Topic: Big bore..... what to do??

[BrianAdair]

I’m having trouble deciding what I want to do to my engine, I have been reading and searching on this site for over a month now.
There is a lot of good info on here, it is hard to find specifically what I am searching for.
Money is an issue…. Lol yea I know if you don’t have deep pockets you shouldn’t be doing this, but it’s what I enjoy and my hobby. I Traded a bunch of Harley parts to a guy at a local shop, and have store credit there

I have a  CB750 K7 and to my understanding it has the F1 head…..? I know the rings are not the same as the 71-76 models and the pistons have a slight dome to them, with 9:2:1 C/R?
 
My dilemma is,
At the shop where I have credit, he has all kinds of old stuff still in boxes, I found 3 or 4 Henry Abe 900cc kits with sleeves, pistons, rings, and gaskets. The only reason I am question going with this kit is I don’t want to split the lower case to have it bored for the sleeves. Is it possible to have the sleeves machined to fit in the stock holes in the case? With this kit would I be able to find replacement rings?
Are stock rods with this kit a bad idea…?

I was also reading on this site  http://www.satanicmechanic.org/bigbore.shtml 
And at this shop he has a set of  64.5mm S-12 pistons, and rings if what I read on that site is correct would make it a 823cc, but these are flat top pistons, so will I lose compression and power?
IMHO 823cc really isn’t worth boring it……

Also on that site I see they say that you can use Honda CBX 1000 pistons, I think I found a set of those also…..

My engine is in very good shape, has 28k miles on it, I’m not building a race bike but I do want it to have good low end and also not be wound out on the interstate….
My plans were to, bore, “to what ever I end up going with”  Hot street cam, HD cam chain, HD valve springs, HD case/cylinder studs, port the head, and running ‘73 model carbs.

Anyone have any advice or help I would appreciate it.

Thanks
Brian

[dave500]

if your going that far do the bottom end aswell,its only a half baked job otherwise.any over bore results in an increase in compression.unless you have dished pistons to reduce it,like in a blower setup.

[mick7504]

The opening in the top case should be about 72mm and if the skirt diameter on the sleeve is greater than this, I can't see a problem machining it to fit, providing that it has has about .020" (.5mm) clearance all around to allow for expansion.
That would leave approx 2mm of skirt thickness.

As Dave mentioned in the previous post, it doesn't stop there.

The reality is, if you want a good strong reliable engine that will kick butt, it's gunna cost dollars.
Another alternative to consider would be to getting some headwork done with a mild camshaft and a new set of rings to freshen the engine up.
EDIT: And HD Head Studs.
The costs can mount up if you want to go the whole hog which doesn't just stop at an overbore.

Here are a couple of links to give you some idea of the goodies available.
http://cbrzone.com/
http://www.cyclexchange.net/Engine%20Parts%20Page.htm

Mick

[Bodi]

There's no point in trying to do this without splitting the cases if you have to bore the top case for sleeve clearance. It is just not possible, even if you could get the rods off through the sump somehow, to keep all the chaff out of the engine.
Going to 900cc is a big step and you want to have a good long look at the primary chains and the transmission anyway. High strength primary chains would be a wise investment, and any worn main/rod bearings will cause trouble sooner with the extra load from your overbore.
Sounds like a cool project anyway. I wish there was a shop I know of with NOS performance parts lying around for my bike!

[BrianAdair]

Ok, you talked me out of the 900cc kit....

I just picked up a set of Rocky S-12 pistons, rings, and gasket, I think they are 64.5mm, taking it to an 823cc?
I couldn't find a specs sheet on clearances for the boring, but I found online around 0.0010 for street....?

I also found, and I'm in the process of getting a Russ Collins 315 cam.