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25 DB Piston Went Bye-Bye. Looking For Advice.


Guest oldodgeboys

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Guest oldodgeboys

<span style="font-family: 'Arial Black'">

I have a problem with my 25 DB engine piston and I hope some of you can help.

I was driving at 35 MPH when one piston came apart. Oh man…that was a <span style="font-style: italic">really</span> unpleasant sound.

The piston involved is #2. Last year #2 cylinder showed a compression of 25 PSI, while the others were 45-55.

<span style="color: #000099">I pulled the piston and measured a few things and came up with the following information:

The wrist pin and top of the piston stayed in place and are still connected to the connecting rod, but most of the skirt was shredded by the crankshaft.

The oil pan was filled with about 20 pieces of piston, including the steel spring that was inside the piston skirt. The spring is bent in one place and broken in another. I can’t tell if the broken spring is the primary failure or a secondary failure.

The piston markings show PN 15551, patent date 3/4/19, and 'I' stamped on top. The piston looks to be in the range of 'standard size’ pistons from the factory.

The piston rings on #2 are very worn and weak.

The cylinders look fine with a few minor scratches from flying debris. The rest of the motor looks fine with the exception of aluminum crumbs sticking all over the inside of the motor.

The #2 cylinder measures at 'standard size’ (3.873) near the bottom of the cylinder, and the top of the cylinder measures .010 larger diameter due to wear. The cylinder was measured in both the x-axis and y-axis and was found to be round (+/- .001). Measurements were taken with a Telescope Gage and a 3-4 Micrometer. These are preliminary measurements taken from underneath the motor for now.</span>

I think metal fatigue of the skirt spring or the aluminum piston was the primary cause of the piston failure. What do you think about this?

<span style="color: #660000">I have several repair options at this point and I would like your opinion:

A) Rebuild the engine. Normally I would do this, but the economy strangle-hold is preventing that right now.

B) Replace the broken piston with another used piston. Use new rings. I think this would be the lowest cost, but the highest risk. This would not fix the piston metal fatigue issue.

C) Replace the broken piston with a new piston and rings. This may present issues with engine piston balance. It would not solve metal fatigue with the remaining 3 pistons.

D) Replace all the pistons and rings with new pistons and rings. The cylinders can be honed while the motor is in the car, but the cylinders would not be bored. I would use ‘standard bore’ pistons and rings…if I can find them. This option solves metal fatigue issues. It would restore most of the lost compression. </span>

Option #D is my favorite at this time. What do you think about installing pistons/rings without boring the cylinders?

What are your thoughts? Your advice is valuable to me and I believe in learning from the experience of others.

Thanks for your help,

Gus</span>

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Gus, I would think your decision depends on how you use your car. I know you like to drive alot, but it seems you go short distances. If you do or plan long tours a full rebuild is THE way to go. .010 taper seems to be at the extreme .limit, you may want to consider a bore for the good job. Personally I'd go the B route and get it back on the road. Maybe start gathering parts for a rebuild "down the road".

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Guest oldodgeboys

<span style="font-family: 'Arial Black'">Hi Doug-

Option B was my first inclination also, but the metal fatigue issue would still be there (if that really is the root cause). The rest of the motor is in very good shape...even the bearings need little or no adjustments.

If I go with option D the cost would be around $400 ish, I think that is acceptable for now. I think the engine would hold up well for a long time.

Like you said, the cylinder taper is a worry. I really don’t know what the effects are of putting new pistons and rings in a .010 taper cylinder??

Also, I found that the skirt spring was broken due to impact, not fatique.

Gus</span>

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This is just a dum observation from 3000 miles away, but why buy new pistons if it probably should be bored? I've never seen acceptable wear limits for an ol' DB but .01 is too big for a modern engine. The rings have to adjust to the differant diameters every time they go up an down. AND if you replace the pistons and rings you definately need new valves and cut the seats. None of it is tough work, I'm sure you can do it. But then what about the water pump? Might as well put a new shaft in that too. Oil pump? And, and, and.

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Guest oldodgeboys

<span style="font-family: 'Arial Black'">Please remember that the cylinder measurements are preliminary. After the head comes off I can be more accurate.

The current measurement of .010 taper shows two things: 1) the bottom measurement shows that the cylinder was never bored, 2) the top measurement shows there is .010 wear at the top of the piston ring wear zone.

What is not known at this point is how much taper is in only the piston ring wear zone of the cylinder. So the amount of potential ring movement is still to be determined.

Looks like I need to get off my duff and pull the head so I can get the final measurements. In the mean time I hope someone can chime-in with some thoughts on acceptable cylinder taper in regards to piston operation and ring operation.

As I said, the rest of the engine looks good so no need to get into any of that.</span>

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The one question I always ask before I rebuild an engine is what type of life will it have for the next ten years. If you are going to really drive the car then you do not want to do it again. You will be driving it faster then it was designed for with modern rings with are made of harder material and designed to seal better. This means the cyc will wear faster. I would bite the bullet and completely rebuild it. That taper will only get worse not better. Make sure you flush everything real good to make sure there are no metal shavings left. A good engine is hard to nfind. Have a nice day

Jan

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Guest oldodgeboys

<span style="font-family: 'Arial Black'">Thanks for the input. Your statement about new rings vs. old cylinders is well taken.

Due to time and financial restrictions, a complete rebuild is not an option at this time. This car needs to be movable soon. I know what the ideal path is, I just cant do it at this time.

Now this is interesting: I found the root cause of the piston failure...someone in the past had reinstalled all the pistons backwards. The piston skirts were facing the right, and the rod dippers were facing the left. Hard to believe. Amazingly, the rod bearings and crank look fine.

Here is an interesting link. This shows that a .010 cylinder taper is a candidate for a re-ring job without re-boring the cylinder. It also has good info about ring gaps.

http://www.motorpartscentral.com/ProductModelDetail.cfm?ProductModelId=14215

I'm still working on getting the head off the block...its been a while since it was off and she's a stubborn @*!&*!. At this point it looks like I will use new pistons and rings. A complete rebuild would be done next year...I hope.</span>

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