Top End Re-build

[KyleB]

Hello all, I am in the process of rebuilding the top end on my 96 XP. The engine that is currently in it is a SBT installed a few years ago. Will I need over sized pistons? What is the standard size pistons so when I measure I'll know what to order? Thanks.

Kyle

[oldbugger]

Re: Top End Re-build

Kyle,
There are several questions that need to be answered in order to give you a complete response. What engine do you have? What caused the need for a top end rebuild? are a couple. Still, you may be re-sleeving or replacing cylinders. If you are replacing cylinders the SBT kit will come with the correct size pistons. Share some more details and you will likely get a quick/accurate response.

[KyleB]

Re: Top End Re-build

I apologize for not giving enough info. The reason why I need the top end rebuilt is that I apparently experienced detonation due to the engine running too lean for too long. I bought the boat used and quite honestly I don't think the previous owner broke in the replacement engine correctly because I only have 115 psi on the MAG cylinder and 140 on the PTO. The engine is a 787 from a 96 XP. Actually I thought about just putting new rings on her but when I pulled the dome off the MAG piston was burnt pretty bad and think that a top end would be the way to go. The PTO side piston looks good but while I'm in there I thought I would go ahead and rebuild that one as well. I measured the cylinders last night and they are 83 mm. I assume this is 1mm over OEM correct? The condition of the cylinders is actually very good. They are not scored at all. Thank you for your reply. If you think I need to do something else other than a complete top end please let me know.

Kyle

[oldbugger]

Re: Top End Re-build

Kyle,
There is cylinder taper and out of round measurements that come into play as well. Cylinder taper wear limit is 0.1mm and out of round appears to be 0.08 mm. there is also a piston to cylinder wall clearance spec of 0.15mm on the 787. Unfortunately I am not near my most recent pistons or I could measure them and get a OEM stock diameter for you. If no one replies by later today i will measure and respond. IF all measurements are in tolerance I would order the top end kit 60-107 from SBT and you will get everything you need if you call them with your measurements they will match up the piston size. The second option would be to order matching sleeves and have your cylinders re-sleeved and bored, this can be a bit of work depending upon your local machine shop availability/capability. You could always dot eh cylinder exchange as well, a very clean option and over all only costs slightly more than having all the machine work done!

[oldbugger]

Re: Top End Re-build

Kyle,
It looks like 81.89MM is the stock size.
Some guys recommend having the cylinder head machined when you go 1 MM over. I don't have specific knowledge of this but I would certainly want to check it out before I hand anything re-sleeved or cylinders replaced. Depending on what you find with the taper and out of round checks you may be able to just get new +1 MM pistons.

Hope this helps