Page 1 of 2 1 2 LastLast
Results 1 to 10 of 11
  1. #1
    Zeke says hi! BLK MAJK's Avatar
    Join Date
    Mar 2009
    Location
    Haskins, OH
    Posts
    112

    WEIRD Squish Readings...

    So I decided to check my squish today on my 96 XP because I am looking at milling the head. I got some really weird numbers and wanted to see if anyone has an explanation or any ideas...All were checked multiple times with solder through the spark plug hole, one side of the piston at a time. I looked through the rave to make sure I was in line with the wrist pin....onto the numbers

    PTO
    front of piston (towards front of ski) - .061, .062
    rear of piston - .076, .075

    MAG

    Front of piston (towards front of ski) - .063, .062
    rear of piston - .071, .071

    Those numbers have me very confused....I can't imagine it is normal for that much variation between the front and rear of the same piston. During the build, the exhaust manifold was put on first to ensure proper alignment of jugs. Any ideas on why this would happen and how to fix it?? Maybe a different/better way to measure? 96 XP, fresh rebuild, Pro-K's and rejetted carbs, no other engine mods



    EDIT: was just doing some reading....my engine was REALLY hard to turn over with the solder in there, and I read a page stating that if the solder was too thick, you would get weird measurements....sound right? I will have to check again today after getting more solder.


  2. #2
    Lake Mead Bum & BTLS Mark starflight's Avatar
    Join Date
    May 2007
    Location
    Boulder City, NV
    Posts
    3,364
    +1
    1
    From Graham Bells Performance Tuning guide:

    To find accurately what the squish clearance figure is, the barrel must be tensioned
    down on a standard thickness base gasket. Clean all traces of carbon from the head and
    piston. Place a strip of modelling clay 20mm wide by 3mm thick across the piston crown.
    TABLE 2.1 Minimum squish clearances
    Cylinder size (cc) Clearance (mm)
    50-80 0.6-0.8
    100-125 0.7-0.9
    175-250 1.0-1.4
    300-500 1.1-1.5
    Fit the head gasket and head and turn the crank to move the piston just past
    TDC. Remove the head and then cut the clay down the middle with a sharp, wet knife.
    Carefully pull one strip of clay off the piston and then measure the thickness of the clay
    left on the piston. You have to be accurate, so use the end of your vernier calipers. As a
    cross-check also measure the clay thickness on the other side of the piston. If the
    thicknesses vary this would indicate that the head gasket surface has been machined on
    a different plane to that of the combustion chamber. Also at this time measure and
    record, for future reference, the compressed thickness of the base gasket and head
    gasket. (FIGURE 2.3).
    After the clay thickness is measured you can work out how far the head must be
    machined to give the desired squish clearance. As mentioned previously, the combustion
    chamber must also be machined deeper into the head to keep the compression ratio at
    an acceptable level. If you wish to keep the compression ratio the same as standard, the
    combustion chamber will have to be machined twice as deep as the amount skimmed
    off to reduce the squish clearance, assuming a 50% squish band. Therefore if 0.9mm is
    removed, the combustion chamber will have to be made 1.8mm deeper. A 50% squish
    band is one having an area equal to half the cylinder bore area ie. an engine with a
    54mm bore would have a squish band approximately 8mm wide (FIGURE 2.4).
    To check that the machine shop recuts the combustion chamber to the original
    contour when it is deepened, you will have to make a template of the chamber shape
    before you send the head off. The template can be made out of any light gauge metal or
    even stiff cardboard. (FIGURE 2.5).
    Most people like to see the compression ratio pushed up as high as possible. High
    compression has always been equated with high horsepower. I agree that the
    compression ratio should be made as high as practicable, but often the manufacturer
    has already found the limit and built his engines accordingly. All you can do in this
    instance is check that production tolerances have not lowered the ratio significantly
    below that which the manufacturer intended.
    Something you must always remember when dealing with two-stroke engines is
    that increasing the compression ratio will not give a power gain equivalent to that
    which you would pick up with a four-stroke engine.

  3. #3
    Lake Mead Bum & BTLS Mark starflight's Avatar
    Join Date
    May 2007
    Location
    Boulder City, NV
    Posts
    3,364
    +1
    1
    Here's the whole guide. It's old school, but alot of the info is still valid.

  4. #4
    Lake Mead Bum & BTLS Mark starflight's Avatar
    Join Date
    May 2007
    Location
    Boulder City, NV
    Posts
    3,364
    +1
    1
    Here it is...
    Attached Files Attached Files

  5. #5
    96XPSS's Avatar
    Join Date
    Feb 2006
    Location
    Warner Robins, GA Home of the 2007 All-Star Little League World Champions!
    Posts
    4,361
    First of all, you don't check squish one side at a time...because of piston rock.

    So, install two pieces of solder at the same time accross the pin area of the piston and post it up.

