Page 1 of 2 1 2 LastLast
Results 1 to 10 of 16
  1. #1

    Top End Rebuild - compression ring seating question


    I've just completed a top end rebuild on my SLT 780.

    It's an SBT rebuild kit. I've had great success with previous cylinder kits from them.

    Right out of the box I'm hitting 118 PSI on all three cylinders and haven’t fired it up yet. This seems a little low.

    I know that I can expect a ramp-up in compression after the rings seat properly. But, is it reasonable to expect an increase from 118 to 140-145ish?

    Is achieving that much increase in compression a stretch? And, if it is..does anyone have any suggestions on how I might get the compression up?



    Any input would be appreciated.

    Bobby




  2. #2
    burtonrider10022's Avatar
    Join Date
    Jul 2012
    Location
    Orland Park, IL
    Posts
    396
    +1
    13
    Does the 780 run @ higher compression than the 650/750 Fuji's? I was under the impression that compression was usually around 120 PSI for seated, in tact pistons.

  3. #3
    I'm assuming it does because my last rebuilds in my other two SLT 780's the compressions are running 145 to 150s.

    However, on my last rebuild I never checked the compression on the rebuild until AFTER break-in. On this rebuild the compression seems very low right out of the box. And, if it is..might you have any suggestions on how I might get the compression up so as not to have to go through all the trials and errors in order to hit my target after break-in.

    Maybe larger rings?

    Thanks for the help

  4. #4
    BlueFishCrisis's Avatar
    Join Date
    Aug 2011
    Location
    Medina, Ohio, United States
    Posts
    4,988
    +1
    140
    Compression is directly related to multiple variables. How does your cylinder bore compare to factory? Is the bore out of round at all? What is your piston to cylinder clearance? What is the end gap clearance of your rings? What is your squish?

    All of these can raise/lower compression ratings.

    I don't think you sill see an increase from 118 to 140 psi by simply breaking the rings in.

    There have been recent posts about wrong pistons, new rings shipped out of spec, etc that may prompt you to check all of the above instead of simply taking it for granted that you have been shipped all components within spec.....

  5. #5
    ujustlost's Avatar
    Join Date
    Oct 2006
    Location
    Howell Michigan
    Posts
    1,597
    +1
    6
    I would start with something a little more simple like are you doing the compression test right and is your gauge accurate. 118 does seem a bit low, however I have never seen comps in the 140-150's on Polaris triples from a fresh stock rebuild. Those are numbers you generally see on seadoo twins but not stock triples. If it were me I would like to see it between 125-135. People use compression numbers to help them determine if the engine is in good condition or not, in your case we know everything is new on the top end but what about the lower end? Original crank ? What happened to the ski that you had to rebuild the top end? If everything else end up being good on the ski and you still would like more compression I would split the head gaskets, that should give you another 2-4 psi

  6. #6
    Thanks for your help.

    SBT specs are shipped .5mm over factory (Cylinder Exchange Top-End Kit) I Checked before I installed. And, they checked before they put them in my car as this is the second time I've installed the set on the same machine in less than a week.

    I returned the first set for the same reason AFTER break in. (SBT lived up to their warranty expectations and took good care of me) However, I don't want to keep returning top end kits they'll think I'm an idiot lol and it's not likely to solve the problem. Piston clearance is identical to my other machince with the same kits in them.

    I've purchaed the same top end kit's from them before ... no problems..got lucky I guess

    What would happen if I bumped the ring size to 1.mm over?

  7. #7
    Thanks, Yes, my gauges are new and accurate as I compare them with my other rebuilds with the same SBT top-end kits.

    It was rebuilt simply because of a holed piston (my son thought he knew more about how to make it run faster than the carb manufacturer) Anyway, I figured why not just do a top end rebuild while I'm at it because compression was running in the low 120's on the other two.

    Would bumping the ring size up a notch help?

