New member, here to educate myself .. Just bought a '72 CB750 project!!

[The_Austrian]

Hello fellow CB750 owners!

My name is Simon, I'm 23, Austrian and just traded a ragged out Porsche 924 for a '72 CB750 which was a former drag bike!!
I restore Mercedes/Porsche from 50s through 70s for a living, that's what brought me from Austria to the United States.
I got the bike, basically a pile of parts, this week and already tore the engine down for rebuild and upgrades.
And that's the reason why I'm here!

I'll turn the bike into a vintage cafe racer over the next months, so I'll highly appreciate all tips and tricks you guys have.

I'm going to make a build thread with tons of photos .... and I hope you can share some enthusiasm with me!

And here's my first question, the bike came with two engine. On of them is bored a pretty good amount and has 71mm "Venolia 262" Pistons. Does anyone know anything about those Pistons or how much displacement that would be? Looks like 1000cc but the rods etc is stock, which would be ridiculous.


Thanks on advance for all your help, looking forward to post on here!

P.s. What's the easiest way to post photos, it doesn't really let me upload them?

 

[dirtdigger]

71mm would make 1000cc. They are Probably RC Engineering if they are Venolia pistons. Take a look on the cylinder in the lower front right corner and see if it is stamped RC Eng. Take a look at the rods and see if there is a RC stamp on the big end. If they are stamped RC then they are upgraded stock rods with bigger bolts and should have bronze bushing in the small ends. RC used these a lot until they came out with the golden rods. Unless they are lower compression and you use an oil cooler these motors run pretty hot for continuous use. What does the head look like? Does it have a RC Eng stamp on it? For a streetable higher power motor I would suggest 8366 or up to 915cc kits. They are dead reliable and dont overheat like the big motors will. Any motor questions let me know. I am always interested in buying performance motor parts if you want to sell...especially RC Engineering parts.

As far as posting pictures look here http://www.cb750.com/threads/18-How-to-Post-Pictures

Let me know if you need help with the motor as far as suggestions, etc

 

[amc49]

I'd sure be worried about the cylinder block bored that big, the sleeve used would determine how much of the aluminum casting is left to support it all when torqued up. If bored too much to suit liners then the cylinder block often cracks at the thin spots which can be paper thin in some places with a bore that big. Those can tend to blow head gaskets if used in day to day driving when the cylinder does not hold up head as well as it should.

On one that big you'll need an oil cooler for sure. A drag motor compression ratio is going to kill you on pump gas.

15 mm. piston pins pretty much suck at a bore that big too. Bronze bushings in small ends only make that worse.

 

[dirtdigger]

Cylinder block at 1000cc is not really much of an issue, it has been proven to work, there are more 1000cc motors running on the street then you would think. With a switch to mls gaskets and good flat machine work...blown head gaskets on these big bore sohc are a non issue.

15mm pin not an issue. Pins have never been a breakage issue on the sohc even with the big bores. RC ran them on nitro and the piston gives before the pin. Have been running big bores for many years no issues. If you are concerned APE has tool steel thick walled 15mm pins they guarantee wont break. The RC built stock rods with bushings also a non issue they are time proven and many original examples still live to this day, I have two sets on my shelf taken out of running drag motors I bought for parts...these modified stock rods by RC have proven to work in big bore motors even thought they dont look like they should. With proper parts selection and build technique the big bore sohc are far more reliable then people give them credit for. There are still RC cobra motors running that have never been touched and with todays parts they are even more reliable.

 

[amc49]

I do not concur, you are removing a solid 1/4" to bore that far, no way are those blocks that thick and I heard of some of them cracking between the cylinders, or leaking oil mist from the cracks from the liner pressfit. Why the later DOHC went to zero cooling slots and a solid block of aluminum with the 70 mm. bores, as well as crankcase mods to make the cases more solid under the cylinder blocks too. The so-called 'donut' cases.

The pin not the issue at 15 mm., it's the rod end, the smaller pin allows the rod to break MUCH easier. Many think that is a rod related issue but commonly not. A smaller pin by virtue of size pulls the small end of rod out of round easier to then try to seize slightly, then the rod snaps from that. The pin will come out of the blown engine looking fine. The later DOHC used exact same rod castings but for length and they became lethal in the 900s with that super long 69 mm. stroke (check that piston speed FPM out!) and why most 900s end up in the scrapyards with holed crankcases. Even dead stockers blew them and common, I've run across them in the scrapyards. I contacted RC back in the day (early '80s) and asked for the same beefed up OEM rods and they told me they dropped reworking OEM rods on the DOHC because the extra power the motors made broke most of the few DOHC sets they made. The big end not the problem, they could fix that, the pin end was, they couldn't. Why Kawasaki and Suzuki never break there, they use much bigger pins and why the Honda DOHC 1000s and 1100s went to a bigger pin, those later OEM rods are pretty much as good as Carrillo ones at $1400/set. Other people (Falicon? IIRC) who refinished rods also told me the same thing, the 15 mm. pins broke rods bad enough they quit refinishing to put bushings in the top of rods on that size. MTC made a good turbo rod that worked with 15 mm. but it was very heavy. APE strongly suggested to me when I contacted them about thicker 15 mm. pins the same as well, the thicker then being heavier helped the rods seize slightly going out of round to again rip top of rod off. 'We suggest you not do that' was what I was told.

