Making one engine from two

[beergood]

Some of you may have read a thread I started yesterday about replacing an engine. I think I may try a different approach.

The bike is a 78 750. For clarity: The current engine in the bike will be referred to as Engine 1, the spare as Engine 2.

My current engine block seams to be okay (runs fine, no noises or oil loss) and doesn't look terrible, but my valve cover leaks and looks like crap. I have a severely busted and hopeless exhaust stud problem and some other cosmetic issues with the head. My cylinders just plain look like crap. And all the other covers are also pretty wasted.

I have a second engine that I was considering rebuilding and swapping out, but upon further reading and contemplation I think that I would rather not spend the time and money to completely rebuild the second engine. And although the PO assured me that it was a runner I have never seen it run, and don't know if there are any internal issues.

So here is what I am thinking. I can refurbish the valve cover, head, cylinders and other covers from the second engine and make sure they are sweet and then swap them with the first engine's parts. That way I don't have to yank a whole motor, and I can probably accomplish the lion's share in one day. I also need to do some carb maintenance and replace the points with my 'new' Dyna. I am also thinking about purchasing a Real Gasket, although I need to be sure that an 850 one will work on my 78 750 (some sources say they will, gonna read some more about it).

Does anyone see any problems using the cylinders from one engine and the pistons from another? I did a preliminary check on the cylinders, and they look fine. I haven't looked at the pistons from engine number 1, but it only has 16,000 miles on it, so I imagine they have to be OK (I will check once it is all apart). I could also swap pistons, but I think I would rather just clean the ones that are already in the first engine.

Also a quick question about gaskets. I am thinking I am going to make my own. Anyone have any quick advice? I did do some searches on this and found some good info, just throwing some more feelers out there. Also, what is the skinny on re-using the metal head gasket?

Thanks in advance for any input.

[greg78gs750]

hey there,

i JUST TODAY put a realgasket on my 78 750, it definitely fits. i havent made any serious gaskets, but i did put one today under my shiny, newly polished points cover, just got a sheet of the paper gasket material and cut it out with an x acto. seems fine. cant wait to watch your progress.

greg

[tkent02]

I would keep the pistons with the cylinders they've been in.
fitted by the factory and worn in together and all...

As far as re-using the head gasket, it probably wont work but all you are out is time when it fails. New headgaskets are like $60....

If your engine #1 runs well keep it together, fix the leaks or whatever, shine it up but why open Pandora's Box if you don't need to? Chances of the other engine being as good are just that, chances.

You might pop the second engine in the bike temporarily to see if it's any good.

OK, your troubles are all in your head, so take the head off the engine #2, disassemble, inspect and send it off for any work it needs, hopefully you can just lap the valves and install new stem seals and call it good...
Clean all the covers ahead of time and then you can just swap heads in a day and be done with it. If you find a set of 850 pistons by then so much the better.

The realgasket for a '79 850 fits the 750, the two heads are identical, same part number.

[psyguy]

piston rings / sleeves wear is specific to each cylinder
if you want to swap the barells, i'd hone the cylinders and install new rings

generally, i'd choose one engine and do a rebuild (while the other engine can be in the bike so you can use it)

[beergood]

piston rings / sleeves wear is specific to each cylinder
if you want to swap the barells, i'd hone the cylinders and install new rings

generally, i'd choose one engine and do a rebuild (while the other engine can be in the bike so you can use it)

This was my first instinct as well, but the more I think about it the more I am unsure it is necessary.

If you were to take off a set of cylinders for inspection, deemed them to be safely within spec and reinstalled them, the odds that you will have the piston rings in the exact same orientation as when they came out is incredibly small. More than likely they won't even be close (and nowhere have a found where this is even discussed much less worried about). Therefore your rings and cylinders aren't "mated" any longer.

Neither engine appears overly used/abused, so I am leery about honing when it isn't needed.

[tkent02]

If you were to take off a set of cylinders for inspection, deemed them to be safely within spec and reinstalled them, the odds that you will have the piston rings in the exact same orientation as when they came out is incredibly small.

A good reason to use a hone even if the old rings are going back in. Only takes a minute, borrow a hone from Autozone if you don't have one. The amount of metal removed is infinitesimal, has no effect on the piston clearance unless you hone it way too much.
Then give it a good hard break in when you start running it again.

I think my 750's PO went honeless when he put it back together last time, there are not much signs of a cross hatch pattern in the bores, fairly polished. Not much sign of compression either.

[beergood]

That is some rock solid advice that I can't argue with.