Please Help and comment on my Pistons (PHOTOS) 1979 85HP

[mz3bo]

Hello,

I have a 1979 Johnny 85 HP. The engine is running fine except ? high idle event at 4-6deg and everything liked and synced. However, I have on piston showing 90psi while all other 3 are showing 120psi. Today, I decided to open the top portion and took some pictures. I am not engine expert but willing to learn. The top piston shows some marks as well as the head.

My questions:

1- can I change only 1 piston ?
2- What about the mark on the head ?
3- If engine is running strong, can I live with it like that ?
4- Do you think the low compression is because of the mark on the piston or something else ? I can wiggle the piston 1/32 left and right
5- Those thiny hair lines on the walls, are they ok?

6- Any other opinions are greatly appreciated.

 

[mickyryan]

hairlines on walls are crosshatch that's not worn off from honing,
the top piston looks to be chewed up like a foreign object was in the side of it it can be replaced the head doesn't look badly damaged but you can have it magna fluxed to be sure I would fix it since you have it torn this far down.

 

[interalian]

You ate a ring on the top one. The head can be salvaged, but you'll need new pistons and rings. Measure the bores to see if they're over the wear limit - if within, standard pistons can be fitted after honing. If over, it would need a re-bore to oversize and suitable oversize pistons/rings.

A full check of the cooling system is in order as well, and look at all the water diverters in the block (rubber hose-like bits).

 

[emdsapmgr]

According to the factory it's perfectly acceptable to change only one piston, but you'll want to replace all 4 ringsets and re-hone the block. The low compression is due to the fact that one of the top rings has disintegrated. Bits of the ring have worked their way around the piston groove and into the combustion chamber. As the piston went up and down, the bits of the ring have damaged the piston and have wedged themselves between the piston crown and the head face-pitting he head. You cannot continue to run the engine the way it is, as another ring failure is probably just around the corner. That failure could irreparably damage any good remaining parts, so plan your overhaul now and quit running it. 2 stroke pistons do have considerable movement within the cylinder, as you noted. This extra clearance gives the pistons room to expand when they heat up. A normal condition. The head that has the damage would be "scrapped" by normal "factory" standards. I do know some folks that clean the head up and re-use it. ( the problem is if there are any ring bits that are stuck in the head. These bits can come loose after an overhaul and do more damage unless the damaged head face is perfectly cleaned up.) All factory pistons (both standard and oversize) weigh the same. So when you select a replacement piston, make sure it is factory, so that it matches the other 3 already in the block. The piston ate the ring for some reason. You need to do an autopsy on the powerhead to determine just why it failed. Could be due to excess heat (I agree with post #3 to check the water diverters) If all 4 are not in perfect position, one could cause a single piston to overheat. Also, Pull the carb apart that feeds that bad cyl. Specifically, check all three jets in that one carb throat that fed the damaged cyl. If one jet is plugged, the engine could be running lean enough to generate enough heat to cause the ring failure.

 

[Faztbullet]

My questions:

1- can I change only 1 piston ?
2- What about the mark on the head ?
3- If engine is running strong, can I live with it like that ?
4- Do you think the low compression is because of the mark on the piston or something else ? I can wiggle the piston 1/32 left and right
5- Those thiny hair lines on the walls, are they ok?

6- Any other opinions are greatly appreciated.

1) Yes you can change just one if needed,,from the shadow in cylinder its near the wear limit. You will need to remove the powerhead to change the piston so mike the cylinders. If out of round or past limit it will need to be bored. If this is the case do all 4. 1979 was the first year the 85hp had the 3.5 bore and these models had the high ring pistons. If they are original you will need to replace the other 3 anyway.
2) Clean it up with Dremel tool and take a magnet and check for ring particles...the dents will fill with carbon over time and will not effect running.
3) It cannot be running too strong as that cylinder will be down on compression and running like that can cause more damage if remaining ring rotates and hangs in port(what your seeing now)and scores walls..
4) Low compression is because ring is broke and rolled out of piston land(groove)
5) Those are the OEM hone marks and no they are not ok as worn ,,,,

 

[gm280]

Yes I see a ring that broke up and worked over the piston as well. I have to wonder what the cylinder looks like for that chewed up piston. I doubt you can simply hone and go back together with new piston and rings. But that is just my guess. As for the head, It look usable and will clean up nicely. But I believe you are in for a boring and honing job and new parts. JMHO

 

[interalian]

You can see the two pistons are different - the top one has a milling hole in the center, and the bottom has a grid of dots. The photo of the cylinder head shows extensive impact damage on the lower cylinder, perhaps from a prior issue on the motor. The top cylinder head is less marked than the lower. Looks as though the motor has been repaired before, and the lower piston replaced at that time.

Replacing just one piston may be a bad idea as the ring lands will be worn and the rings will have more movement than they should, allowing early failure.

The water deflector at the bottom of the block has been displaced to the left. It should be near the bolt hole as shown by the arrow: