Topic: Big bore sizes

[Sdsbassist]

What were common overbore sizes for the 750 motor?

[Dimitri13]

836, 915, 1000.

[Retro Rocket]

810, 836, 850, 888 {900} 915, 970, 1000, 1080. and a few more in  between....

[754]

 812, 823, 836, 850, 890, 915, 970, 1000, 1050, 1080,and have heard of bigger!
 add about 100cc if you add stroker crank. Bigger bores will happen with billet cylinders..

[754]

 I left out 749 or 750, but is used at times..

[HondaMan]

I left out 749 or 750, but is used at times..

One more!
760 (1.00mm oversize)

[750K]

Good thread for us rookies, what would you say is the best balance of power vs reliability, longevity for say a street ridden bike? I have a 72 750 that's going to get a bigger bore at some point for a future project.

[Don R]

812 was popular but the Ebay 836 seems to be a bargain. Mine is going together soon.

[RAFster122s]

For the 836 kits, how much larger do you have to bore the sleeves, presuming you aren't having to change sleeves?  (I'm a 550 guy and am curious.)

Big bores can be expensive, just for the machining, they can make parts cost look cheap by comparison.

David

[Jerry Rxman Griffin aka MuthaF'er]

The 836 is the easy way to go for beginners. 65mm vs 61mm stock.  Plentiful with inexpensive kits available. Above 850 you have to get into resleeving and then rods are a good idea to replace.

[Terry in Australia]

Depending on where you live in the world, quite often the machining costs more than the pistons. Cliff (Maduncle) was quoted 275 bucks to bore a CB450 twin out to second oversize (.5mm/ 20 thou) so you can only imagine how much the same shop would charge to bore a CB750 cylinder block out a full 4mm to 836cc. Cheers, Terry.  

[trueblue]

Damn, that's expensive, after boring my 650 jugs out to 61mm I have change out of $100.

[Terry in Australia]

Damn, that's expensive, after boring my 650 jugs out to 61mm I have change out of $100.

You did well mate, I remember shops in Melbourne were charging 50 bucks per hole for an 836 kit back in the 1980's! Cheers, Terry.

[trueblue]

It's not what you know, but who you know .  But in all seriousness, even at full retail it would have only been $175.

[Terry in Australia]

That's right mate, I've got my own Repco boring bar in my garage, so I don't have to worry about paying someone else, but most small boring bars will only do 5 thou cuts, so for every 1mm you're looking at 8 passes, and an 836cc big bore kit means removal of 4mm per cylinder (16 passes per cylinder) and time is money.

A more modern machine can probably do 10 thou or even bigger cuts, but they're horribly expensive to buy, so what you make up in labour time, you have to offset with recovering your investment. Cheers, Terry.

[trueblue]

I had mine done through one of the businesses my boss owns, the boring bar they have there can do a 0.040" or 1mm cut in one pass, makes poking holes nice and quick.  Oh and Terry it is a good thing you're the spelling Nazi not the maths Nazi, when I went to school 8x4 was 32 not 16 .

[ekpent]

Wiseco made a nice kit that was an 811 that worked real well.

[Doo Bee]

Yup 836 is the way to go. Best performance least amount of money spent.
Should replace rod bolts or get better rods if you are gonna hammer it with
a good cam----high RPM's. Better valve springs for that cam, heavy duty
cam chain ETC. The CX-3/63A  is the biggest I would go for the street cam.
You want it to idle and have some low end. See my post for Cycle-X 970
kit Sweeeeeeet, just bumped it to the top.

[754]

 Any boring bar can  cut 40 thou or more..

[flybox1]

The 836 is the easy way to go for beginners. 65mm vs 61mm stock.  Plentiful with inexpensive kits available. Above 850 you have to get into resleeving and then rods are a good idea to replace.

but that's really not the end of the conversation for us rookies.     I know absolutely nothing about the mods that can be made, or all the other stuff that is needed once an engine is bored.  rods, sleeves, springs, cams, cam chains....its all foreign language so far.  I just know that my bottom end leaks oil, and one of the 3 PO's of my 750 used RTV sealant on my cases (luckily none of the red stuff in my oil pan, yet)

Thankful for guys like HondaMan/MReick and others who offer these engine services here on the board. Trying to glean as much info from their threads as possible for when the day comes to pull the trigger.

