View Full Version : Turbo CB750 SOHC build

05-13-2010, 5:53 AM
Well, I got this build going on a couple other sites, but someone said I oughta put it up here too. Hope I'm not whorin' myself out too much. I've been collecting parts for this build for years now, always something got in the way (usually money) of finally starting on it. Enough people made fun of me that I resolved this was the year. I tried to get it going about 2 years ago, sent a frame and a bunch of money to an asshole who ripped me off of pretty much all of it, and gave up. This time around I think it's gonna happen right. Ok, enough backstory, here's Part 1,the beginning. Sorry, it'll be a bit long, it's been a long time coming.

This crap all started a few years back when I was trolling eBay and found this bike. These are the original and only 2 pics that were in the auction listing:



Yeah, that's a turbo on it. Called the dude up, and after figuring out his accent (drunken Long Islander) we negotiated a price and he pulled the auction down. Me and a buddy drove the hellish trip up there on a rainy Friday, hitting the bridge at just about rush-hour, and it was all mine. BUT, the pics were a bit misleading (go figure, as large and clear as they were). The story on the bike, according to the now-hungover guy who helped us load it, is that some dude was building it in his basement and passed away. That part I could believe, since nothing was tight (anywhere) and more than a few things were not gonna fly. The frame was spray-bombed black with the motor in there, and there was a lot of corrosion on everything. Apparently the basement was damp :( But, all was not lost. There were some AMAZING parts on that bike. Both rims are invaders, and the rear is a one-off hallcraft "galaxy" from their last year in business. I figured it out talking to a long-time Triumph shop owner near DC who has the only set ever made for a Triumph on a show bike in his store. He ordered them brand new and got 'em like 2 months before the company went bust, it was their last new wheel idea. The spokes on it cross like a laced wheel, very cool. In fact, I still don't have a bike worth running it on. The front was an absolutely cherry, NOS 10-spoke 21" invader with dual Honda discs, but sadly that wheel was also lost when I got fucked outta my frame/money by that guy in Indiana. Still got the back wheel though. The Amen Savior frame was also NOS (for real). Not a single hole in it, prefect bushings in the rear suspension, not a spec of bondo; perfect.

Ok, that's getting too long. Long story short, I parted out most of that bike keeping only the wheels and the turbo kit. I got enough for the other parts that the turbo kit became free. That's where the trouble started. I wanted to just throw it on my stock CB chop and roll with it. Kinda wish I had done that. The American Turbo Pak literature (they made the kit) said up to 4-5 pounds of boost was fine for a bone stock motor. Wouldn't even have to change the timing. Well, that never happened. I didn't wanna make my only bike at the time unreliable, so I decided to build a motor for the turbo kit and later put the whole thing into my chop. I started wasting my money on any interesting parts that came by for the motor. Last year I got serious and picked up the real goodies.


Vintage MTC forged low-compression 836 pistons. At the same time I picked up a second complete ATP turbo kit with the competition-style piping (stainless and equal length). I had the turbo rebuilt, and all looked good. Nope! I sent the MTC pistons off to be refurb'd by an excellent machinist, Mike Rieck, and he informed me that the pistons had had it. Too many years of running, and the guy who pulled the motor apart to ship 'em to me had bent the oilrin groove on one. Another strike. Took me 2+ years to find that set, where the hell was I gonna find another?

I'm writing this at work, and I gotta run for a bit. More to follow...

05-13-2010, 11:47 AM
holy crap

05-13-2010, 12:06 PM
holy crap


05-13-2010, 12:19 PM
mmm boost that should be a monster when your done any plans on carb mods your gonna do?

05-13-2010, 4:00 PM
Alright, I'm back home and got a bit more time (and access to more of my photos). So, scratching the engine for the time being, what about a bike? I picked up an Amen rigid frame that needed a ton of love for $150. It had been in a wreck, cuz the backbone was tweaked, but that's ok since I wanted it long and low anyway. Here's the start of my first attempt at a frame for this monster:

Here's the tweak in the frame rail:


Here you can see what's left of the nasty backbone:


Anyway, I started cutting and cutting.


