Finding True TDC

Finding top dead center can be done numerous ways.  You can use the stock markings on the engine block and the advance mechanism, you can use a dead stop, you can watch a screw driver move up and down, you can even yank the head and use a dial indicator on the piston.  All of these are valid methods... To a degree (or several).  But when you're setting things up, why not do it the best way possible?  Especially when this precise method doesnt take much longer than simply lining up the stock marks.

The stock marks on the CB750 can be off by a few degrees.  Ever notice that the little pin on the back side of the advance mechanism doesnt fit very snug into the detent on the end of the crankshaft?  This adds one error.  Another error is the size of the arrow casted into the top case that you line up the T-dash mark with.  That casting mark imay not be held true to the line boring of the crankshaft over the years.  Then there's the distance between the dash mark and the casted arrow versus the angle of your eye through the viewing window of the points plate.  Etc. etc.  Propagation of error.

The errors in using the stock timing system can add up.  Whether it's significant or not is up to you.  For me, and since the dead stop method is so simple, it is significant.  So heres how to get true top dead center using a degree wheel and a dead stop.

The above picture shows you all the tools you need to find true Top Dead Center (TDC) using a dead stop.  A degree wheel and a dead stop.  The dead stop I'm using is made from a gutted spark plug and an M8-1.00 fine thread bolt.  A dead stop is a tool that is placed into the spark plug bore and is used to stop the pistons travel in the cylinder.  The bolt can be adjusted until it is in the path of the piston.

First mount your degree wheel on top of the advance mechanism using the long M6 stud.  Remove the large nut and mount the plate against the teeth protruding from the tip of the advance mechanism.  You dont need to tighten it down very much, but make sure that it doesnt move easily and stays mounted solid to the advance assembly.  Next take a small piece of wire and secure it to somewhere on the engine.  Adjust it so that it acts as a pointer on the degree wheel.  Again, make sure that the pointer is very secure and will not move.  Set the degree wheel so that the pointer is at 0.

 

Next place the dead stop into cylinder 1 or 4 (shown above, between the fins).  I use cylinder 4 since it's on the side of the engine where I'm working.  Run the bolt down a bit and turn the engine by hand until it stops rotating (i.e. hits the dead stop).  DO NOT USE THE STARTER TO ROTATE THE ENGINE.  If the engine rotates without hitting the dead stop, adjust the bolt down further.  Again, rotate until the engine hits the dead stop.  Once you've hit the dead stop, look at the degree value on the wheel.  Mine is reading 51* ATDC in the picture below.  Since we've set the degree wheel to zero initially, we simply divide the number indicated by the pointer by 2.  51 divided by 2 is 25.5. 

 

Move the degree wheel so that the pointer is now at 25.5*.  Moving towards 0* of course.  Following along, my movement can be seen in the picture below.  Only move the degree wheel during this process, not the crank.

 

Now, rotate the engine in the opposite direction until you hit the dead stop from the other direction.  When the piston hits the dead stop, look at the value on the degree wheel.  It should be opposite of your other reading, but identical in value.  My reading, shown below, is now 25.5* BTDC.  If everything was done correctly, the value you adjusted the wheel to previously and this new value should be the same. 

Now we know where true TDC is.  Remove the dead stop and rotate the crank until the degree wheel is at 0*.  This is true TDC.  See the below picture... and my knee in the clutch cover.

Now, just to show you how much my plate was off, here is a picture of the alignment of the T-dash mark on my advance mechanism lined up at true TDC.  It's a little bit to the left, which means that if I used this to mark my TDC, I would have been a little retarded (~2*).  No pun intended.

 

Again, the significance of the difference between true TDC and the TDC determined using the stock marks is up to you.  When installing a new cam, making ignition adjustments, etc, etc.  precision is key.  Especially considering you're already working with a few degrees in either direction.

Free Horsepower: Reducing Friction Losses in the CB750 Valve Train

There have been many subtle but significant changes to the internals of the CB750 engine over the years.  Many of these changes were to increase the "comfort" and "feel" of the bike rather than increase performance.  Manufacturing processes and tolerances drifted over the years, which with all these things combined, effectively dropped the output from the noteable 65 rwhp of the sandcast engine, to the 40-45 hp dyno pulls seen from some of the mid 70's bikes.  By reversing some of these changes, one can regain this horsepower using stock parts and a little machine work.

