CB750 Oil Change Tips

The CB750 is the only SOHC/4 model with a dry sump.  As a result, the oil change process is more involved than just removing the sump bolt since you need to drain the oil tank as well.

Excellent set of instructions posted by Jonesy:

Here’s how I do it (FWIW…)

  • Start up bike and let it run for about 10 minutes. This warms up the oil and gets things roiled up inside, but not so hot you’ll burn yourself. This helps lets the oil pick up crud and let it drain away with the oil, rather than having it settled out and stay in the engine.
  • After the bike is warmed up a bit, shut it off and put it on the center stand.
  • I start with the oil tank first. I remove the sidecover and temporarily undo the rear brake light switch to get it out of the way. I also crack the filler cap. Have a drain pan ready and remove the plug. I have a piece of aluminum that I bent into a small trough to keep the oil from running all over the frame.
  • After the tank is drained, replace and tighten the plug. Proceed on to the oil pan plug and drain the oil from there. Again, replace and tighten the plug.
  • With the oil pan plug out, kick the engine over a few times to pump out the remaining oil
  • Lastly, remove the oil filter housing. If all the parts are in the housing, there is a thin washer between the oil filter element and the spring. This likes to stick to the filter and before you know it, you’ve thrown it away. (This is probably why 90% of them are missing)
  • In other words, when you remove the innards, don’t lose it!.
  • Wash out any sludge or particulates that have collected in the oil filter housing. Take a good look at what’s in there, as any sizable bits of metal might be a warning that something’s on it’s way out. In some cases it’s worthwhile to drop the oil pan to give it a good cleaning and look for any signs of trouble.
  • If you bought an oil filter kit that includes new O-rings, fit the small O-ring on the oil filter bolt and seat the big O-ring in the groove of the filter housing. Apply a bit of clean oil to the oil filter bolt O-ring to make it easier to reinsert into the housing.
  • With the bolt back in place, reassemble the oil filter housing with the spring first, then the washer and finally the new filter. Reinstall the filter on the engine, being careful not to overtighten the bolt.
  • Reinstall the sidecover over the oil tank and put the brake switch back in place.
  • Fill the oil tank with 3 quarts of oil. According to the owner’s manual, the oil level will settle into the correct range when the engine is started. So far, I’ve found this to be true.
  • I like to hit the starter a few times with the emergency stop switch off to circulate the oil into the empty filter housing before running the engine. After doing this, fire up the bike and make sure the oil light goes out in a few seconds.

Again, this is how I like to do it. It doesn’t HAVE to be done this way, but I hope it’s helpful.

From jaknight:

If you happen to want or need to drop your oil pan (I obviously don’t know how familar you are with these bikes), just a heads up to keep track of what holes the different bolts come from; they do vary in their length.

If you happen to need the torque specs for the different bolts:

OIL TANK DRAIN PLUG – - – - – - 24 FT LBS  (ideal median)

CRANKCASE DRAIN PLUG – - – - – - 24 FT LBS (ideal median)

OIL FILTER HOUSING BOLT – - – - – - 20 FT LBS (ideal median)

OIL PAN BOLTS – - – - – - – 7 to 10 FT LBS (pick your median)

From toycollector10:

On an older bike you will probably have a lot of gunk sitting in the bottom of the oil tank.

After you have drained the oil put a screwdriver down there and see what you bring up stuck to the tip, and use a flashlight to have a look inside the tank. If it is a mess down there I recommend you pull the tank off the bike and clean it out properly.

and Hondaman:

Although the torque on the oil bolt is listed at 20 ft-lbs: for many years, I’ve only tightened them to (snug + 1/8 turn). This works out around 7-8 ft-lbs on a T-wrench. I’ve always done this because the oil filter housing distorts and locks that $%#*! bolt in something awful, often causing its destruction the next time around.

…just some experience. It won’t leak if your large o-ring is new…

CB750K and CB750 Engine Differences

This table of CB750 engine modifications was compiled by Axl Griessman at satanicmechanic.org, and is based on Reinhard Hopp's book, "Honda CB 750 - Die Geschichte einer Legende".

