Normal idle???

[mach0415]

Back to the origional problem, intermttent miss at idle, I also have this problem. I have traced it to the vacuum port for the dizzy, if I back the idle down far enough (300 rpm) the miss stops, I checked the port for the dizzy and had 0 vacuum, raised idle to 500 rpm and had full engine vacuum at that port. right now I am congitating a solution to this problem:confused:, have several thoughts but haven't had time to experimant with a solution. Also note that engine bone stock, but have the HEI dizzy in it from Stovebolt. the dizzy gave awsome performance improvement:eek:.

The original problem is from a compression loss on my engine, due to #1 intake valve spring broken, causing the valve to not fully seat. The busted #3 piston isnt helping any either.

If you block off or disconnect vacuum to your advance, then your miss at idle should go away, if that is what is causing your concern. I wish it were that simple for me.

[Don Coatney]

have the HEI dizzy in it from Stovebolt. the dizzy gave awsome performance improvement:eek:.

How did you quantify that awsome performance improvement?

[james49ply]

smoother running, better pick-up, plugs show cleaner combustion. the origional dizzy might have been worn and caused the less than ideal performance.

[Don Coatney]

smoother running, better pick-up, plugs show cleaner combustion. the origional dizzy might have been worn and caused the less than ideal performance.

I am not sure how you measured the smoother running and better pick-up but if you are happy so am I.

Is your intermttent miss at idle related to your distributor? Suggest you contact forum member "martybose" as he also had HEI vacuum advance issues. I dont understand nor agree with his "fix" but if it works go for it.

[mach0415]

Well I pulled the pan and pulled #3 piston and rod. It appears the engine ingested something in #3 and then proceeded to beat the ever loving hell out of it, cracking the piston.

The rod bearing has some wear, but the cylinder wall looks good. Visually, all of the 4 rings looked good and were not stuck.

I need to check diameter, specifically with a bore gauge for taper and roundness.

I am dismayed with my initial inspection of the upper compression ring. I was able to easily fit 0.067" in the end gap at 1 inch before TDC

Did all these engines have split skirts?

I have some numbers on the pistons. Are there any stampings to indicate if they are original?

The numbers on the top are 3-561, a dcpd logo, and a large 10 and a large C and a little 4:confused:

Edited February 10, 2012 by mach0415

[Don Coatney]

What you found is typical of a high mileage Mopar flathead. The top ring broke and then broke the top ring land with the pieces ending up on top of the piston breaking it. I was sitting at a stop light a mile from home and my engine started ticking. The ticking quit but I was only running on 5 cylinders. I drove home, pulled the head and found this pictured piston.

[mach0415]

Thanks, Don. So I suppose I should heed wisdom in that a whole set replacement is in order. Basically a rebuild. I think it may be time to part her out or find a "cheap" used engine.

[mach0415]

Thanks, Don. So I suppose I should heed wisdom in that a whole set replacement is in order. Basically a rebuild. I think it may be time to part her out or find a "cheap" used engine. Here are more pics.

[Rusty O'Toole]

Don I have seen that type failure before and thought it was piston failure from too high revs. Are you saying the ring breaks first, breaking the upper ring land? In that case would more modern molly rings eliminate the failure point?

[Young Ed]

I suspect the 10 cast on top to indicate over sized pistons. Make sure you measure the bore before you buy anything.

[mach0415]

I pulled all 6 pistons and found only one to be the original style, with the hole below the lower oil control ring, a cut, and another hole in the skirt area. (the others have a split skirt the whole length) The difference in distance from the top of the upper compression ring land between the two styles is approx. double the thickness ...whatever.

I measured the bore with my trusty bore gauge and found only .007-.010 difference from stock. I measured the pistons and despite the "10" stamped on all but the mismatched one, all were stock, at 3.25". My taper is less than .005"

So, I decided to check the old rings' engap and stopped when .067" worth of feeler gauge still slipped easily in the ring gap at the lower point of the stroke, while in the cylinder bore Its amazing this thing even ran! But, it does explain the ridiculously low compression readings I was getting. The valves appear to be okay, but I will re-guide and lap them. Lots of slop in the guides.

The question is, Can I get away with putting a stock set of pistons in this and go to a .010" oversized ring set and file as needed, given the lower compression ratio? or do I absolutely need to go up a size on my pistons too?

Thanks,

Mark