replace pistons and bore oversize?

[ribcage]

I need some advise. I took apart the power head thinking that I only needed a new piston in cylinder #3 with it bored. After looking at the other two pistons I notice some scoring. Please let me know if you guys think I should go ahead and replace the other two pistons? I have included pictures...

 

[pnwboat]

Re: replace pistons and bore oversize?

How do the cylinder walls look? If they're kind of scored then definitely. Before you buy anything, have the cylinders measured to make sure you can bore them out. These motors are pretty old now and it's not uncommon to find that a previous owner has already bored the cylinders out to the limit.

 

[ribcage]

Re: replace pistons and bore oversize?

The first and second cylinder dont look bad. The third cylinder feels like it definately needs bored as it has a deep spot where the piston ring blew in the third cylinder....So you think i can continue using the pistons above in the first two cylinders.

 

[Justinde]

Re: replace pistons and bore oversize?

My 1988 125 has number 4 oversize 30thou, the rest are stock, so, up to you if you do them all.......

 

[jerryjerry05]

Re: replace pistons and bore oversize?

No,they are toast.
The cylinder walls???Maybe???
Can you feel ANY grooving or cuts?
Take the block to a machine shop and have them check the cylinders.
Possible you can hone the 2
There are lots of used, GOOD pistons out there(I might have a couple).
Profile?Location? You might be close to someone who can help.

 

[ribcage]

Re: replace pistons and bore oversize?

I am in louisville Kentucky. I am going to take it to a machine shop tomorrow and get their opinion on boring the cylinders. SO I guess I will need to replace all 3 - pistons....??

 

[Frank Acampora]

Re: replace pistons and bore oversize?

Here is your decision tree: The piston on the right is scuffed relatively badly but probably still serviceable. The piston on the left is less damaged. Now, you know you are going to bore at least one cylinder and buy an oversized piston. This will cost about 200 dollars.

IF the other two cylinders are scored you only have one option: bore all three and replace all three pistons for a total materials cost of around 500-600 bucks (including boring charge)

If the other two cylinders are not scored, you have three options:

1. replace the pistons at about 100 bucks each resulting in a total cost of around 400 bucks.

2. clean and re-use the pistons for a total cost of around 200 bucks BUT--The engine probably will not deliver its rated horsepower. How much of a loss? Difficult to estimate, It may not even be noticeable. But, that's a chance you would be taking. You need to decide if it is worth the money to gamble.

3. Bore all three cylinders and install three oversized pistons. Total cost around 600-700 bucks depending upon boring costs and brand piston you buy. However, in essence you will have a completely new engine.

So now you have enough information to make an informed decision when you go to the machine shop. BTW: Take those two pistons with you and let the machinist tell you if they are serviceable or not. First hand is always better than a photo.

When you reassemble the engine pay particular attention to cleaning and setting the carbs. You didn't say why the #2 cylinder was scored but if the piston was melted, you had carb damage.

I strongly urge you to NOT buy a stock piston. These pistons have a semi-keystone top ring and they do tend to break more often than a flat ring.

 

[ribcage]

Re: replace pistons and bore oversize?

Well im taking it to get looked at today. I found someone that will bore and hone each cylinder for 36.00 / piece. Also I found new pistons for 70.00 with the rings. Total cost 70x3 = 210 + 108 is 318. You mentioned the key ring on top of the piston. Do you think I can get a flat head piston and it will be fine??? Please let me know. What would be the benefit?

Here is your decision tree: The piston on the right is scuffed relatively badly but probably still serviceable. The piston on the left is less damaged. Now, you know you are going to bore at least one cylinder and buy an oversized piston. This will cost about 200 dollars.

IF the other two cylinders are scored you only have one option: bore all three and replace all three pistons for a total materials cost of around 500-600 bucks (including boring charge)

If the other two cylinders are not scored, you have three options:

1. replace the pistons at about 100 bucks each resulting in a total cost of around 400 bucks.

2. clean and re-use the pistons for a total cost of around 200 bucks BUT--The engine probably will not deliver its rated horsepower. How much of a loss? Difficult to estimate, It may not even be noticeable. But, that's a chance you would be taking. You need to decide if it is worth the money to gamble.

3. Bore all three cylinders and install three oversized pistons. Total cost around 600-700 bucks depending upon boring costs and brand piston you buy. However, in essence you will have a completely new engine.

So now you have enough information to make an informed decision when you go to the machine shop. BTW: Take those two pistons with you and let the machinist tell you if they are serviceable or not. First hand is always better than a photo.

When you reassemble the engine pay particular attention to cleaning and setting the carbs. You didn't say why the #2 cylinder was scored but if the piston was melted, you had carb damage.

I strongly urge you to NOT buy a stock piston. These pistons have a semi-keystone top ring and they do tend to break more often than a flat ring.

 

[Frank Acampora]

Re: replace pistons and bore oversize?

NO, your engine is old school and you must get the piston specified for it. It has a baffle built into the top. The top ring is not a key ring but a semi-keystone ring. That means the top is tapered and the bottom is flat. It looks like a wedge in cross section. The theory is that combustion pressure gets behind the ring forcing it into the cylinder wall and losing less pressure. Yeah, it does work but yeah, these rings do tend to break easier than regular flat rings. HOWEVER: You can not switch rings in a piston. You must use the ring that it (the piston) was designed for. So, whatever pistons you buy, if they do not come with rings, buy the matching rings.

The prices I quoted were worst case and I was assuming 60 dollars a cylinder for boring and about 125 for Wiseco pistons bought direct. Shopping around, you can usually do better as you already have. Good luck with your project.

 

[HotTommy]

Re: replace pistons and bore oversize?

As this thread is not too old and my situation directly relates to it, I'll add my question here rather than start a new thread.

I have a 1993 Force by Mercury 150 HP two cycle outboard engine. The top piston was damaged, apparently by detonation caused by a lean situation. After examining the piston I noted two score lines the length of the cylinder aligned with some damage at the edge of the piston. I can see them and feel them. None of the other pistons or cylinders show signs of damage.

I live in rural Alabama and the nearest reliable machine shop is about 60 miles away. I spoke with them and they'd prefer to see just the engine block (so they can begin work right away if it is needed). I'd rather not take the block apart if I don't have to. If the scored cylinder only meant a mild reduction in power, I could live with that. So here is my question. Is there any way for me, an amatuer, to evaluate the scoring and its likely impact? For example, is there a tool I could obtain to measure the depth of the score, and would that provide me with useful information?

 

[Jiggz]

Re: replace pistons and bore oversize?

Hot Tommy, you are actually better off starting your own new thread despite the old thread being directly related with your case. The reason being, with an old thread forum members tend not to bother reading them because they are already old. A new one makes it more interesting to respond to knowing it is a new case. Anyways, on your new thread, post pics of the piston and also of the cylinder. Usually, if the cylinder is not badly damaged, you can just hone it yourself to lessen the adverse impact to the new piston although having it rebored will be preferred if damage is really bad. A honing kit from Harbor Freight costs less than $30 while a new set of piston is around $125 with rings and another $15 for new wrist pin bearing kit (is using the highly recommended Wiseco pistons). Anyways, post pics and start it from there.

 

[HotTommy]

Re: replace pistons and bore oversize?

If this thread doesn't yield a timely answer, I'll start a new one. I added to this one so the old heads wouldn't have to repeat all the sage advice already given above.
Thanks for the advice.

 

[Frank Acampora]

Re: replace pistons and bore oversize?

Tommy: I will answer on your other thread about head gasket since this is directly related.