Topic: 71 cb750 new engine build

[orkid1989]

alrighty i have a 71 cb750 and im building the motor up. ive already made a topic in another part of the forum (http://forums.sohc4.net/index.php?topic=87872.0) but they suggested that i ask here. for starters i got the cyclex 849cc pistons, heavy duty cam chain, heavy duty valve springs, and the best cam cyclex sells. ive torn the motor down and had all the machine work done and now its time to put it back together and i want to make sure i dont forget something in the process. also i am wondering what is the best way to break in the motor once i finish it and install it in the bike? and just a side note i was wondering if anyone knows about how quick my bike will be after all is said and done? i mostly just want to outrun my friends 1200xr sportster. any advice would be greatly appreciated

[Retro Rocket]

Just saying guys, i sent orkid here as most of the hipo engine builders reside here, this was mainly concerning how to run in his engine.....

[orkid1989]

Just saying guys, i sent orkid here as most of the hipo engine builders reside here, this was mainly concerning how to run in his engine.....

thanks for the tip. just want to make sure i know what im doing before i mess something up

[orkid1989]

also another thing i was wondering is should i get new rods? im not drag racing the motor or anything but all i need is to snap a rod as soon as i build the motor

[Doctor_D]

You can easily run stock rods.  It's the rod bolts that are the weakest link in a stock rod.  Get F2 rod bolts for a bit of insurance.  Mike Rieck can probably sell you a set.

As for break-in, if you check out my build thread, linked below, you'll see a video of one of three heat cycles and then a video of us breaking in the motor - on the dyno.  If everything is fresh or new, bearings, rings, etc., the motor will be tight and run hot -- but that's the only concern with breaking it in this way.

[orkid1989]

You can easily run stock rods.  It's the rod bolts that are the weakest link in a stock rod.  Get F2 rod bolts for a bit of insurance.  Mike Rieck can probably sell you a set.

As for break-in, if you check out my build thread, linked below, you'll see a video of one of three heat cycles and then a video of us breaking in the motor - on the dyno.  If everything is fresh or new, bearings, rings, etc., the motor will be tight and run hot -- but that's the only concern with breaking it in this way.

o ok sweet thanks. as far as the rods i found s set of forged aluminum connecting rods made by MC Japan. are those any good?

[orkid1989]

actually i just read up on those rods and seems like nobody likes them so they r probably out.i am having a hard time finding the bolts though. i called up carpy and he mentioned peening them?

[MRieck]

actually i just read up on those rods and seems like nobody likes them so they r probably out.i am having a hard time finding the bolts though. i called up carpy and he mentioned peening them?

I have F2 bolts and nuts in stock. The beams can be polished and than shot peened as well. I'd strongly recommend balancing them at that point as well.

[orkid1989]

actually i just read up on those rods and seems like nobody likes them so they r probably out.i am having a hard time finding the bolts though. i called up carpy and he mentioned peening them?

I have F2 bolts and nuts in stock. The beams can be polished and than shot peened as well. I'd strongly recommend balancing them at that point as well.

any idea of anyone who will do that?

[orkid1989]

if the main problem with the connecting rods is the bolts then i guess ill give it a try. im not exactly sure how they break or where they break and that is what is worrying me. so just need the bolts? also MRieck how much would you charge to polish, shot peened, and balance the rods?

[Doctor_D]

I did the works on my stock rods. Polished, shot peened, cryo-treated, balanced and F2 rod bolts. (Still not as nice as Carillos, but anyway...)


Shot peening can be done by your local metal finisher (look up heat-treating and shot peening in the phone book).

Balancing and polishing are easy enough for you to do by yourself.  There are instructions on the web.  Here's a picture of the jig I used to weigh the big ends.


That's a stand on the end of a beam scale and another stand that's on my work bench.  It's a little picky to get accurate #'s, but it worked.

[orkid1989]

I did the works on my stock rods. Polished, shot peened, cryo-treated, balanced and F2 rod bolts. (Still not as nice as Carillos, but anyway...)


Shot peening can be done by your local metal finisher (look up heat-treating and shot peening in the phone book).

Balancing and polishing are easy enough for you to do by yourself.  There are instructions on the web.  Here's a picture of the jig I used to weigh the big ends.