Engine #Changes
CB750E-1000001K0
CB750E-1000140Bolt at the final drive changed from M8x80 to M10x82
CB750E-1000220new lower case, engine case only available as set
CB750E-1000425Transmission changed for better shifting
CB750E-1001081new crankshaft
CB750E-10017607 instead of 8 clutch disks
CB750E-1003528new final drive shaft and sprocket
CB750E-1005307fixed right bearing of main tranny shaft
CB750E-1007220new lower case, deeper oil pan
CB750E-1007415introduced circular opening in lower case
CB750E-1007500End of sandcast engines
CB750E-1009554finned oil filter casing
CB750E-1010336cam holders with additional oil passages, new rocker shafts and valve cover
CB750E-1014996new exhaust valve guides, new valve guide seals
CB750E-1026144new cylinder head, cylinder block: new central M6 bolt, cyl. block w/8 rubbers i/o 12. Case with guard to prevent damages from broken drive chains. New sprocket cover, new shift mechanism w/11 parts i/o 7
CB750E-1042806weaker clutch springs
CB750E-1044813K1
CB750E-1044805new case, final drive 18/48 i/o 16/45 or 17/45, chain oiler
CB750E-1044812carbs w/links i/o individual cables
CB750E-1044968improved neutral position
CB750E-1056080new clutch basket
CB750E-1064903introduced special washerless bolts
CB750E-1068376cramp for scavange hoses
CB750E-1071336fuel hoses 5.5mm dia. i/o 5.0mm
CB750E-1113723grooves in gear shift drum changed from 0.5mm to 1mm for 4th and 5th gear
CB750E-1114461new head gasket
CB750E-2000001K2
CB750E-2061311shorter oil hose w/white mark. Main jets 110, smaller cutouts in air filter case
CB750E-2200001K3
CB750E-2200001new cylinder head, new valve guide seals, improved oil passages to camshaft, new piston rings (three part oil scavenge rings), new cam chain guide, larger piston pin circlips, new final drive shaft, new sprocket cover
CB750E-2228679new cylinder head, new valve guides, new valve guide seals, no more rubbers between fins
CB750E-2300001K4
CB750E-2304501new cylinder block, smaller cam chain tunnel
CB750E-2352923new cylinder block
CB750E-2348093crankcase mission cover now has gear indicator
CB750E-2372115K5
CB750E-2428762K6
CB750E-2470427clutch almost indentical to F1: new clutch basket, new clutch cover and chrome cover
CB750E-2434657 to CB750E-2439607new gear shift drum, new center shift fork for improved shifting. New gears on countershaft w/bronce bushings, new case
CB750E-2700001K7 - carbs w/accelerator pump, shorter intake rubbers, new final drive shaft, sprocket w/center bolt, wider chain line (as F2), final drive 15/41, new clutch, new camshaft (as F1), new pistons (as F), compression ratio 9.2/1. New transmission: 4th gear 1.133 i/o 1.087. Final drive gear 50 teeth i/o 56 (as F1), primary drive 1.985 i/o 1.708
CB750E-2719530new valve guides
CB750E-2719997new cam carriers (as F2)
CB750E-3000001K8 - new intake and exhaust valves, new valve spring retainers and cotters
CB750E-3021913new cylinder block w/larger cam chain tunnel
CB750FE-2500004F1 - new camshaft, carbs w/additional idle air. New pistons, compression ratio 9.2/1. New case w/o primary chain oiler. Final drive gear ratio 43/50 i/o 48/50. 4th gear on main shaft 31 i/o 30, 5th gear on countershaft 31 i/o 32
CB750E-2600014 & CB750GE-1000014F2 - new cylinder head with bigger valves (34/31mm i/o 32/28mm), larger combustion chamber, larger carb mount rubbers, stronger cam chain, new camshaft, stronger valve springs, new retainers, cotters, new pistons compression ratio 9/1. New rod bigend bolts and bearings, stronger clutch springs, additional fins on crankcase, larger fins on oil pan, "oil cooler" - finned plate between oil filter case and engine. Final drive 15/43 or 14/43.
CB750E-3100001F3

CB750 Engine Removal & Replacement Tips

Quote from: jdigga on June 08, 2007, 11:19:07 PM

So the weeping from my fins is at the point where stuffing half a Shop Towel in between isn't enough to stem the spray.  I'm a bit tired of having to wear a specific pair of oil-stained riding jeans, so it's time to make the necessary repairs.

