787 bore specs.

[92rslt1]

Hi guys, Trying gather some data in regards to my 787. I got the ski as part of a package deal, it ran but wasn't winterized so the rings set up. I removed the old pistons, chucked them in the trash and am trying to measure the bore. I have the pdf shop manual, but cant seem to find the out of bore specs. I'm also getting bazaar readings.

PO said it began to foul plugs so I suspect it needs a bore.. Not sure If I want to spend a ton of money on it or just sent it to the parts pile.

I ran a ball hone through both for a second to clean them of any debris and here is the measurements I am getting.

Magneto cylinder
top 81.23
bottom 81.51

PTO cylinder
Top 81.4
bottom 81.26

They seem to have worn in opposite fashion for some reason.

There is no gouges or scratches, so if they are usable I would like to reuse them as is, but it seems like they might be worn judging by the numbers.

 

[racerxxx]

What are you measuring with?

 

[92rslt1]

I have bore indicators, but just did a quick check with my calipers, as they usually yield a close approximation.

 

[92rslt1]

Ok I'm getting 88.8896 on one and 88.9 on the other at the top.
Bottom 81.76 on one bottom, 81.83 on the other.

That's with the bore tool.

 

[racerxxx]

Yeah, I'm gonna say something ain't right.

Bore taper should only be 0.05mm (0.002") on a new bore, wear limit should be 0.1mm (0.004")

Cylinder out of round should be 0.008mm (0.0003") New, wear limit 0.08mm (0.003")

787 piston to Cylinder wall clearance should be 0.10-0.012mm (0.004-0.005") new, wear limit is 0.15mm (0.006") I have my cylinders bored to 0.0045" wall clearance.


Your numbers are just wacked out. You're measuring 9/32" difference (0.281"), that's over a 1/4" of difference. That thing wouldn't even run with those numbers. Now I have a set of first over pistons (new never fired) and they measure 82.14mm with a mic and my fresh never fired bore measures 82.23 (0.010mm clearance) with calipers-those are close numbers. A standard bore piston is 81.89mm, so that bore should measure 81.99mm+

If you're tools are digital I would change the battery, when they get old they get wacky.

 

[92rslt1]

Yeah, I'm gonna say something ain't right.

Bore taper should only be 0.05mm (0.002") on a new bore, wear limit should be 0.1mm (0.004")

Cylinder out of round should be 0.008mm (0.0003") New, wear limit 0.08mm (0.003")

787 piston to Cylinder wall clearance should be 0.10-0.012mm (0.004-0.005") new, wear limit is 0.15mm (0.006") I have my cylinders bored to 0.0045" wall clearance.


Your numbers are just wacked out. You're measuring 9/32" difference (0.281"), that's over a 1/4" of difference. That thing wouldn't even run with those numbers. Now I have a set of first over pistons (new never fired) and they measure 82.14mm with a mic and my fresh never fired bore measures 82.23 (0.010mm clearance) with calipers-those are close numbers. A standard bore piston is 81.89mm, so that bore should measure 81.99mm+

If you're tools are digital I would change the battery, when they get old they get wacky.

I typed that incorrectly, sorry about that.

top is 81.8896 mm
bot is 81.76 mm
----------------------
.1296 mm
.005 inches

top is 81.9
bot is 81.83
-----------------------
.07 mm
. 003 rounded up in inches

The numbers still look odd though, I measured with a fresh battery too, about 4 times each. I will try a friends tool, or run them to the shop tomorrow.

 

[racerxxx]

There are specific locations to measure the bore called out in the service manual, I would go by those guide lines. Premium members have access to the manuals or just use google if you don't already have the manual.

But your bore by your numbers is still a standard bore. Next time, don't throw out anything until the project is completed. Ya never know what you will need for a reference. Wait.......I never throw anything out. LOL

 

[92rslt1]

Ok second question.

The previous owner said he had to change the plugs each time he took it out.

Is there anyway to pressurize the crank case to make sure the crank seals aren't leaking into the bottom end?

What solvent would you use to clean everything up, I usually tear engines completely down and use mineral spirits and replace all the seal so I usually don't worry about them. Given this likely will be a topend and carb rebuild only, I don't want to damage any seals with cleaner.