  6. #6
    Zeke says hi! BLK MAJK's Avatar
    Join Date
    Mar 2009
    Location
    Haskins, OH
    Posts
    112
    Quote Originally Posted by 96XPSS View Post
    First of all, you don't check squish one side at a time...because of piston rock.

    So, install two pieces of solder at the same time accross the pin area of the piston and post it up.
    I got it figured out with some help from a member on PWCyesterday. He mentioned that the head bolts don't actually center the head on the bores. So sure enough, I came home and it was shifted to the rear by about 0.025". I loosened up the head bolts, shifted it forward, torqued it back down, and now my measurements make sense! EDIT: I also forgot to mention that I used 2 pieces of solder at the same time to avoid piston rock. I think I am going to pull the head bolts out tomorrow and give them a good cleaning and retorque them. It seemed like as I was tightening them they all got to like 25 lb/ft then just wanted to keep turning like they were about to strip out. I didn't keep torquing cause it kind of freaked me out, but I really don't think I could have ALL of my head bolts strip at the same time

    Anyway, here are the new numbers

    PTO - 0.061"
    MAG - 0.058"

    This is with a 6 hole base gasket. Also checked compression. 156 psi in both holes...Dumonde Tech must work, as I was only expecting 150! That seems like what most people get after a rebuild. Glad I took your advice! I think it's time for some more compression Any suggestions on what route to take? I found a buckshot head with 43cc domes that should be pump gas safe or have my stock head milled by PPG, or the local auto machine shop (not too sure about this option). Cost difference between the buckshot and PPG milled stocker would be about $20. I have 1 concern with both options

    With the PPG modified stocker - do I let them just mill to their specs, or should I give them how much to take off (numbers of which I have NO idea)
    With the buckshot - I think the domes are rated at 180 psi. If I am already at 156, will that actually put me to 186? Isn't that getting out of range for pump gas?

  7. #7
    good job

  8. #8
    96XPSS's Avatar
    Join Date
    Feb 2006
    Location
    Warner Robins, GA Home of the 2007 All-Star Little League World Champions!
    Posts
    4,361
    Whatever. Good luck taking shortcuts and doing what you've been told isn't right.

  9. #9
    Zeke says hi! BLK MAJK's Avatar
    Join Date
    Mar 2009
    Location
    Haskins, OH
    Posts
    112
    Quote Originally Posted by 96XPSS View Post
    Whatever. Good luck taking shortcuts and doing what you've been told isn't right.
    I'm confused by that statement. Sorry, I also didn't mention that I did use 2 pieces of solder this time. It definitely made a difference, so you were right also, sorry I didnt mention it. I did end up having to shift the head too because it was off center. Didn't mean to offend you, I was actaully trying to say a big thank you for recommending the Dumonde Tech. I have always taken your advice because you are one of the people who I know this better than alot of people.

  10. #10
    96XPSS's Avatar
    Join Date
    Feb 2006
    Location
    Warner Robins, GA Home of the 2007 All-Star Little League World Champions!
    Posts
    4,361
    Quote Originally Posted by BLK MAJK View Post
    I'm confused by that statement. Sorry, I also didn't mention that I did use 2 pieces of solder this time. It definitely made a difference, so you were right also, sorry I didnt mention it. I did end up having to shift the head too because it was off center. Didn't mean to offend you, I was actaully trying to say a big thank you for recommending the Dumonde Tech. I have always taken your advice because you are one of the people who I know this better than alot of people.
    Thanks man, and thanks for the PM. You can see that you never mentioned using two pieces of solder. If you did, the measurements would have been something like .062, .058, .063, .059, front to back. This would have told you that you need to loosen the head and shift it slightly rearward and retorque. Then rechecking the squish, it would have come up something like .060, .060, .061, and .060, front to back. This was why I was trying to get you to use two, so you could get it real close to perfectly centered.

    And I'm glad you got the same results I did using the Dumonde Tech. It should also perform much better. When you get it dialed let us know what she's turning, I'd be interested in knowing.

    Congrats on a job well done!

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. GPr 1200 Squish reading ?
    By dewzee1200 in forum Yamaha How To & FAQs
    Replies: 1
    Last Post: 03-10-2016, 05:05 PM
  2. 1300 hull crack...need advice please read
    By RXPNJ in forum 4-Tec Performance
    Replies: 3
    Last Post: 06-28-2005, 10:30 PM
  3. ROCX Members, read this!
    By Thomas in forum Sea Doo Open Discussion
    Replies: 2
    Last Post: 06-16-2005, 04:43 PM
  4. Every 4 Tec owner should read this!
    By MikeTrin in forum Sea Doo Open Discussion
    Replies: 0
    Last Post: 05-16-2005, 12:41 PM
  5. EVERYONE READ THIS ASAP !!!!!
    By CRAZEDRXP in forum Sea Doo Open Discussion
    Replies: 2
    Last Post: 05-11-2005, 01:09 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
  •