I started the tear-down around 5:00pm.  Most everything came off without any problems.  I did have to use an impact driver on the front sprocket cover and found to my dismay that the sprocket bolt was loose!  Luckily it appears that the cover has a little shaft on the inside to keep the bolt in check.  I also discovered that the rear brake splined shaft is slightly bent, but it functions just fine.

I knew the actual removal of the engine would be challenging, but I had no idea what I was in for.  I tried putting the bike on its ride side but I could get the right angle to pull the frame off.  The engine was getting hung up everywhere, and as soon as I'd release one spot, it would hang up somewhere else.  I must have put the bike upright and back down again half a dozen times.

By now it's around 8:30.  Took a breather for dinner and browsing this forum for some tips.

Went back out for one last attempt before calling it a night.  The bike was on its side and while staring at it I realized the engine wasn't going to come out that way.

I put it back upright on the centerstand and broke out my set of lady fingers.  Shoved one in the lower right rear mount hole, one underneath the rear of the engine, and one in the upper right rear mount hole to act as a handle.  Pried the rear up to clear the lower mount.  Pulled up the front of the engine to move it towards the right a bit.  Basically I walked it out of the side, front-back-front-back.

Turned off the garage lights and closed the door at 10:30pm.

In the process of putting the bike down so many times, my points cover took a beating (even though I put it down on those rubber puzzle mats).  The right bar on my clubmans is pointing slightly more downward than the left.  I forgot that I left my key in the seat lock and it broke off--I was able to retrieve the bit inside the lock, but I only have one key.

Honda made a huge access improvement in 1979 with the DOHC bikes.  The lower right frame rail is detachable and the engine pretty much falls right out.  I pulled my other engine the other week, and other than it being a heavy SOB it was hardly a challenge.

And to think I'm only 1/4 of the way there.  I still have to take the engine apart--I have no doubt I'll run into problems there.  Plus all the cleanup of old sludge buildup and repair the busted up bits.  Then put it all back together again.  Don't get me wrong--I love tinkering with my bikes and learning about them.  But sometime it's just frustrating as hell.  I'll feel better when I'm back on the road again.

Just had to get that off my chest...

Before you put it back in....there is a flange on the motor mount on the lower right side. Grind or file the top of the flange off to the height of the rest of the mount, then shoot a bit of paint on it. This makes the whole assemble-disassemble job MUCH easier. This little flange has probably left several notches in the rib on the bottom of the engine where the bolt goes thru, which can actually jam the engine so badly on the way out as to require a frame cutout to unjam the whole thing (don't ask me how I know that....). But, removing this tiny little flange top will prevent all of this...

Rings for the Henry Abe 900cc Piston Kits (CB750)

Quite often the guys that have the 900 Henry Abe kits for the 750′s are looking for ring sets. I just got off the phone with Ed at Total Seal (www.totalseal.com) I shipped my 4 NOS Henry Abe pistons to him to fit or cut & fit for 3 piece oil rings. The good news in that Total Seal can supply all 3 rings (top, 2nd, and 3 piece oil) without cutting the piston. Henry Abe made 2 different 900 pistons however and my qualifier here is that I have the smaller/lighter of the 2. Axel’s site (www.satanicmechanic.org) lists my pistons as the slipper type.

Here are the Total Seal part numbers:

Top Ring           203778
2nd ring            201336
Oil Ring 3 piece  001549

Hope this can help others.