Until all I had left was this:


And then I started putting tubes back til it looked like a frame again:


And had a friend do the final TIG welding:


And we ended up with this (ignore that goofy-lookin guy holding it up for the pic):


05-13-2010, 4:14 PM
I was really digging that frame...until I started lining stuff up. It seems that I didn't get quite all the bent parts cut out. The rear section was tweaked too. Since I pulled my lines off that for aligning the new front section, the front is crooked too by a couple degrees. I dunno if a couple degrees in a bike with that much rake is a problem or not, but I need this bike to not kill me, so I shelved that frame again. It's still up in my rafters, someday I'll build a jig and fix the tweak the right way, but for now I had no engine or frame. Just that stupid turbo kit, nagging me.

To assuage my irritation (word of the day, BTW) I decided to start in on the engine. Now, this one isn't the one for this build, but just to show you that no engine is too far gone here's one we built for my buddy's chop.

Started as this:




And ended up as this (seriously):


Now, fortunately not all my stash of engines are as bad off as that one was, so this engine was less work. I started with a '76 engine because I wanted a K-model bottom end but I wanted a late one with all the good improvements. It has the larger output bearing (which is gonna be important) but not the extra built-in offset so I can put it in any frame I want (since I didn't have a frame yet).

I had the '76 motor clean in no time:



05-13-2010, 4:27 PM
Oh yeah, those pistons I had that were no good, remember those? Well, I decided to just say "f*ck it" and start buying the parts I knew I was gonna need to make this sucker right. Here's why a free turbo kit might not be a good thing.


Custom JE pistons. Only set around. 836cc, 8.0:1 static compression ratio, forged and beautiful. Ok, more pics (cuz I can't get enough of these damn pistons)



Oh baby. So much better than those 30 year old abused pistons. And the rest of the goodies? Well, how about a prepped crank and forged connecting rods?


CycleXchange did the crank work, and those are their rods on it. Chamfered oil holes, new bearings. The crank was not lightened, as per my request, cuz this is a street bike. I'm gonna put a lightened alternator on it, but I didn't feel lightening the crank was necessary at this time.

Now, gotta get all that power from the crank to the wheels right?


Back-cut transmission, also done by CycleX. All new ball bearings throughout courtesy of myself and the local Applied Industrial. Those primary chains are crazy special ones from Germany. Allegedly "unbreakable", but we'll see about that later :) Standard hi-po parts not pictured are HD cylinder studs, HD lower-end studs, HD cam chain, all new tensioners and such, etc. Basically everything you can think of, it's goin into this motor. Why? I have no idea. I coulda bought and parted out a damn crotch-rocket and had a much more reliable powerplant that was probably more hp too, but I didn't. Instead I wanna see 3-4x the original rated power from a 35+ year old aircooled antique. But then, I coulda skipped all this and bought a new geezerglide too right?

05-13-2010, 4:36 PM
That's about where the engine still is. I'm waiting on the machine work to be completed in the topend (any day now). New larger sleeves, bored topcase, etc, etc. I'm going to start assembling the bottom end this coming week. Oh, one more pic, the ported head:


Stainless oversized valves, titanium keepers, port work on the intakes, and just a little smoothing on the exhaust to keep velocity up. Woo-freakin'-hoo.

Ok, that's enough wasting money, I've still got 2 more frames to get through on this build :) I picked up yet another Amen rigid cuz they're my favorite frame. I decided to keep it simple and make a nice compact bobber for this engine. Something to really raise some hell 'round town on. Here's the mockup:


Again, I was really diggin' on this setup. It just fit together nicely. But, once again, fate intervened though fortunately not in a bad way. I was trolling eBay again and came across another frame and thought "man, that's gotta be the one".


It's a CycleOne Manufacturing rigid frame. Set up for a 250 rear tire with a jackshaft :) I knew it had to be the frame for this build, and I got it for a steal on eBay. Now we're getting somewhere...