One such change to the valvetrain of the CB750 was the modification of the rocker shafts.  The rocker shafts on the early engines (K0-K2) were not bolted in and were free to spin within the rocker tower.  This freedom provided significantly less friction and wear at the rocker pivot.  Less friction = more power.  However, with this freedom came a tad bit of top end noise.  Based on some information in My CB750 Book by Mark "Hondaman" Paris, Honda was interested in reducing the overall noise and increase ride comfort after noticing that many people were using this bike for touring.  That or quell some warranty issues.

In order to reduce some of the top end noise, Honda bolted the rocker shafts in place.  This prevented them from spinning and eliminated the chatter created by the shaft walking side to side as it rotated in the rocker tower.  It also prevents the shafts from completely walking out of the tower if they broke.  Simply bolting the original shaft down created wear problems on the underside of the shaft where the rocker pushed against it.  The wear was significant and Honda responded by adding a small groove on the shafts under the rockers to catch and hold the slung oil in the top end.  This reduced the wear from the original bolted shaft, but since this shaft was also bolted down, the wear was focused on the underside of the shaft.  Preventing the shaft from rotating may have reduced noise and increased the "comfort" of the riding experience, but it increased wear and friction losses.

Making the rocker pivot on only one side of the rocker shaft focuses the wear and can cause the shaft to become oval at this point.  The chatter from the original unbolted shafts was minor, and if you can live with a little bit more engine noise, you'll be rewarded with less top end wear and friction losses. 

Below is a picture of three different rocker shafts.  The lower one is the early model, with no hold-down bolt holes.  The middle one is a Honda factory replacement shaft produce from the early 2000's.  The top one is the final design with the oil grooves under the rockers.  It's interesting to note that Honda made a replacement shaft with a beefier middle, but without the hold down bolts "to minimize top end noise". 

Here are a few more pictures of the rocker tower assemblies as they developed.  The first picture below is an early model rocker tower.  Note the abscence of the 5 mm bolts.  The second picture below is the final design, with the 5 mm bolts and grooved shafts.

According to Hondaman, the ideal fix for this in order to maximize power to the wheel and reduce friction and wear, is to unbolt the shaft and free it from the shackles of those small 5 mm bolts.  The early shafts can be put in place of the bolted shafts, using the later rocker towers with the bores for the 5 mm hold-down bolts.  This is one fix, but if you already have the later model shafts with the bolt holes and the oil groove, ithey can be used to increase oiling.  Take the late model shaft, deburr the 5 mm bolt holes, and reinsert into the rocker tower.  Don't put the 5 mm bolts back in.  Let it rotate and the bores for the 5 mm bolts through the rocker tower will collect oil and lubricate the shaft where it runs through the longer bore of the rocker tower.  The grooves under the rockers will aide in preserving oil between the metal surfaces. 

Unbolting the rocker shaft and allowing it to float does present a risk.  There is potential for the shaft to break in the middle where the groove is cut to clear the 6 mm bolt.  If this happens, the shaft can walk out and jam the rockers and the cam chain.  This would destroy your top end and potentially send lots of metal down into the cases.  If you are using a high lift cam, heavier spring rates on your valve springs, or if your engine typically sees rpms near or above redline, it may be safer to keep them bolted down.  If you really want to keep the shafts floating with a high lift cam or other mods, find a suitable way to block the shafts from walking out in the event they break. 

Additional modifications are discussed in Hondaman's book My CB750 Book - on putting them back on the road.  I firmly suggest that everyone who owns a CB750 have this book as a supplement to the Honda shop manual.  One of the other modifications that Hondaman suggests is adding more oiling holes to the rocker tower by drilling small (approx. 1/8") holes into the 8 round bosses on top of the rocker shaft bores.  In the picture above of the late model rocker tower two of these bosses are between the 5 mm bolts.  These holes will add more oil to the now rotating shaft and may reduce the friction even further.  It is also suggested that the four holes in the small outer rocker shaft bores be omitted if a high lift cam is being used. 

For the work I recently did on the top end of my engine, I used late model rocker towers with very low wear early model rocker shafts.  I did not add the extra oiling holes as I am running a very high lift cam and also feel that the 5 mm bores are sufficient oiling cups.  Though a few extra SMALL holes are certainly not going to hurt.  I will put up a few pictures of my final setup soon.

