787 squish test

[Black67tb]

Hello. Need some advice. I'm now in the process of doing the squish test on my 787 engine. I have new WSM pistons +1 mm oversize. What would be the measurment that l need to see on the solder squish test? I will be doing this without a base gasket. I am also using .065" solder? No flux core, correct?

 

[mikidymac]

0.965-1.067mm (0.038-0.042 in) for peak performance. So measure it without base gasket then figure out what base gasket will get you to the spec clearance.

If you don't care about max performance you can use the Seadoo Book clearances. 1.2-1.8mm (0.047-0.071")

 

[Black67tb]

Thanks mikidymac, l think l'll go with 1.2 - 1.8. My +1 mm oversize pistons would be ok with this?

 

[mikidymac]

Yes

 

[Black67tb]

Thanks again.

 

[Black67tb]

Yes

Hello again. Would like your opinion. Did the squish test, and l measured 2.15mm with no base gasket and no rings. I have a 6 hole gasket which is .6mm. Now my measurment of 2.15 subtract .6 is 1.55mm. Am l doing this correctly?

 

[mikidymac]

No, something is wrong.
At 2.15 you are already at too much squish with no gasket. So if you add .6 gasket you are even farther off at 2.75.

You have to put the rings on.
The solder has to be over the piston pin so you measure at the front and back ofe each cylinder, not on the exhaust side.

Finally you measure the thinnest part of the solder where it’s crimped between the head and piston.

 

[Black67tb]

Yes, l thought something was fishy. I did measure above the pins. Couldn't get any squish with .065 solder. Had to use .125 solder. I will install the rings and give it another shot.

 

[mikidymac]

I use thick plumbing solder from Home Depot. Oaty is the brand.

 

[Black67tb]

Well tried it again with the rings. Still 2.15. Maybe l should give you more history. This engine was an old SBT rebuild when l bought this ski. Broke the counterbalance shaft and cracked the case last summer. Decided to replace cases, new crank, rebuilt balance shaft, new 1mm oversize WSM pistons. Now the problem of my large squish test l don't get! Before the tear down l had a 5 hole gasket probably done by SBT. Today l looked at the underside of the head. Looks like the machining is not stock. The step down is deeper (1.65mm). I compared it to my neighbor's old stock head. It had a step of only .65mm. I'm thinking this is why l have such a large quish! Check it out.

 

[etemplet]

Yep I had a head like that on one of the engines. Sure explains your dilemma. That won't stop it from running.

 

[Black67tb]

So l can't use the lead test with this head. Is there another way to test what size of base gasket i need? Do l just use a 5 hole gasket like the one was in there before the tear down?

 

[mikidymac]

You found the issue.

SBT cuts the head to lower compression and my guess, to reduce the amount of warranty returns. This significantly reduces the compression and performance and is a really crappy way to do things.

If it was me I would get a clean used head and test squish.

If you just want to slap it together and run it then don't bother with squish just like SBT and put another 5 hole base gasket. This is the kind of stuff that gives SBT a bad reputation.

 

[etemplet]

As mentioned, I had one of these machined heads and it was my only good one so I installed it on a 787 RFI. My point was to determine the impact on the engine and they are easy to replace if I didn't like it. So after building I did a compression test on the engine. I got 150 PSI. After I break the engine in a bit, I'll take another compression reading.

 

[Black67tb]

Well, l want to thank you for your responces. I guess l have to make a choice. Also l would like to know the compression once the machine is broken in.
Thanks

 

[etemplet]

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