[754]

 The 836 will use up most of your sleeve, but now with the 850 you have an oversize..cheap going 836-850..
 Barrels are cheap.
 If you have any or all of these you will have  a huge improvment, bore lit , cam, springs, headwork, header, good clutch..
 You can kill them with stock rods & 836, but 2 of us here have a thousand strip passes saying they can stay together..

[chewbacca5000]

The 836 will use up most of your sleeve, but now with the 850 you have an oversize..cheap going 836-850..
 Barrels are cheap.
 If you have any or all of these you will have  a huge improvment, bore lit , cam, springs, headwork, header, good clutch..
 You can kill them with stock rods & 836, but 2 of us here have a thousand strip passes saying they can stay together..

What kind of power are you putting down on those stock rods with a thousand strip passes?  Just curious.  Got my engine apart and want to make wise decisions.

[754]

 I was running an 836 with cast pistons, RC 315 cam, springs, good valvejob, otherwise stock heads. Usually with Webers, and a 4-1.
 My buddy ran cam, 836, springs, valve job, stock carbs..
 We usually shifted between 10 and 11, 000 rpm.... both motors used stock rods. Mine was together for about 25 years, not touched except splitting cases to change a blown countershaft.....TIP, dont run sprockets without the wider hubs, if you must check  splines frequently...

[Terry in Australia]

Any boring bar can  cut 40 thou or more..

Not all of them Frank, my little light duty machine only has a 1/4 HP single speed motor and while 10 thou cuts are do-able, I prefer not to over-load it. I have thought about roughly boring cylinder sleeves on my lathe or on my Mill, then finishing them in the boring bar, but as I only do one once in every blue moon, no biggie really. Cheers, Terry.

[754]

 Its all in the tool geometry and knowing what to use..
 I can peel off 100 -120 though in steel, on my little South Bend lathe, other have trouble cutting 60 thou.. Biggest problem resulting in inability to cut is RIGIDITY, if machine is badly worn or missing parts or needs adjusting, it can cut poorly..

[Terry in Australia]

Its all in the tool geometry and knowing what to use..
 I can peel off 100 -120 though in steel, on my little South Bend lathe, other have trouble cutting 60 thou.. Biggest problem resulting in inability to cut is RIGIDITY, if machine is badly worn or missing parts or needs adjusting, it can cut poorly..

Nope, mine's in very good condition Frank, and the original handbook recommends 5-10 thou cuts.

[JD_G]

I have a 1976 CB750F and have the entire engine torn down. The motor had 8500 miles on it and I am thinking of boring up to 836cc's but have been trying to figure out if I should replace the stock rods for aftermarket forged aluminum rods. The challenges for me is finding new bearing shells, the machine work on the cylinder skirts for the extra clearance needed with the rods, and trying to determine if I need to balance the crank. Has anyone here done an overbore to the 836cc bore with stock rods and bearing shells? Should I replace everything? I am already doing a top end rebuild but was planning on using the stock cam. The valve guides, springs, retainers, and valves are already on my list of things to do as well as to port/polish the intake/exhaust. Any help would be appreciated.

[754]

Many of us have run 836 with 10.5 to 1 compression on stock roads and lived.
 You got to decide , the cheap low compression is fine for  budgets if you want to cam or headwork now or later, get the higher compression, it works better with those  other things. The high compression sets care often at least triple the price.,

[HondaMan]

IMHO...The 'sweet spot' of the 750's engine design is the bore size of 2nd over, 749.8cc (won Daytona in 1970!). Honda has long designed engines at the 2nd oversize, then stepped down 0.5mm for the original size while allowing room in the head gasket for 0.5mm over the 'nominal' (2nd oversize) bore.

[Jerry Rxman Griffin aka MuthaF'er]

Use your stock rods with 836. Perhaps your bearing shells are still good too. With only 8500 miles you should be good. Your valve guides are probably fine too. The F2/F3 was the one notorious for the valve guide issues and not the F0/F1. I floated a valve on mine when I blew her up. I installed stronger valve springs to help prevent that. Of course now it helps if I let off the throttle for shifts too vs power shifting which is not very forgiving if you miss a gear.