That's a stand on the end of a beam scale and another stand that's on my work bench.  It's a little picky to get accurate #'s, but it worked.

wow those r some pretty looking rods. ill look in my area to see who does shot peening cuz im sure their is someone. or see if mike will do it.

[orkid1989]

also if i balance the rods shouldnt i balance the crank aswell?

[Doctor_D]

The crank balance shouldn't have changed since the motor left the factory.  So, as long as you weight-match the individual parts going back in, you're fine. However, it's a nice extra to send it to Jay at APE to be lightened and balanced.

For more answers to your basic questions, check out some of the build threads for people who did extensive engine work and you'll pretty much see what needs to go into a hotted up engine if you'd like it to last. (Tweakin, Sparty, MCrider, etc.)

[orkid1989]

i think ill just end up getting the f2 bolts for it and put it together and ride. just feel like it is taking forever and i just want to ride the dang thing

[turboguzzi]

i'm all for dyno running in these days, way more controlled than being in traffic and having to vary engine load accordingly....

my dyno guy does 3 X 50% power runs, cool down, 3 x 75%, cool down. 3 X 100%. Ready to Race. That's all

[HondaMan]

Those "F" rods should be good for 9500 RPM with the 849 pistons, but I wouldn't go for the 11k very often that the top CycleX cam will reach (with bigger carbs).

I'm curious: we (or at least, I...) regularly beat the Sportsters, even the S&S-built Sportsters, with box-stock K1 bikes in the 1970s. What is the HP of this Sportster of which you speak?

My first 750 (old factory K1) was a 12.8-12.9 second bike on the strip after 5k miles and fresh tuneup, ran 132 MPH top end as well. So, you have my attention....?

[orkid1989]

from what ive heard they are a solid 12.0 bike obviously depending on the rider.

[orkid1989]

ok havnt updated in a while but i decided to bite the bullet and get the cyclex super rods. also getting the dyna s ignition. hopfully soon i will be able to buy some cr carbs as well

[HondaMan]

ok havnt updated in a while but i decided to bite the bullet and get the cyclex super rods. also getting the dyna s ignition. hopfully soon i will be able to buy some cr carbs as well

If you're looking for STRONGER ignition, skip the Dyna. It makes WEAKER spark above 6000 RPM. There are many posts about this issue.

[orkid1989]

ok havnt updated in a while but i decided to bite the bullet and get the cyclex super rods. also getting the dyna s ignition. hopfully soon i will be able to buy some cr carbs as well

If you're looking for STRONGER ignition, skip the Dyna. It makes WEAKER spark above 6000 RPM. There are many posts about this issue.

Thanks for letting me know. Which ignition would you suggest?

[orkid1989]

Just got the rods in and they r awesome. Assembling the motor this weekend. What oil should I use and what weight for break in and after break in? From what I have read honda gn4 10-40 is a great oil. I figure seeing as I rebuilt the motor mine as well not cheap out on the oil

[lucky]

The crank balance shouldn't have changed since the motor left the factory.  So, as long as you weight-match the individual parts going back in, you're fine. However, it's a nice extra to send it to Jay at APE to be lightened and balanced.

For more answers to your basic questions, check out some of the build threads for people who did extensive engine work and you'll pretty much see what needs to go into a hotted up engine if you'd like it to last. (Tweakin, Sparty, MCrider, etc.)

No such thing as a "balanced " crank.
A crankshaft is balanced so that at a particular RPM it is "balanced".
That particular rpm is usually a cruising speed. A calculus formula is used.
Modern engines often have balancing shafts that further balance out vibrations.

But a law of physics says that  a unbalanced assembly will accelerate the fastest.
A light weight crankshaft is not always the best way to win. You need torque also.
Harley Davidsons won a lot of races and they are crude engines, even when built by pros. Ask the people at Megacycle Engineering in San Rafael Ca.

[orkid1989]

I never knew that or would have ever thought of it. Makes no sense to me but also I am not a physicist.

[Retro Rocket]

No such thing as a "balanced " crank.
A crankshaft is balanced so that at a particular RPM it is "balanced".

That is still balanced, every place in the world that does crank work calls it balancing including our esteemed members Big Jay {APE Engineering} and KOS {M3 Racing}, not to mention the bike manufacturers themselves. Balance shafts are common but mainly used to make a bike smoother running for road use.  I understand what you say but it is universally referred to as balancing.....