New piston doesn't look like old one

[jbs007]

Hi all,

This is my first annual on my 66 M20E. I've flown it about 100 hrs this year. Before that it flew 80 hrs the previous 5 yrs. 450 hrs since overhaul. One compression was really low (40/80). On pulling the cylinder we found the piston rings were stuck all the way in the groove on one side of the piston. The cylinder looked find though. We got an overhauled cylinder with piston in it. Anyway...

The "new" piston does not have dimples next to the wrist pin whole like the old one does. The older piston looks to be of much higher quality. Also, the "new" piston looks to have been pitted and then sandblasted and has nicks in the skirt. Pictures attached. My IA says its fine but I want more opinions. Also the wrist pin has much narrower ends than any other Lycoming wrist pin I've seen. Any info on this would be great too. Thanks!

Jared

Ps. Since I know it will be asked: the last ~20 hrs my airspeed has intermittently decreased about 8 kts and my oil consumption went up to 3hrs/qt. it was intermittent; after 10 min or so it would come right back up. There was no real sign of roughness. The engine would just barely sound different. This made me think it was some fouled plug not having good spark. All spark plugs got replaced this annual.

[Awful_Charlie]

I would be most concerned about the relative weight of the piston & pin assembly - if they're within a couple of tenth of an ounce, then probably good to go - if they're half an ounce or more different, be prepared for some vibration!

The nicks look as though they are there intentionally, probably for oil retention, or maybe just as a result of the manufacturing process, and I wouldn't be worried about them particularly.

Not sure bout the pin - I'm guessing the caps still fit up ok though?

Finally, I do hope that you or the A&P has checked that the cylinder and piston part numbers are eligible for your engine? If you're still unhappy, call the people that supplied or made it with your concerns, if they can't address them, you might have another problem

[captainglen]

I would not wory about theextra features on the old piston but you are right about the pitting. The reconditioned piston does not look smmoth at all at least on the one side. It will probably be all right but I would see about exchanging it for a better one. The old cylinder is consistantant with your observed symproms but there is another concern and that is that the ring galling on the old cylinder is due to the fact it was overheated. The cooling baffle rubber seals need to be checked to make sure that cooling air is not bypassing that jug allowing it to get hotter than the others.

[N601RX]

Lycoming sells their pistons in 3 weight grades. After manufacturing them they are all weighed and grouped into 3 groups, Heavy, Light, and service grade. Do not mix heavy and light. The service grade falls in the middle and can be used to replace both heavy and light if you do not know which one you have. There is a service bulletin or instruction that describes this. Superior and ECI also manufacture pistons for Lycoming engines. They may not be identical to each other. Lycoming also made many design changes over the years. SI 1243

Lycoming has also produced 3 different versions of wrist pin caps that are not to be mixed. If I remember correctly they may have said to remove the small ones from service. There is a service bullet or instruction that talks about this also. SI 1267

If you can't find them online let me know and I can e-mail you a copy when I get to work tomorrow. All of them are not online, but I had purchased a subscription to them all last year while overhauling my engine.

[N201MKTurbo]

If you are really ambitious, clean the old piston real good and weigh both of them. If the new one is heavy then remove some material from the bottom of the skirts, so it shakes just like the old one. If you are really neurotic, have someone make you a long piston pin and put it on precision knife edges on a surface plate and balance both sides of the piston