For more nearly free modifications and everything you've ever wanted to know about the CB750, check out Mark "Hondaman" Paris' book at: http://forums.sohc4.net/index.php?topic=65293.0

Or Directly from the publisher:

Hardcover:

http://www.lulu.com/shop/mark-paris/my-cb750-book-hardcover-edition/hardcover/product-16107855.html

Paperback:

http://www.lulu.com/shop/mark-paris/my-cb750-book-paperback-edition-85x11-size/paperback/product-15148673.html

 

Dyna 2000 for the Flute

Mmmm. Digital. I have both types of coils to see which works best. The minicoils are a tiny bit smaller and weigh a little less but I don't see them creating a lot more usable space over the traditional coils. They do line up the outputs a lot better though.

I'm also working on putting together a frequency-to-voltage ic with a bar graph driver ic  to build a very small digital tach with an adjustable sequential shift light. A few other tweaks and I could make an adjustable rev limiter with this circuit; however, with the Dyna 2000, an external rev limiter is not necessary. I might just build one to play around with the circuit.

Presents!

Replenishing the dwindling gasket inventory... and a few other goodies.

Rock Flute: cylinder rehoning and top end install

I originally started to honed the cylinders with a ball hone but the quality of the finish was lacking and there wasn't really an effective removal of some spots. I went through another 20+ passes with a 3 stone hone and things cleared up much nicer.  I honed until the discoloration from combustion at the top of the cylinder was nearly gone. This was suggested by Mark Paris aka "Hondaman" in his book, as well as a few others I've spoken to. The goal was to remove the lip that is created on the cylinder at TDC. The lip is the upper endpoint of the ring travel.  Once I noticed that the bottom edge of the dark spot had faded significantly, I gave a few more passes with the goal of using the flat stones to blend away any lip that was there. It was suggested to get rid of all discoloration, but since this engine doesn't have many miles on it, I felt I could be conservative with the honing.

In the pictures below you will see that I installed the rings into the cylinder by leveling all of the pistons and clamping the rings with hose clamps. Again, this is another trick I got from a few old heads, Red Good of Cycle One Manufacturing, and Hondaman. Sand the inside of the hose clamps smooth first.  You don't want any small burrs to scratch your rings up.  I assembled after washing the cylinders until tissue paper came out spotless and used only enough oil to prevent the cylinders from flashing. 

In order to slide the rings into the cylinders, just rest the jugs on top of the hose clamps and loosen each clamp a little at a time. Tap on the cylinders GENTLY and the whole assembly will eventually drop over the pistons. What was easily an hour job by hand only took about 5 minutes.

Next came the head. I'm using APE cylinder studs that have already been run. They have already spent a significant portion of time in tension and going through heat cycles. This is important because new studs need to be set and re-torqued for their initial application since they will stretch a bit and loosen their hold on the head. Typically new cylinder studs are re-torqued after a few hundred miles or they are assembled, allowed to sit for at least 24 hours to let them to stretch and then re-torqued.  Even though my used studs have already yielded to their operating length and are ready for final a final torque, I torqued with wet threads to 20 ft-lb and let them sit for 72 hours.

Keep in mind that oiled or wet threads will apply more force with the same torque applied as dry threads. This is because some of the torque applied when tightening a nut onto dry threads is used to overcome the friction between the stud and the nut.

Once the studs have set at 20 ft-lb for 72+ hours, I loosened each nut individually and torqued to 19 ft-lb. I marked the torque pattern on the head and followed this for both steps. The APE studs can be torqued up to 22 ft-lb but I feel this would only be necessary for high compression or turbo applications. Even then its still a lot. To ease the stress on the studs, I torqued to 19 ft-lb with a tiny dusting of oil on the threads.

Single Overhead Cam

New cam showed up in the mail today. Megacycle 125-75.
Lift: .400 int .375 exh
Dur: 262 int 257 exh

Grrrr.

Ramflo Knock-offs

John bought a set of these a while back for one of his bikes, but I stole them. They came blank so we could cut them for any carb rack. I'm putting them on a set of vm29s.

I have very limited space for filters on Rock Flute so I am mounting the 44 mm boots backwards on the base plate. This allows the filters to sit further in on the carb body and the clamps will be hidden inside.

I used a fly cutter to make the holes on the base plate. They'll be snug enough to where I don't need clamps to hold them on as space is limited inside the filter.

I think they turned out pretty good!