  8. #8
    Quote Originally Posted by ujustlost View Post
    I would start with something a little more simple like are you doing the compression test right and is your gauge accurate. 118 does seem a bit low, however I have never seen comps in the 140-150's on Polaris triples from a fresh stock rebuild. Those are numbers you generally see on seadoo twins but not stock triples. If it were me I would like to see it between 125-135. People use compression numbers to help them determine if the engine is in good condition or not, in your case we know everything is new on the top end but what about the lower end? Original crank ? What happened to the ski that you had to rebuild the top end? If everything else end up being good on the ski and you still would like more compression I would split the head gaskets, that should give you another 2-4 psi
    Thanks, Yes, my gauges are new and accurate as I compare them with my other rebuilds with the same SBT top-end kits.

    It was rebuilt simply because of a holed piston (my son thought he knew more about how to make it run faster than the carb manufacturer) Anyway, I figured why not just do a top end rebuild while I'm at it because compression was running in the low 120's on the other two.

    Would bumping the ring size up a notch help?

  9. #9
    Quote Originally Posted by polaris780slt View Post
    Thanks, Yes, my gauges are new and accurate as I compare them with my other rebuilds with the same SBT top-end kits.

    It was rebuilt simply because of a holed piston (my son thought he knew more about how to make it run faster than the carb manufacturer) Anyway, I figured why not just do a top end rebuild while I'm at it because compression was running in the low 120's on the other two.

    Would bumping the ring size up a notch help?
    Forgot to mention that piston clearance is identical to my other machines with the same kits in them. So, it's not likely to be issues with the crank.

    Thanks for your thoughts

  10. #10
    Let me recap

    I've just completed a top end rebuild on my SLT 780. It's an SBT rebuild kit. I've had great success with previous cylinder kits from them.

    I've installed the second set on the same machine in less than a week. For the second time right out of the box I'm hitting 118 PSI on all three cylinders and I haven’t even fired it up yet.

    This seems a little low. I returned the first set for the same low compression reason AFTER break in. (SBT lived up to their warranty expectations and they took very good care of me)

    However, I don't want to keep returning top end kits they'll think I'm an idiot and it's not likely to solve the problem.

    I know that I can expect a ramp-up in compression after the rings seat properly. But, is it reasonable to expect an increase from 118 to 140-145ish? I’d settle for 125-135 psi after break-in.

    Is achieving that much increase in compression a stretch? And, if it is..does anyone have any suggestions on how I might get the compression up?

    The ski won’t even get out of the water with these numbers which makes the task of break-in less than optimal.

    SBT specs are shipped .5mm over factory (Cylinder Exchange Top-End Kit) I Checked before I installed. And, they checked before they put them in my trunk as this is the second time

    Piston clearance is identical to my other machines with the same kits in them. So, it's not likely to be issues with the crank

    My gauges are new and accurate as I compare them with my other rebuilds with the same SBT top-end kits. (have video to demonstrate proof)

    What would happen if I bumped the ring size to 1.mm over?

    Any input would be appreciated.

    Thanks
    Bobby

Page 1 of 2 1 2 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Top End Rebuild Compression Issues (02 GP1200R)
    By Johnny5 in forum Yamaha PWC Performance (2-stroke)
    Replies: 11
    Last Post: 11-04-2010, 11:18 PM
  2. Yamaha GP 800 top end rebuild questions
    By koryk in forum Yamaha How To & FAQs
    Replies: 2
    Last Post: 05-21-2010, 10:19 PM
  3. Newbie: 787 Top end rebuild question
    By kamikaze in forum 2-Stroke Performance
    Replies: 3
    Last Post: 02-12-2010, 07:34 AM
  4. few questions about top end rebuild kits for 1050
    By rbridges in forum Polaris PWC Performance
    Replies: 21
    Last Post: 03-21-2009, 12:23 PM
  5. Top end rebuild question
    By VegasRatt in forum Polaris Open Discussion
    Replies: 3
    Last Post: 02-17-2008, 11:01 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •