787 Acceptable Rebuild Compression

[lilngineer]

I have a 96 787 that I rebuilt top and bottom.

I first built the motor with the same 82.25mm cylinder jugs (re-honed) and new pistons, rings, pins, bearings, & crank.

I got about 5hrs into the break-in and I found compression was only 125/140. It ran ok, but I figured I would repair the top end while I was ahead.

Today, I swapped my jugs for a new set machined to the same .25mm over. I did reuse my 5hr pistons/rings. I checked compression cold before starting and found it to be 140/~152. I also did not have the throttle wide open when testing.

Is this acceptable for a top end rebuild? I find it hard to believe my previous 5hrs wore down the rings to account for the pressure difference.

Could this be because the pistons/cylinders were not specifically matched? How important is this in reality?


I appreciate any help/insight you can provide. Not too crazy about ripping it apart yet again.

 

[glenwhitt]

Must do check at WOT or you will get some low readings. Turn it over till the gauge stops going up. do it three times per cyl to get a good average. Drop a little oil in there also and try it to see if things change a bunch.

Did you measer squish on the head/piston? was it in spec? If so you should have about 150 in each cylinder.

Why did you swap jugs and not rings? Was there damage to the jugs?

How are you breaking in the top end? I have been told by many that if you baby it the rings will not seet right. I did mine with a quick warm up and then ran pretty hard for the first 15 minutes with WOT, 3/4 and some 1/2 throttle mixed in there. I have a good 15 hours on the new top now and still have 150/150. Checked them after the lake run on last Sunday night.

 

[lilngineer]

Glen,

I swapped jugs because my first set were near the wear limit, I assume because of the bottom end failure I had. I took a chance with them, but once I checked compression during the break-in, I decided it was best to pull them.

Trying to avoid boring and buying the next size over pistons/rings (.5mm) (and also scrapping my newly purchased pistons (.25mm)), I worked out a cylinder exchange to stay at .25mm. I figured since they were only ran for about 5hrs they had to be ok.

The break-in guide I followed was on SBTs site. Started with idle, then idle to 20%, then up to 50%, then 90%, etc.

Right now I am using the same size 0.6mm base gasket as was originally in the engine before I rebuilt. I wanted to check squish, but was having a hard time getting the solder into the edges of the dome through the spark plug hole. Is there an easier way? What diameter solder do I need? The service manual doesn't spec it.


Thanks,

Jason

 

[lilngineer]

Also, just to clarify...

I have not began to break-in yet since changing the jugs. I only started briefly.

 

[glenwhitt]

Have you done compression check at WOT yet? I would start there and make sure they are even should be at least 145 or more with new top end. After the rings set you may get few pounds more out of them.

I had some readings that were low and not even like yours so I pulled the head off to inspect after just a few hours on it. I could see no damage. I then put it back together with new rubber gaskets and did check with carbs off. I got 150 in each.

I do not remeber the size of solder I used. I know not to use the stuff filled with rosin or something like that. Use solid core. The size i used was listed in a seadoo racing manual. Just make a big L and adjust so it is hitting the side. After that just turn the motor over by hand.

 

[glenwhitt]

you can also lay a piece of solder in the bottom of the cyl on top of the pistons, bolt head on, turn motor over by hand, remove head, measure for squish band.

 

[Dr Honda]

on a fresh rebuild, you will have low compression until the rings seat... but 5 hours is not a fresh rebuild.

also... do not put oil in the jugs to check compression. it will give you a false, high reading.

 

[lilngineer]

Ok, I checked again with the throttle open. I got 140/~146.

Still seems a little low, but maybe because it hasn't really been ran yet?

I am thinking I may pull the head and change the orings just to see if that may be the problem.

 

[Dr Honda]

Ok, I checked again with the throttle open. I got 140/~146.

Still seems a little low, but maybe because it hasn't really been ran yet?

I am thinking I may pull the head and change the orings just to see if that may be the problem.

No reason to pull the heads. 140's are fine for a fresh engine. The rigs will seat over the next few tanks of fuel, and come up to 150. (as long as they were honed properly)

 

[seadoo GTS 1993]

I just looked at your location, im in Sterling heights.. we live about 15-20 mins from eachother....

Anyway, i think 140-146 is fine, Take the ski out again and do the break in procedure, and see if it goes up

 

[lilngineer]

Squish gap

Well I got a chance to check the squish tonight. I used a 1.6mm rosin core solder and could not get any squish out of it. I even tried pulling the head and taping the solder to the top of the piston at TDC and retorquing the head.

I am currently using a 0.6mm base gasket and I think the squish is a tad larger than the upper limit. Do you think I should change to a 0.3mm to put me in spec? I believe 0.3 is the thinnest gasket available.

Also, I noticed when pulling the jugs that there were only small puddles of oil in the bottom of each of the rod cavities of the crankcase. How much should be down there? Keep in mind the bottom end was ran for approximately 5hrs.

 

[glenwhitt]

use solid cor. I read if you use rosin core then it will get some on the pisting and that is not good.

If you put this in the middle of the piston then you will never get any squish. These are domed heads so you have to place the solder all the way on the edge were there is a flat surface on the head.

 

[lilngineer]

use solid cor. I read if you use rosin core then it will get some on the pisting and that is not good.

If you put this in the middle of the piston then you will never get any squish. These are domed heads so you have to place the solder all the way on the edge were there is a flat surface on the head.

The service manual does state to use a cored solder, just not acid core for the reason you mentioned.

When I did the squish, I had the solder laid across the piston end to end to get to the flats on each side of the dome. I also performed the test with the head installed.

Any comments on how much oil should be in the case?