Topic: Pistons in from top?

[disco]

I’m rebuilding a CB750 K2 at present. On my last Engine build, I did it the ‘conventional’ way and installed con-rods onto crank & then closed up the bottom end. After that I inserted the pistons/rings onto rods. All four Pistons/rings then went into the jugs from the bottom as per Hondaman book.

If building an 836cc with 65mm bore with standard rods, another option may be to leave the crankcase open and install jugs onto top half, then pistons/rings/conrods down thru jugs from top. Do up big ends and then close the crankcases? Might be easier than struggling to get all 4 pistons/rings installed from bottom? Make it easier to deal with spreading ring gaps at 120 degrees too.

[754]

I would say no unless maybe you use the tapered install sleeves they use on V8s .. maybe. .

[disco]

Thanks Frank. Can you explain a bit more?

[754]

Look at maybe summit racing., they use a tapered ring, one end being bore size, they other end tapered bigger
Maybe 2b1/4 inches tall. It looks like you push the piston in just til, the rings hit the  bottom, then insert into the bore.
Then I am thinking you get it a smack with a soft hammer and it slides into the bore.
 Not having a chamfer may? Make a difference?
 Be easy to make.

[HondaMan]

Have the machine shop chamfer the bottoms of the bores for the 836 kit, or they will be REAL hard to put together(!).

[DaveBarbier]

Since the cylinders don't actually fasten to the case you'll need to clamp it. Not a huge deal. Maybe just shorter bolts or head bolts with long spacers.

But installing all through the bottom like Honda intended isn't that big of a pain. Who cares though, go ahead and experiment.


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[1976cb750f836]

U r trying to fix a problem that doesn't Exist!
Stop wasting time and put them on the rods and have a helper to lower cylinder down while u get the rings Compressed! It ant that big of Job! But a helper is a Must!
Just my 2 Cents!
Ps if this is u r first 836, your going to love it!

[DaveBarbier]

Don't listen to him!

If you want to try another way go for it. Shouldn't matter to anyone here if you do it one way or another. If you get stuck or have to do it again the normal way, big deal. Try it, document it and post how it went.


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[madmtnmotors]

Yeah, they could be installed from the "top", but with the lower case open the engine would be inverted and the piston/rod assembly would actually be installed from "below" with the cylinders pointed towards the floor. You also have to secure the cylinders in place and the squared edge of the top of the cylinder bore makes it more important that the rings are fully compressed to prevent snagging one. Seems like more work than sliding the cylinders down over the pistons with a fully assembled bottom end since they are done two at a time from below and the bottom of the cylinders are (or can be) chamfered to help prevent snagging a ring. I've assembled numerous piston cylinder assemblies as suggested in the manuals without any drama and without any assistant.

[disco]

Thanks for the replies. I built my last Engine which was an 836 in the normal manner single handed & it wasn’t extremely difficult. My machinist did put a good chamfer on bottom which made things simple. Most difficult part was trying to keep the rings gaps spread evenly.
The Engine I am working on right now has has 0.5mm oversized CI pistons. So this method won’t work for me right now. I guess I’m just putting it out there for anyone who is building an 836.

[greenjeans]

Have the machine shop chamfer the bottoms of the bores for the 836 kit, or they will be REAL hard to put together(!).

Yes. Chamfer.
  Getting the rings at 120 took a little patience, but once you get a feel for how your are going to compress them, it ain't so bad.

[disco]

Yep I got a good chamfer on the entry. I was just putting this out there, because it’s an option provided your bore is more than 63mm. See the pic.