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#884944 07/07/22 1:02 pm
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Beach Offline OP
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Lost compression on left cylinder of my 66 Atlas. Tore down and found left piston scuffed really bad and right piston mildly scuffed. Left cylinder had scuffed marks. Thinking previous owner may not not have set clearance to .0045". Any thoughts?[Linked Image][Linked Image][Linked Image][Linked Image]


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Obviously it has siezed. If it has been rebored you may have egg shaped cylinders What is the clearance now??.

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Beach Offline OP
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That may be. Haven't measured the bores yet.


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Significant difference between the 2 piston tops, one nice and clean the other carboned up. pre ignition from oil getting past the rings may be the issue not lack of clearance. Any aluminum bonded to the bore can safely be removed using hydrochloric acid, then measure the bore.

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The right side piston scratches may be from honing grit or maybe debris from the other piston.

The left one looks like someone persevered after a seizure. That would let oil into the combustion chamber as Mr K said, maybe leading to detonation that chewed the edge off the piston top.


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Was on interstate cruising when this happened. Noticed smoke in mirror and pulled over. Guess the main thing is to make sure cylinders are bored correctly this time. Pistons are .040" over now. Figure there is enough metal to run .060 over pistons.


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Flat top Commando or dished atlas pistons?

Mag AA unit fail to retard making over advance at low rpm & big throttle?
Magneto timing difference between cylinders?

Coming to the NY rally?


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[Linked Image]Flat top pistons with 2 base gaskets and spacer. Checked timing on ramps when setting magneto and advance is working fine.

Not going to make the rally, wife has had medical issues we're dealing with. Looks like it will be a good one.


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Curious that a lot of the scuffing seems to be on the sides of the pistons, not on the thrust faces.
Makes you wonder if the pistons were cam-ground ???

That does seem to be a big difference in appearance between pistons ..
Twin carbs ?

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Beach Offline OP
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Yes twin carbs.


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Originally Posted by Beach
Yes twin carbs.

curious ? 66 atlas 389-689 or chopped Monobloc?

I've got a 66 also 1189xx not running, but a complete basket case.


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389-689 Got motor on bench and going to check gaps and timing on mag(just in case), check sealing surfaces on carbs and insulators.


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Gap big at .017(should be .012) but was same on both ramps. Don't think this would cause a lean condition. Next check of timing.


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Originally Posted by Beach
Gap big at .017(should be .012) but was same on both ramps. Don't think this would cause a lean condition. Next check of timing.


If the gap increases, that advances the timing.


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Yes just checked it out. The moving points (coil power) while closing, could beat a loose stationary points (armature ground) causing them to rotate and open up. not good ! ! !


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Beach Offline OP
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Maybe that's the problem.


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Got motor out and completely apart. It will be correct in a few weeks. Is there an easy way to check the oprv? Want to eliminate all possibilities.


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It is more likley 10 or even 20:1 it is the pump is crappy rather than the OPV opening early. I can check them on my oil pump tester rig.. I can shut off the exit and send pump output 100% to the OPV and have a pressure gauge only on that circuit...you can see the response as the regulation starts to kick in...
55-60-65 ?


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Originally Posted by Beach
Lost compression on left cylinder of my 66 Atlas. .....left piston scuffed really bad ...... Any thoughts?[Linked Image]
I'm thinking detonation happened first. Detonation can collapse the ring lands which pinches the rings which stops them sealing. Hot gases destroy the lubrication and seizure results. It can happen quick.

Things to check or consider:
Ignition timing variation side to side.
Full advanced ignition timing.
Carb synchronization.
Fuel blockage.
Jetting.
Fuel quality.
Compression ratio.

Last edited by Stuart Kirk; 07/08/22 10:46 pm. Reason: Thought of something else.
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Wouldn't a bigger points gap retard the spark.
Quite considerably, for 0.017 "

Which can also make a motor run hot
The fire doesn't even get started until it should be *half over ...

*approx, or a bit less ...
**assuming it wasn't set/timed like that.

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Originally Posted by Rohan
Wouldn't a bigger points gap retard the spark.
Quite considerably, for 0.017 "

Which can also make a motor run hot
The fire doesn't even get started until it should be *half over ...

*approx, or a bit less ...
**assuming it wasn't set/timed like that.


No, a bigger points gap advances the spark.


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Beach Offline OP
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Figured out what happened. The original pistons when I purchased were hepolite with groove in side for oiling(bad idea). Replaced with JCC pistons .040 over clearance at .0045. Last year I pulled my points to check for proper mating and clean contacts. Put back in at, I thought .012 gap and timing should have remained the same. Checked gap yesterday and found they were at .017, enough to advance timing and cause detonation and the damage to piston. Don't know how I did it but glad to have found the problem.
Thx for all the helpful feedback.


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So how would the gap go from 0.012 to 0.017 ?
If the spark had advanced, muchly, you'd have heard it pinking and carrying on ?
And it would be a pig to start, if not downright cranky !
Now if you had accidentally timed it at 0.017 then the dwell would be less and it would/could produce a weak spark.
This can delay the onset of the fire, and engines will run hot from this.

Probably a bit late for a proper post mortem ...

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Originally Posted by Rohan
So how would the gap go from 0.012 to 0.017 ?
If the spark had advanced, muchly, you'd have heard it pinking and carrying on ?
And it would be a pig to start, if not downright cranky !
Now if you had accidentally timed it at 0.017 then the dwell would be less and it would/could produce a weak spark.
This can delay the onset of the fire, and engines will run hot from this.

Probably a bit late for a proper post mortem ...


When we accidentally over-advance our ignition, by having too big a points gap, the problem is over-advanced ignition. That is the opposite of “delayed onset of fire.”

The rider doesn’t always hear pinking at road speed.


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Originally Posted by Rohan
So how would the gap go from 0.012 to 0.017 ?
If the spark had advanced, muchly, you'd have heard it pinking and carrying on ?
And it would be a pig to start, if not downright cranky !
Now if you had accidentally timed it at 0.017 then the dwell would be less and it would/could produce a weak spark.
This can delay the onset of the fire, and engines will run hot from this.

Probably a bit late for a proper post mortem ...
I explained in previous post how the points ended at .017 and it started easy and there was no noticeable noise at highway speed as Triton mentioned.

Last edited by Beach; 07/12/22 12:16 pm.

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