05-13-2010, 4:43 PM
Whew, I gotta type faster if I'm gonna get you all up to speed here. The frame was awesome, but since I'm not into the whole "billet" scene it was gonna be tough to do a wide-tire CB750 without it getting too modern looking. If that's your thing, that's cool, but it's not where I wanted this bike to head. First problem was apparent pretty soon. The turbo piping didn't fit the single downtube. The collector on the turbo kit is right in the middle above the oil filter and it hit the frame. So time to cut apart my brand new frame.

Look ma, no downtubes:


Notched and tacked in a new neck:


This was the plan for the downtubes:


but I didn't have the necessary bending equipment to do it the right way, so it went to a buddy of mine who builds racecar chassis for bending and welding.

05-13-2010, 4:48 PM
Here's the first mockup. Please ignore the bars, they came with the frontend. It's a '90 sportster 39mm frontend, 19" front wheel and 18" x 250 rear wheel with new Dunlop tires. The tank is a GME that I split and threw on to see how it'd look. I wanted split tanks for the build after all.



Now, the last time I showed people those pics I got ragged to death about the bars, so I yanked 'em off and threw a set of tillers on. I intend tillers for the final build, although a bit more pullback.



Some specs on the setup. It's right at 45* of rake, stock length frontend, and the curved backbone keeps the neck real low. It was also about this time I decided those tanks were just too short and started figuring out what else I could use. I tried a set of stretched fatbobs, but they looked like shit.

05-13-2010, 4:55 PM
I scratched my head and tried on different tanks for a couple days and got nowhere. Then I remembered that Paughco sells raw tank halves. Ding ding ding, we got a winner. I bought a pair of raw fatbob halves and threw 'em on with a fatbob fender and dug the look.




The fatbobs follow the backbone nicely, and the rear fender gives it a cool squatty look. Totally changed the entire direction of the bike, but I was happy with where it was heading.

Turbo kit fits like a glove :) That's the mockup turbo, the rebuilt one is a lot shinier. The carb is a bendix off a shovelhead which is what the kit came with. I'd like a Mikuni flatslide 42, but I'm getting broke now so I may stick with the bendix for now. It's easy to change it later anyway


05-13-2010, 4:59 PM
First order of business, that fender was too narrow for the stupidly-large rear tire. It needed another 2" added to it. My buddy Matt, owner of the aforementioned red chop, is seen here helpin' out:


Here he is caught being too short to weld the top of it :)


Before welding:


And the finished product, minus a bit more smoothing (and bondo).


05-13-2010, 5:03 PM
Man, this 5-pic limit is a killer. Only a few more updates til the thread is caught up. Marathon posting session. The tanks were too tall. I wanted them to be no higher than the backbone. So I cut them apart and made new panels for the bottom and inside.


Here's the bottom tacked on the first one (fit better than I expected, considering the amount of beer consumed that afternoon).


And both tanks with all 4 panels tacked:



05-13-2010, 5:08 PM
Now that I had lowered the tank to where I wanted it, it hit the gooseneck at the front of the frame. I needed to clearance them for the tubes. I cut both tanks til they fit


Then I bent some sheetmetal on the curved edge of my welding table and fit/tacked it in place:


Here's the finished pair:


Then they needed filler bungs. I don't like the screw-in ones so I bought some new bayonet-style and cut holes/welded them in place:



05-13-2010, 5:12 PM
Alright, last update for now. I needed 3 bungs. Two for the crossover, one for the petcock. This sucker is gonna go through a lot of fuel (hence the 5 gallon tanks) so I wanted 3/8 bungs. I scrounged two off of Jeff, the guy doing the bitchin' XS build a few threads away, but I didn't have a third or time to wait for an order. Time to make one. Ok, don't poke fun at my lathe chuck. I know it's a bit rusty, it came to me that way and since the moving parts are clean I haven't made time for the cosmetics. It's a Buck adjust-tru, and I like it alot.





So that's where I'm at. The bungs were tacked in, and the tanks went to the TIG welder again (damn I gotta get me a TIG machine). More updates as they happen (at a much slower pace than what I posted so far). Thanks guys.

