Cylinder and piston clearance

[justinb]

Hi All

I recently did a bottom end replacement of crank seals and bearing the whole 100 yards

Same time I did the top end , I bought new piston and rings standard size as nothing was damaged on cylinder head I bought SBT ones,i was told I only needed to hone the heads which I got an engineering firm to do with the pistons and rings so they could measure all up .

I now recently tried starting but the one head had 90 psi, after striping it apart I noted the top ring had a big gap I then adjusted the ring .

See first pic which has the ring in cylinder showing the ring gap , I belive this is in spec with SBT chart 0.05

Now look at the 2nd pic around the red circle the ring in this space is not sitting flush against the wall it has about a 0.002 space

Does this mean anything? In the 3rd pic with the piston on and in the sleeve I can't get a 0.002 in it any more .

However the SBT chart says I should have 0.005 clearnce

In the book seadoo book it says 0.001 which I don't have that side feeler gauge but it if look at the 0.002 I think it would fit if I had it .

Which should I be going off if I am using SBT rings and pistons or the book ?

The last 2 pictures are the inside of my cylinder

Thanks I don't know if it will work as is or do I need to have the cylinder wall honed deeper to fit the tolerances of the SBT rings and pistons

Or after riding it in this will change ?

Thanks

 

[tobytd]

Not seeing any chamfer on the port openings…good way to chew up new rings.

 

[justinb]

Not seeing any chamfer on the port openings…good way to chew up new rings.

Hey Toby , not sure I understand the meaning of chamfer, how do I fix that ? Your thoughts on everything else is that ok

 

[burtshaver2021]

There should be a chamfer in the ports ( the holes in the cylinder wall ) chamfer is a beveled edge on the ports so they aren’t on a 90 degree.

 

[mikidymac]

Let's back up a minute.
What was your compression before the rebuild?

You need to measure the bore and cylinder diameter with a bore gauge to see if the clearance is in spec, at this point ring gap isn't important.

If you honed a used cylinder then bought STD pistons I can almost guarantee you have too much clearance and will never get your compression up.

On used cylinders you need to bore to the next oversize and install oversized pistons.

 

[justinb]

Let's back up a minute.
What was your compression before the rebuild?

You need to measure the bore and cylinder diameter with a bore gauge to see if the clearance is in spec, at this point ring gap isn't important.

If you honed a used cylinder then bought STD pistons I can almost guarantee you have too much clearance and will never get your compression up.

On used cylinders you need to bore to the next oversize and install oversized pistons.

It was sitting at 145 psi on each one , when we investigated further we found the rotary gear was damaged.

We then opened further and found the gear on the crank had moved over which drives the roatary gear.

If you recall something fell in like a bolt from the carb box , this is what was assumed that did the damage.

I then went on to buy a refurbished crank with new bearings seals etc ,this came from the states

I then decide since I was doing the bottom all up I may as well do the top so I bought SBT rings and pistons ,it was measured and we bought accordingly .

I then sent the new rings and pistons with the cylinders to engineer to have them hone according to the spec of everything.

They did so as I assume

After rebuilding it it took me months to finally start it because at that stage I decide to paint my ski.

When I tested it 2 weeks back it would not start hence the electrical thread I put on

I then did a compression check and found the readings of 90 and 135

I then opened to see what was going on and noticed the 90 psi on the top ring was not seated nicely I assumed it was loosing compression there.

I have since taken it to an jet ski shop and they looked at it and they confirmed that the rings where slightly damaged.

I need to be honest here the chances are I handled them to rough when installing been the first time


I have now ordered new rings , I have learnt about the ring gap clearance since then so I will check that amount with the clearance on ring in piston and pistons inside the cylinder.

I have to assume the engineering place Checked my new piston size against the cylinder

Is there away I can check ? Insert just the piston into Sleve and measure the gap against it and wall

 

[mikidymac]

At 145 psi you should have just kept the OEM pistons and rings and not touched anything.
If it was mine at this point I would just buy OEM rings and use the OEM pistons but that's just me.

Or roll the dice and run it with the new SBT stuff.

As Tobytd said, I don't see any chamfer on those ports so how much material did they remove when they "honed" it?
Clearly they are not 2-stroke experts. With no chamfer you will keep ruining rings and pistons.

 

[burtshaver2021]

Trust nobody. Is there a auto shop close to you that would loan or rent dial bore gauge and 3 to 4” inch caliper so you can measure the bores, pistons, check your clearances. Does anyone else see scoring on the cylinder walls?

 

[justinb]

At 145 psi you should have just kept the OEM pistons and rings and not touched anything.
If it was mine at this point I would just buy OEM rings and use the OEM pistons but that's just me.

Or roll the dice and run it with the new SBT stuff.

As Tobytd said, I don't see any chamfer on those ports so how much material did they remove when they "honed" it?
Clearly they are not 2-stroke experts. With no chamfer you will keep ruining rings and pistons.

