Thread: 1992 750SS top end rebuild
07-27-2008, 11:32 AM #1
- Join Date
- Jul 2008
1992 750SS top end rebuild
Friend's 750 SS ...
Number 2 piston and rings shot - rings welded into piston with heavy scoring on cylinder, number 1 piston and rings fine with no scoring.
I am somewhat experienced at car and chainsaw engines (but not a pro), so most of this is pretty straightforward - couple of questions at items I could use some help on:
1. I expected to see scoring at the exhaust port, but was actually at the very back (90 degrees away from exhaust port) - consistent with failures on these engines or should I look for secondary issue?
2. The #2 crank counterweight (rear most) had "hammering" marks (like someone had beat on it with a hammer and implement) on it near the smooth portions, and these smooth portions were shiny, vice being duller like the front crank weights ... I am sure the engine has been apart before, but can't figure out what would cause these marks ... any ideas?
3. Rebore only the bad cylinder or both? Had one mechanic say yes and another no ... 10-30 thousand shouldn't matter much, but since this is an old machine, cost vs value is always at stake!
4. Best place to get machine shop work done in northern Virginia or via shipping? I am in Fairfax County and my friend uses the ski in Northern Neck area of Virginia.
5. Do the top end gasket kits have everything needed? Where's best place to get top end kit (and pistons) from?
6. Any special tips for reassembly? I noted the case to cylinder assembly gasket had sealant between the cylinders, but not around the perimeter ... any harm in light RTV coating everywhere on gaskets? Which color/kind does everyone like?
7. Headgasket had a rubber coating on it that was pretty messed up - could this have been failure mechanism to cause the damage to #2 piston and cylinder?
Thanks in advance - cool engine design and easy to work on so far - if this is successful, think I'll look around for a blown engine and get it myself!
BTW - friend called one sales and service place in our area and was told they don't work on machine older than 10 years any longer!
12-23-2009, 10:32 AM #2
Ok talk to John Zigler about parts he is a VERY trustworthy person and he ships parts fast!
The number 1 reason for piston melt down is the carbs are running lean causing over heating then in a matter of minutes you can melt a piston EVEN A NEW ONE
You will want to bore both cylinders because with one bieng heavier than the other it can cause bad vibrations and tear more up
you will want.
1 Top End Rebuild Kit that has all the gaskets for the crankshaft up
2 Carburator Rebuild Kits
2 pistons .5 mm over
heres everything besides carb rebuild kit you need just tell your machinest to bore .5 mm oversize OR half a millimeter OVER SIZE
The kit above has pistons gaskets everything you will need for a TOP END REBUILD
get the crankshaft polished and hopefully it will remove those marks
Heres carb rebuild kits
Buy one or two depending on if you have dual or single carbs
12-23-2009, 06:19 PM #3
Did the same thing to mine:
I got away with just a hone. but if there is scoring, ask the machine shop to verify the depth of he scores before you buy the pistons. the have them match the teh pistons to teh jugs. If you ask around at the local bike shops to see who they use.
T-Hoff here in Raleigh has done a few for me and did a good job. These engines are very similar to old VW air cooed engines. so look fro a vw restorer and see what they do for their jugs.
Definately rebuild the carb and I would recommend:
Switch to premix
remove and block off the case drain
post pics, we like pics
12-23-2009, 06:27 PM #4
pto cylinder: could be a rear crank seal that casued your lean condition, or a cooling issue.
Since you have a single carb, both cylinders get the same amount of fuel in each stroke, so both piston crowns should look the same
Post a pic so we can see the wash pattern
12-23-2009, 10:35 PM #5
Pix pix pix!!!
09-16-2011, 08:57 AM #6
- Join Date
- Sep 2011
Hey guys, sorry this is a desperately old thead but I'm having trouble finding good information on somebody that has completed a top end rebuild on a 750 SS recently. My ski has done the exact same thing, 2nd cylinder not seized but dropped compression to 40psi while the 1st cylinder still exhibits 160psi ish. The stop occurred when I was having a drag with a mate across a long channel so top of the rev range held way.. too long probably just ran lean and burnt the piston out big time. The bore is scored but not deeply and rather than in a particular spot its the entire circumference of the bore. Anything else I should look at besides a top end rebuild? The bottom end seems pretty tight and the cranks look fine. Any help would be much appreciated. Here are two photo's of each cylinder bore. (the scored bore looks a lot worse in the pictures trust me...)
09-16-2011, 05:59 PM #7
Weird patterns on the piston crowns. There are usually black deposits, I'd suspect to lean.
These things should be able to run all day at full throttle.
09-17-2011, 02:52 AM #8
- Join Date
- Sep 2011
Hah sorry thats exactly what my dad said when I showed him the pictures, ''They're funny looking piston crowns.. they look more like concrete than anything else.." Strangely enough he was right its not actually looking at the top of the pistons, thats the concrete that the block is sitting on. the only thing showing is the bore photographed from the crank side of things looking up the bore. Any more thoughts? One piston looks fantastic nice and shiny and smooth, the other has full circumference wear.
09-17-2011, 11:40 AM #9
Ha! Well you sure had me fooled!
Got pictures of the piston? Has the top ring land broken off? Did the ring(s) break? Are the rings aligned with the pins? What did the spark plug look like?
This info would sure help.
09-18-2011, 03:45 AM #10
- Join Date
- Sep 2011
Ah no strangely enough there was little damage to either of the rings on the siezed piston. I was looking at pulling the whole block out of the ski as I've only got the barrels out so far, and I discovered that the oil feed off the stator cover to the carby was bone dry. I would have thought that oil should stay up in the hose without flowing backwards through the oil pump..? Real question is, is it possible for a single cylinder to seize due to an oil feed failure on a single carb engine? I'm hoping its not a crank seal that ran one piston lean, I don't think it is because both spark plugs prior to the seizing looked more or less the same. If one were running lean I would have noticed it before. I've had a look around but are there any good top end/bottom end rebuild sticky threads on this engine? or possibly a reassembly guide?
Here are some photographs of the offending piston and its corresponding spark plug. The flash somewhat washes out the colour on the ceramic but it looks like a light straw colour, Its slightly lighter in colour to the other cylinder but not drastically so.
And here is a picture of the piston crown of the offending piston, they both look exactly the same with the same amount of carbon build up on each crown.
Thanks for all the help, I'll make sure I post plenty of pictures when the ski is all back together and top end (hopefully all) is rebuilt.
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