05-13-2010, 5:29 PM
Ill still say we take a pool on the first test ride, weasther you will run over, flatten something, or impale yourself.

It will be so cool to see that thing fire & be ridden. You & Matt side by side will be a sight.

05-13-2010, 8:44 PM
You seem to care ENTIERLY TOO MUCH about what other people think. Cool build though.

05-13-2010, 10:26 PM
Turbochargers are fun and terrifying at the same time. I can't wait to see where this goes....

05-14-2010, 1:13 PM
kick ass, john!

05-14-2010, 2:11 PM
gawd damn John,,u runnin outta MC forums to post on or what??? HAHHAHA
get off the puter ,, quit dicken around an finish the fuckin thing already!!! lol

05-15-2010, 8:10 AM
gawd damn John,,u runnin outta MC forums to post on or what??? HAHHAHA
get off the puter ,, quit dicken around an finish the fuckin thing already!!! lol

yeah what he said LOL

05-23-2010, 7:19 AM
Alright, some pics are up. Kicked the day off with a shop cleanup and a couple beers :)


Matt was doing some stuff on his bike, so I plastigaged the bottom end on mine. Nice new set of green shells


Laid a strip of green plastigage in each half. This is the top case, cuz you gotta do it all upside-down.



Managed to get it assembled, torqued, and pulled apart without moving anything.


~.0015" clearance across the board which is great.


After it was done I cleaned the plastigage off with brakleen and wiped the shells and crank with a bit of motor oil to keep 'em until I was ready to move on. I'm still waiting on primary rubbers for the clutch (last minute decision) so this is as far as the engine's gotten for now even though I do now have all the topend machine work done.

05-23-2010, 2:22 PM
So, the tank mount was tricky to figure out. I wanted the backbone to be pretty clean looking, and with the tanks sitting so low on the frame I didn't have much I could do to make standard tabs without them looking really out of place. Matt and I hashed a few ideas out and ended up with this. Started by making a pair of bungs for recessed 5/16" capscrews.


While I worked on that, Matt drilled the backbone. That was the only drill I had with a 3/4" chuck in it :)


Here's one welded in and ground flush, with the capscrew in it.


With both of 'em in we made a set of tabs that bolt in place with clip-nuts so you don't need to get to them to tighten the whole thing down.



05-23-2010, 2:29 PM
Blocked the first side up and tacked it down. It's 3/16" above the camchain cover. I might have to grind the corners down a bit on the final one, we'll see.



There's no pics of the in-between step but we made a set of tabs to hold the nuts in place (since they were welded to the other tank) and repeated the process of lining up and tacking. Here's the finished result of the two tanks.



All in all it was a real clean setup, just what I was hoping for. The two tabs aren't quite enough by themselves. They're plenty strong, but I'm worried about vibration so I need to hide a third mount underneath somewhere. Figure that out later. Here's how she stands for now.


05-23-2010, 10:46 PM
Very cool John. The bikes taking on more of a nasty slick look, I like it. Glad you got the tank/frame thing nailed down & it looks like it was worth the effort.

05-23-2010, 11:12 PM
Any ideas as to how much power you'll get out of this? Looking good.....

05-24-2010, 1:51 PM
That thing is a beast, turbos are a blast.

05-25-2010, 7:14 PM
I think John is shooting for 1 zillion hp......may have been 200hp , I cant remember.

05-25-2010, 8:23 PM
Hard to say. I'll be easing my way up on the boost. Starting with just a couple pounds til the motor's good and broke in. Even then, I'll probably keep it down around 8-10psi for daily use although the kit is good for ~25psi. I got a mag article of a rayjay install on a modified Z1 and at 25 pounds of boost it was running 220hp to the rear wheel. That's out of a 1016, whereas mine's an 836. I'm aiming for somewhere around 100hp for daily use. Cranked up to full potential it oughta be up in the 175hp range, but who knows what'll break first :)

05-26-2010, 12:11 AM
Finding that happy medium in between blasting around and blowing shit up is the key... can't wait to see a video clip!

05-28-2010, 11:19 AM
that tank install is mint man good job