Setting up squish - 787?

[trxxx250r]

I have an spx and I rebuilt the motor. After break in I was only able to attain 120 psi comp. So I tore it back down to see if I could determine what the problem was and re-ring it. It's bone stock but I went with WSM 1st over pistons. They now have maybe 1.5 hours on them. I was running a 4 hole base gasket.

That said this post isn't about the previous rebuild and what I may have done wrong or the present re-ring which I will be doing everything wright.

I want to ask some questions about squish. It's a very important measurement and I know the benefits of a proper or tight squish from my land toy motors I've built.

So, I want to test for squish clearance. I pulled the cylinders and pistons, removed the rings, looked every thing over, then I bolted the cylinders w/pistons back on with no base gasket. I am attempting a lead test to get a clear measurement. I glued a 3/8" piece of .125 lead to both sides of the piston against the cylinder wall and directly over the wrist pin. I then bolted the head on and turned the motor over hard by hand until it hit tdc and then rotated past. I removed the head and WOW, it worked better than I thought! All lead was between .059 - .061 I am disappointed at the thickness of the clearance but the test worked great!

Check the pics -



Ok, I really want a clearance of .045. I'm at about .060 with no base gasket. I have a 3 hole that's supposed to be .012 but it actually measures .010. So, my final squish is going to be around .071 - .073.

Questions:

Are these WSM pistons that far out of spec with the OEM pistons?
What squish clearance do these motors typically come out of the factory with? I believe this untouched motor originally had a 5 hole gasket in it
Will a squish clearance of .075 as opposed to .047 cause compression to be around 120 psi? when it should be 150
I realize tighter squish will increase compression but how much?
If I had a squish of around .045 would that raise compression to where I couldn't run 87 octane?
Actually, what is the limit in terms of compression where I can't run 87 anymore?
Obviously something's wrong if I'm using a .010 base gasket and still at .073 total squish?
Best way to increase squish? Deck top of cyl or flycut bottom of cylinder?
I see the squish band on these heads are slightly recessed into the head, so milling the head in this case, at least till it's flush would increase squish, yes? Or am I wrong? Typically you deck or flycut to alter squish clearance.
Is it better with these motors to cut the cyl or mill the head to take say .025 off???
Lastly and most important - Is there really going to be any noticeable difference between .047 and .075 clearance on this particular stock motor?

I'm sure I will have more questions that I'm forgetting but hopefully they will be answered in the course of this discussion

Thanks!

 

[trxxx250r]

Btw - I need this information as soon as I can get it because I'm not putting this motor back together till i have all the answers and I know it's going to be 100% right. But, everyone's pushing me to get out on the lake with them ASAP...

 

[Dr Honda]

Well..............


OK, squash. In a Rotax engine... the squash band is a critical measurement. (as you know) but it's more than just a way to check clearance of the combustion chamber. If you look at the head, you will see the squash area, and the main chamber in the middle. The squash area accelerates the raw (cool) air/fuel mix into the center, where the piston can better handle the heat and pressure. It also sets the ignition for a centralized burn, and push.

If the squash is too large... you will overheat the top ring, and the lip on the piston will fail. (and you loose performance)

If it's too tight... you could contact the head when things expand, or the static compression can be high enough that you will get detonation.

If you are at 60 thou, without a base gasket... then those pistons were made wrong. I tell most people that using a 4 hole is safe... and it normally gets you very close to spec.


OK... your test method is very interesting... but solder is normally hollow. (with a rosin core) SO... when you pulled it up from the glue... could it have opened some? Also... what was the glue? If it was something like RTV, it could compress, and spring back, giving you a false "Squash".

Personally, I would run the test the normal way. Reach in the sparkplug hole, and roll one point at a time.


To answer your compression question... not knowing what gasket you had prior to the test... but if you take 0.030" out of the deck... then yes... the static compression can jump from 120 to 150 without an issue.

 

[Dr Honda]

I was just sitting here, and I had a little spark of memory......


The 720 and 800 pistons are very close to each other. Same bore, and writs pin is close. I'm wondering if those are 720 pistons? I know the part number stamped on them is right... but they could have had a mix up at the factory?

There was a thread a couple years ago... and we found this to be the issue. The engine would start/run, but had poor performance... and the compression was low. (around 120)


You may want to contact WSM and ask what the wrist pin height is supposed to be on both engines... and then measure it yourself.

 

[trxxx250r]

Thanks Dr. Honda, In my post I did say I ran a 4 hole previously when I was getting the 120 psi.

The glue was a just a rubber cement and it wasn't even dry, just enough to hold the lead in place, there's no way that altered the lead.

I had already planned on putting a call in to WSM today at 9:30 to talk to someone in tech but thanks for mentioning that!

 

[trxxx250r]

In case anyone is wondering, here are some pics of my head -

 

[Dr Honda]

The head looks good to me.

Thanks Dr. Honda, In my post I did say I ran a 4 hole previously when I was getting the 120 psi.

OK... but when you did your lead squish test... did you have a base gasket in? I thought I read you didn't have one during the check.


OK on the glue.

 

[trxxx250r]

No base gasket, all metal to metal.

Call to WSM was a wast of time but I very closely compared the oem piston to the wsm and they are identicle except a very slight difference in the dome but that could just be my eyes. If anything there's a little more dome on the wsm.

 

[mandriva]

You use piece of .125 lead.... it,s way too big!!
You need size rosin core solder .65 for verify the squish correctly!!!

 

[mikidymac]

You use piece of .125 lead.... it,s way too big!!
You need size rosin core solder .65 for verify the squish correctly!!!

This thread is over 2 years old BTW.

 

[etemplet]

The old thread was interesting to me. I had the same issues with deck height when I built my first 787 a couple months back and it caused me some trepidation. My squish was .071" which I checked last instead of first LOL. I wasn't about the pull the top end over .008" and I figured WSM knew what they were doing. Compression check was 175. I'd give my higher number but it may have been a bit wet but I like em' tight. LOL Next top end I do, I'm gonna measure first then deck the cylinders.

 

[mikidymac]

The old thread was interesting to me. I had the same issues with deck height when I built my first 787 a couple months back and it caused me some trepidation. My squish was .071" which I checked last instead of first LOL. I wasn't about the pull the top end over .008" and I figured WSM knew what they were doing. Compression check was 175. I'd give my higher number but it may have been a bit wet but I like em' tight. LOL Next top end I do, I'm gonna measure first then deck the cylinders.

No reason to deck the cylinders. Seadoo makes about 5 different thicknesses of base gaskets to fine tune the squish. THe aftermarket gaskets only come with a 5 hole because this is the safe generic size and works 90%of the time. To do it correctly measure without a gasket then subtract to the desired squish to get the needed gasket thickness. You can gain some performance by doing it correctly.

Also, your compression SHOULD NOT be 175psi. Stock fresh is 150 psi.

 

[etemplet]

I may check the compression again for funzies. As I said the cylinders may have been a little too wet. I still think I'll get 170psi though. Either way, this ski is faster than the other one. It has some giddy-up.

The kit I purchased had around 4 gaskets all with the same number of holes. I didn't pay enough attention as I would have used the .015" instead of the .020". Perhaps the WSM piston dome has a little more dome.

To go for the shock and awe effect... the PTO cylinder pressure was 200psi on first check. LOL I had some oil in that one so I know it was a bit wet. Not sure how much oil makes a difference but dang !! I love working on this stuff.

 

[mikidymac]

At 200 psi you would have to run race gas to not destroy a piston.