Jerry

CB750 Service Bulletins

Bulletin NumberIssue DateSubject
Recall Letter08/13/1971Safety Recall of pre-K1 CB750's to modify the drive train to reduce drive chain shock loading and extend drive chain life.
750 #409/22/1969Drive Sprocket Factory Recall
750 #611/3/1969Tachometer Gear Stopper Bolt Security to Prevent Cylinder Head Damage
750 #710/17/1971Wing Bolt Retaining Nut Security - Early CB750 Air Cleaner Covers
750 #811/6/1969Ignition Timing With Bent Advancer Shaft
750 #905/07/1973Vacuum Guages: Model Differences and Calibration Instructions
750 #1012/10/1969Dead Batteries - Loose Alternator Rotor Set Bolt
750 #1111/01/1971Brake Lever Free Play Adjustment
750 #1301/26/1970Modification to Prevent Oil Filter Case Damage
750 #1410/06/1972Crankshaft Bearing Insert Selection
750 #1501/31/1975Modified Throttle Valve and Cable Kit
750 #1605/20/1970Camshaft Holders and Caps
750 #1806/13/1975Drive Chain and Master Link Selection
750 #1907/02/1970Adjustment of Cam Chain Tensioner
750 #2109/03/1971Drive Chain Maintenance
750 #2208/28/1970After Sales Service: Modified Chain Cover and Chain Oiler Parts
750 #2309/17/1971Brake Pad Wear
750 #2509/21/1971Carburetor Adjustment, CB750K1
750 #2612/23/1970Front Brake Disc Mounting Bolts
750 #2802/09/1971Drive Chain Link Removal and Installation Tool for CB750K1
750 #29Crankcase and Final Drive Bearing Replacement
750 #3005/21/1971Drive Chain Oiler Adjustment and Replacement
750 #3108/28/1974Connecting Rod Weight Selection and Size
750 #3208/09/1971Drive Chain Recall Notice
750 #3311/03/1972Wiring Harness and Breather Tube Routing, CB750K1 & K2
750 #3405/04/1972Disc Brake Caliper Noise: CB750 & CB750K1
750 #3511/17/1972New Molded Countour Oil Hose
750 #3604/06/1973Rear Brake Wear Indicator, CB750K3
750 #3711/19/1973Wheel Sprocket Diameter and Chain Pitch Compatibility
750 #3811/26/1973Muffler Warranty Extension
750 #3902/01/1974Modified Head Gasket Removal
750 #4004/01/1974Modified Shift Drum
750 #4104/05/1974Front Fork Oil Seal Change, CB750K1 & K2
750 #4207/10/1975Cylinder Head Gasket Oil Sealing
750 #4301/10/1975Battery Cable Extra Lead
750 #4404/18/1975CB750F License Plate Bracket Plug
750 #4503/24/1976Soft Mounting Cushions Improve Instrument Accuracy

CB750 Charging Rate Chart

This charging table provides charging system ouput for various engine RPMS during day (headlight off) and night (headlight on) conditions.  This data can be used to test and adjust the output of the charging system.

Engine RPM Night Riding - Amps Day Riding - Amps Battery Terminal Voltage
1000 -0.5 0.7 12.0
2000 0.0 1.0 12.4
3000 2.4 1.0 13.2
4000 1.3 1.0 14.5
5000 1.0 1.0 14.5
6000 1.0 1.0 14.5
7000 0.8 1.0 14.5
8000 0.6 1.0

14.5

Can you use a late F cylinder with the K heads?

Dr Rieck says the following:

…once and for all.

First, the 77/78F2/3 heads shared a common head drain back hole which early K,F and late K heads did not. This eliminated the middle drain back holes in the F2/3 heads. The 4 studs next to the eliminated drain back holes require cap nuts and copper sealing washers on the F2/3 head only.

The late F2/3, 77/78 cylinders CAN accomodate the knock pins(or dowels) and the rubber gaskets which allows you to run these early/late K and earlyF/F1(75/76) heads.

The later F2/3 castings are good as they are more stout but this does add weight.

Trying to run a Wiseco 836 kit with an F2/3 head will result in low compression secondary to the increased volume of the F2/3 chamber.

Bottom line….early/late K and early F1(75/76) heads will work on late (F2/F3) cylinders and early/late K and F1 upper cases with mild opening (joining) of the F2/F3 two inner drain back holes at the base of the cylinder.