. This motor only has 3,000 miles on a rebuild/re-babbit, so I would hope it isn’t done already….

Thanks for any constructive advice.
 
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I think my question would be what spring tension/compression should it have? Cams can make noise as there are no bearings to replace. Some bore and place bushings. but many builders don't do it unless it is requested.
 
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Yes, the cam spring and plunger should continuously press the camshaft back against the block - no clearance. From what you describe it sounds to me like the spring may be broken or missing. This would not be the first engine assembled without that spring.

My recollection is that when installing the timing cover that the last 1/4" or so of bolt compresses the spring.
 
Aluminum gear, it is well fit. I didn’t measure backlash, but it certainly isn’t loose to the crank gear. New crank and cam gears installed during rebuild by engine builder.

After re-read: Too much lash isn’t a problem, but too little lash, that might be. I need to measure that.
 
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Being under load really shouldn't affect the cam thrust much if any at all. Opening the exhaust valves under higher cylinder pressure will add a bit to cam load but that seems a stretch.
 
I had to add a shim to get the proper amount of push from the plunger. I used a couple of washers. Others have drilled and tapped for a screw that can be adjusted from outside the engine.
 
I chased a similar noise a few years ago and discovered that the crankshaft thrust bearing clearance was too large, it was not the camshaft making the noise. So look at and measure your crankshaft movement.
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Dull thud is a sound I had and in preparation for a long trip I figured that I should check bearing adjustment although it was only 3000 miles (and 10 years) from a rebuilt by a long term experienced with old engine shop.
Only a couple small crumbs of babbitt in the drain plug recess.
The babbitt in the rear cap was just a collection of crumbs captured by the intact thrust surface.
The Babbitt was brittle and would shatter if struck with hammer .
Broken in gently,not driven over 50 mph.
I repoured the mains myself, the first time I had done it — didn’t break it in gentle and on the third day of driving I found that the car would do 67 and its still hasn’t failed 40 years later. I have come to the conclusion that there is nothing you can do to “save” a bad babbitt job and that it is hard to hurt a good one.
 
Did you check the main bearings when you had the oil pan off I would have definitely checked the center main bearing I only see that you check the rods.
 
The rear cam bearing can wear unevenly if its an earlier engine before they started drilling the oil hole for the rear journal per the service bulletin. The cam may be harder to move through a thrust check since there could be journal wear plus any valves open also apply pressure. When the engine is running, the cam should move some but the spring & thrust button generally contains that movement pretty well. A motor that's not running is a different story.
 
I would check to see if you are getting Camshaft slap/walk. This happens when the camshaft thrust plunger and spring in the timing cover fail to maintain proper pressure on the camshaft. The helical gear pulls the camshaft gear in but when certain valves run past their respective cam lobes it momentarily creates a reverse force on the lobe which pushes the Camshaft Gear out. The result is a knock.

Having said the above, I am not an engine expert so would very much appreciate other's correcting me or setting me straight.

I have severe cam slap on my Burtz engine, in part likely due to using stiffer valve springs, and I have been doing a lot of investigation on how to fix it with the assistance of Richard Knight and other Burtz engine builders. I have Dan McEachern matched gears.

I have verified that the plunger spring currently being supplied by the vendors is significantly weaker than the original Ford spec (at least based on the 4 springs I tested). I tried putting in a spacer at the end of the spring to increase the spring pressure/force. I also tried spacer washers in the base of the plunger bore to limit clearance between the plunger and camshaft face to 0.002 to 0.005. This didn't work that well because it is so finicky to set up to get the correct clearance. So... I am now going to the bolt in the timing cover as described by Chris Wickersham (https://www.santaanitaas.org/chris-wickershams-tech-articles/) and as advocated by Richard Knight. Butch Chase also has an internal way to set the plunger spacing limit which can be found on the MAFCA Facebook forum.

My cam slap can be detected when hand cranking the engine (belt off). I have found that the best way to check for cam slap is to remove the side cover and when cranking the engine by hand, pull forward on the timing gear. Pushing the timing pin in the hole to apply back pressure on the gear is sometimes not conclusive.
 
Try shimming the plunger spring by 0.100 inch. The shim should be between the spring and plunger, not between the spring and front cover.
 
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Sort'a like this...
 

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Recently i had a badly made new oil pump drive gear that would bind and cause the cam to walk fwd then snap back. Also unrelated, new from ‘ antique ford country’ cam plunger that was made 010 too big to fit in the cover boss Just cause its new doesnt mean its any good
 
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