cb750 dohc big bore help

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forcefanajd

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ive been researching the internet since yesterday trying to figure out the best way of doing a big bore in this bike and a lot of the information is very vague so im reaching out to you guys for help. here is my basic plan, cb750 cases, cb900 jugs with cb1100 sleeves, cb900 cams, cb900 crank, cb1100 pistons, cb1100 rods, and cb750f cylinder head. my question is what is involved as far as machine work, i have a really good machinist so im not really too worried about getting it done its just i want to see what is involved before i jump into the project.

here is what im assuming is going to be done for machine work.

cases machined to accept cb1100 sleeves
cylinder housing machined to accept cb1100 sleeves
deck the head and cylinders

can anyone shine some light down on this crazy idea that i have? i think itd be a pretty stout engine and i would probably back it up with cb750 f2/3 transmission to get the better ratios. thanks for any help fellas.
 
Check the valve sizes.
Can't remember if 750 has smaller valves?
Decking head may not be a good idea until you check CR stock
 
well the cb750f cylinder head has 2 mm larger valves but the combustion chamber is larger than the cb750k dohc. the cb1100 had 9.8:1 compression, the cb750k dohc had 9.2:1 compression, and if my math is correct without decking the head and block im looking at 9.3:1 compression.
 
You need to measure combustion chamber
Overbore and stroking will automatically bump compression in a smaller combustion chamber
 
ahhh, good point, i forgot to account for the flat part around the combustion chamber which would now be at the top of the cylinder. supposedly its pretty common to bring a cb750 up to a 1080cc and this setup would net 1062cc's. i could always use the cb750k dohc head and just run cb900 cams in it, sure it wouldnt flow quite at good but i wouldnt have to worry about v2v or p2v clearance nearly as much.
 
900 was more of a 'tourer'
750 cams are probably better, particularly if you can find the first year K (1978?)
They were pretty quick for an overweight lump
Haven't been into DOHC 750/900 since the 80's
I specialised in them for a couple of years but have forgotten most of the stuff I knew or did (except fitting cam chains and puling motors ;) )
 
900 is a 'long stroke' motor (stroke longer than bore diameter)
Cams 'look' more aggressive on paper but I think the opening and closing ramps are much longer?
Don't know if it was only the 'Euro'750 that got different cams?
US models are frequently tuned different, more torque at lower rpm for the 'stoplight grade prix' ;D
I didn't 'get here' until 1999 so all my experience is on 'British' bikes
 
any idea where i could find some definitive info as to which cam is better? ive looked all over the web and its very hard to find any info on these.
 
May be difficult, Honda stopped putting cam timing in service manuals and there is a lot of bad information /guesswork on cam specs.
I'll check if I have any info
 
Cb750 was 62x62 and 900 was 66.5x69. 100 was 70x69. They look similar but lots of differences. Pistons are taller on 900 than a 750 for example.

Wiseco offer pistons for that bike and I have seen some offered as 67.5 for both bikes, but I have some doubts about that. Last time I had a 750 motor I replaced it with a 900 but I don't recall all the gory details.

I'd call these guys and ask them what they have and what's needed to fit them. http://www.dynoman.net/engine/pistons/honda.html

We spent this afternoon stripping a 900 with shot big end. I doubt the owner will let me strip his 750 just to compare parts though ;-)
 
teazer said:
Cb750 was 62x62 and 900 was 66.5x69. 100 was 70x69. They look similar but lots of differences. Pistons are taller on 900 than a 750 for example.

Wiseco offer pistons for that bike and I have seen some offered as 67.5 for both bikes, but I have some doubts about that. Last time I had a 750 motor I replaced it with a 900 but I don't recall all the gory details.

I'd call these guys and ask them what they have and what's needed to fit them. http://www.dynoman.net/engine/pistons/honda.html

We spent this afternoon stripping a 900 with shot big end. I doubt the owner will let me strip his 750 just to compare parts though ;-)
Click to expand...

alright, so im slightly confused here now. i had a different post here a little while ago but after posting it and reading it there was no sense in it after doing the math so im going to think out loud here a little bit and i really think im going to have to get parts in front of me with some calipers to do some measurements before i know what will work.

the cb750 was a 62mm stroke and the cb900-cb1100 was a 69mm stroke (only differences was the bore). there is a 7 mm difference in stroke so that must be made up somewhere, i know the cb900-cb1100 had a 3mm higher deck height and the rods were 1.5mm shorter than the cb750 counterpart so that leaves us with 2.5mm of difference left. i wonder if that made up for all of that in the piston height or wrist pin height??
 
I'm sure that 900 pistons were taller. I'll see if I can find a pic somewhere.

could you fit a 900 crank and top end by any chance? or get a complete 1100 motor?
 
teazer said:
I'm sure that 900 pistons were taller. I'll see if I can find a pic somewhere.

could you fit a 900 crank and top end by any chance?
Click to expand...

thats the exact plan.

cb750 cases and head
cb900 jugs, crank, and possibly cams depends on which is better
cb1100 sleeves, pistons, and con rods.
 
Swivel said:
I did a CB750F with a 890cc Wiseco kit that was bored and sleeved to 67.5mm with CR's and a Yoshi pipe.It was far better than a CB900 both in RWHP and it revved quicker that a CBX-6.I was like a grenade in power delivery.It was very responsive.It was just like a later GSXR in revs and feel.It had underbucket shims and better valve springs.One of my favorite engine builds.
Click to expand...

What did you get for rwhp? My guess with the setup I'm planning on doing I should be able to get close to 100whp with a decent set of carbs.
 
Well the main reason I wanted to do the 1062 is because I think it would be a fun project to undertake and since I have another bike that I ride on a daily basis I'm in no rush to finish this one, I'm making my own frame for it too so it'll be a long time till it sees the road again.
 
Honda UK used 750 crank in original endurance racer and increase the bore to get 997 or 998cc (forge exact size)
bigger valves are always good as valves are always the limiting factor to flow.
No need to open ports up to crazy sizes, that just looses port velocity
short rod/ long stroke is great for torque at 'lower' rpm but piston can out accelerate flame front at high rpm (its all relative)
Shortest piston and longest rod would be better way to go (generally, short pistons are also lighter so less stress on rods/bearings)
You will probably still be in the 1.4~1.6 rod ratio range, 1.7 or higher will be better though (you can do the math, it's not my engine ;D )
If your building motor as more of a design exercise/hobby, don't worry about stuff just go ahead and have fun
 
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