Honda stayed with 15 mm. pins for way too long and into too big piston sizes, they are way below the engineering recommended pin size for some of the cylinders they are in as far as hi-perf design goes. Even the DOHC 750s can spit rods out due to that and also why the RC DOHC turbo 750 was extremely limited in boost, the first one immediately snapped one of those 15 mm. rods in two. They only used about 6-7 psi of boost there. For magazine cover show only, the realization the engines could not take boost in any great amount killed RC building turbo kits to sell to the world.

When Honda first modded the SOHC with a DOHC bastard one-off head to later morph into the DOHC they realized pretty quick the rod/pin shortcomings and modded rods (titanium) to not blow up the engines but the problem stayed and Freddy Spencer and others commonly blew the official racing superbike DOHCs up over and over due to same issue, sometimes 2-3 bikes down due to it a race. If the rods didn't blow then the timing chains did. Why after way too many years Honda finally remedied the issues with the last gen DOHC motor. Too little too late like so many things Honda does.

No trouble at 100 hp, start making 125+ and those issues will begin to eat you alive, at 150 the engines won't stay together.

 

[dirtdigger]

Many big bore blocks running around without issue. The oil leak issue is not from cracked blocks but results from the machinist not doing the required welding when the boring breaks through on a limited number of castings...it doesnt happen on all blocks and works fine more often then not. If machined to use orings like oem it also keeps the oil inside...something a lot of machine shops will not do that are not familiar with the 750. Blocks are not a big issue, they have been doing it from the start and still do it to this day. I am running two on the street and two in drag bikes right now...no cracks no oil leaks no problems.

Again on the sohc there is not a huge issue with the pin size, you keep referencing the dohc this and that, but if it was a huge common issue the aftermarket, carrillo, falicon, rc eng, cycle x all would have made optional bigger pin rods and pistons for there big stuff and stopped making the dreaded bad 15mm stuff. They make there own pistons, rods...why continue with the same bad 15mm stuff if all they had to do was use a bigger pin...because there is no need, the 15mm stuff works fine. RC continued the bushed sohc rods up till they stopped the parts business, have a couple of used sets on my shelf, still in good condition and know of a couple guys running them in 72mm drag motors...no problems. Again sohc not dohc stuff. I'm not disagreeing with you that bigger pins aren't better. You build a big bore 750 with carrillos you wont break the bottom end. You keep the under 10,000 rpm they will work fine, the heads dont flow at much higher rpm so no need to spin them. And all APE sells for the sohc is thick wall tool steel pins, so guess they lied to you that they dont suggest it. The pin issue has been debated and discussed in depth on sohc forum, many guys with many more years experience than you and I building and racing the sohc including Jay at APE have declared the 15mm wrist pin a non issue in the sohc. Its a non issue. 05 to 08 GSXR1000...using 15mm wrist pins...making 200hp....no problems

So I guess we can agree to disagree but I know from personal experience no issues with any of this on the sohc if you use quality parts and solid build and machine techniques.

 

[amc49]

The ROD is the issue, a proper rod correctly made at the top and then the pins are useable although less than desireable. The small pins though zero in on that issue, they help force it into the light. The aftermarket would want to stay at a common pin size, it sells more parts. Just redesign the rod.

Seems to me we are arguing opposite sides of the same point.

BTW, if you know of ANYONE who can refinish rod upper ends with FIFTEEN mm. pin bushings I'd love to know it. I've gone looking twice and could not find anyone that did bushings that small.

 

[dirtdigger]

APE reconditions Carrillo rods with 15mm bushings so they must have a source for the bushings from somewhere. Might want to check with Kenny at Cycle X because there stock design Super Rods also have bushings, would think they know of a source.

 

[amc49]

Thank you for that.................

I was somewhat surprised when RC told me they were not making a DOHC modded OEM rod at the time, it seemed to be a dead on copy of the SOHC rod and if one can why not the other I asked. They told me the material had changed............

The DOHC 900 stroke is killer on the OEM rods, those when they break almost always break right under the pin at the narrow part rather than in the middle somewhere. Bigger pin forces that narrow spot to be bigger too. In fact on the last rod they filled that in perfectly and then the rods became quality parts, the bottom half pretty much unchanged, it was fine all along. The modded rod does get bolts with no serrations in the sides to make them stronger too. The earlier serrated pilot section bolt would occasionally break under big loads. The size though stayed the same, the better bolts interchange into other rods. Hard to find them now though.