At that time I wanted it all nice and new , I did not know better !!!

The old stuff gas been binned , I've spent 400 dollars on the new stuff , I don't see why they won't work ?

I have to try and see, the 2nd mechanic looked at it and said it should all be fine just get new rings as others was slightly damaged .

He said I can chamfer those ports .

I have no idea how much material was taken off in the honninng process all I know it was done according to the pistons

Is there a way I can check ?

 

[burtshaver2021]

They will work as long as to much material wasn’t taken out during honing. Had you not have binned the OEM parts, they are better quality than the SBT and were pretty much new at 145 pounds

 

[justinb]

Trust nobody. Is there a auto shop close to you that would loan or rent dial bore gauge and 3 to 4” inch caliper so you can measure the bores, pistons, check your clearances. Does anyone else see scoring on the cylinder walls?

I will have to phone around what should it measure ?

You asking me if there is scoring on it or you saying there is?

All I can see is the cross hatch of the honning ?

 

[burtshaver2021]

Third picture from the top, looks like some vertical marks to me, may be ok, but with no chamfer on the ports and a messed up ring you should have a good look at it.
I don’t know the piston to cylinder wall clearance by heart but the numbers are in the manual. Do you have a manual?

 

[justinb]

Third picture from the top, looks like some vertical marks to me, may be ok, but with no chamfer on the ports and a messed up ring you should have a good look at it.
I don’t know the piston to cylinder wall clearance by heart but the numbers are in the manual. Do you have a manual?

Oh that ,I've ran my finer and nail across it , no indents in wall or damage smooth like everywhere else so unsure.

Yeah I have the manual I will have a look .

 

[burtshaver2021]

Oh that ,I've ran my finer and nail across it , no indents in wall or damage smooth like everywhere else so unsure.

Yeah I have the manual I will have a look .

I think the specs are in technical data section of the manual

 

[justinb]

I think the specs are in technical data section of the manual

OK just come from another engineer shop they checked this clearnce wall to piston with this gauge and others it reads 0.0039 inchs

Which is what it shows in the manual,they said it's spot on also agree replace the damage ring more than likely the cause of compression loss along with sorting the ports sharp edge off

 

[burtshaver2021]

OK just come from another engineer shop they checked this clearnce wall to piston with this gauge and others it reads 0.0039 inchs

Which is what it shows in the manual,they said it's spot on also agree replace the damage ring more than likely the cause of compression loss along with sorting the ports sharp edge off

Wow, that’s some fast service.

 

[justinb]

Wow, that’s some fast service.

I still had them in my car whilst driving home found a place not far from my house poped in and they did it in front of me .

Same tools as the manual I even showed them the notes from manual , I have piece of mind now only concern I have is the clearance from the SBT chart is slightly less than the book been a different make .

See attached 1st on is SBT

NOT SURE IF THAT SLIGHT DIFFRENCE WILL CAUSE ISSIUES ?

 

[burtshaver2021]

Are both pages you posted from SBT? On the first page I see nothing about your engine, just Kawi and Yamaha. I’m the second pavé I see .0039, is this from SBT or from the Seadoo manual? What are the allowable limits from the Seadoo manual? .0039 to? And what is the wear limit for piston to wall clearance?

 

[burtshaver2021]

So they measured the piston? Measured the bore and your clearance is .0039. I find that odd seeing as it was just honed enough to remove most of the chamfer, I would have expected the clearance to be slightly larger now. But if that is indeed correct then I would say your good to go. Just push the rings into the bore down to about 3/4” from the top and measure the ring end gap before assembling

 

[justinb]

Are both pages you posted from SBT? On the first page I see nothing about your engine, just Kawi and Yamaha. I’m the second pavé I see .0039, is this from SBT or from the Seadoo manual? What are the allowable limits from the Seadoo manual? .0039 to? And what is the wear limit for piston to wall clearance?

1st is SBT look at the bottom where it days 720 the reading is there

The 2nd one is the manual the wear limit says na for new parts wear limit is 0.08

 

[burtshaver2021]

Oh, and don’t forget to get those ports chamfered first. I’ve heard of people filling them themselves but I’ve never done it so..... if you do, just be careful not to damage the cylinders.

 

[burtshaver2021]

Once you get them back together, rotate the engine with the cylinder cover off a few times just to be sure everything is turning smoothly and don’t forget to oil up the walls and everything else with 2 stroke oil.

 

[burtshaver2021]

Best of luck, I know this has been a long journey for you with this machine. Hoping you get to enjoy it soon

 

[justinb]

Best of luck, I know this has been a long journey for you with this machine. Hoping you get to enjoy it soon

New rings will take about 10 days to come will check ring gap then and do as u say

Hopefully issue resolved ,yet still think I have an electrical issue lol