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fang
Posted: Fri Jul 14, 2006 6:11 pm Reply with quote
Junior member Junior member
Joined: 19 Oct 2005 Posts: 23 Location: Austin, Texas, USA
I just got my '78 CB750 top case back from the machinist -- now my big sleeves fit in there! It is finally time to put my motor back together. It has a Henery Abe 900cc kit. As I went through everything before I gat started I had a few questions come up. (I think it is silly that head gasket sets do not include the six Cylinder Head nut Rubber Seals. What a pain!)


1. I am thinking about the primary chains. I have visually inspected them, but they difficult to evaluate. Axle, what do you recomend to evaluate these? There is some (minimal) visible wear on the metal body of the tensioner, and the roller is slightly chewed up. I have a spare motor (a '76), and it's tensioner looks almost like new. I no longer have those chains. I also have another spare motor, but I have not yet split the cases. I am running low on money at this point, so I don't think I can afford new primary chains (for about $160.00) if they are not needed. I have never had a primary chain fail, but this is also my first fairly modified CB750 motor to build. (I have build several basically stock ones.)

2. M3 Racing and other places offer a few things that they advertise as "must have" upgrades for big bore/high performance motors like I am trying to build. What do you think about stuff like their "Heavy Duty Primary Drive rubbers," and stuff like that? Or in other words, what are the upgrades that you have identified as necessary for builds like mine? I have a heavy duty cam chain. Do you think that the M3 Racing cam tensioner is really important?
(click image to link to M3)

Here's my list:
Abe Henry 900cc kit
CB1000 sleves machined to fit (non-tapered at bottom) -- they're thick!
APE heavy cylinder studs
heavy valve springs w/ Ti retainers
Viton valve seals
55-422 Webcam (for big bore)
heavy cam chain
Web Cam adjustable cam gear
copper head and base gaskets
forged con rods
what might be a R.C. crank -- came with rods, and seller claimed it was ballenced...
maybe some other stuff I forgot

Thanks for your input. Axle, you're the best!
-Fang (in Austin, TX)
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Satanic Mechanic
Posted: Sat Jul 22, 2006 1:44 pm Reply with quote
Moderator Moderator
Joined: 04 Sep 2005 Posts: 309 Location: Munich, Germany
First, congrats on your motor! It's a pity the 'donuts' didn't come with the gasket kit - some incude them, some don't, but if they are old, you get mysterious oil leaks around the head.

As for the primary chains, I would first check them for equal length. Just put them on the crank, or just arounf something eles and let them hang free. Are they different in length? Or, if you have installed them, do they have different slack? If they do, you will have to bite the bullet and go for new ones.

If they look ok, you can so another check: Put them on the primary drive sprocket (on the cltuch basket) and try to lift them off. You should not be able to lift it for more than halt the tooth height.

If they passed these two tests, you are pretty safe to use them again. You may want to shim the tensioner spring a bit while you have the engine apart.

The 'HD primary drive rubbers' are a good idea - the rubbers tend to harden over time, and if they do, they put additional stress on the primary drive. It's not as important that they are stronger than the stock rubbers but the rubbers have never been available from Honda seperately.

As for the cam chain and the M3 tensioner, it much depends on the camshaft and the valve springs. In the old days, some racers were using very stiff valve springs - sometimes they had to because of poor cam profile design Rolling Eyes but if your are using a modern cam and I would think the web cam 55-422 qualifies for that, altough I have no personal experience with it, youz should be ok with the stock tensioner and even the cam chain. (But since you already have it, I would install the HD cam chain - its always better to be safe than sorry Wink )

Be sure to check the valve clearance when installing the cam - the Henry Abe pistons have only small pockets which most probably need some machining to allow for the Web Cam cam.

While you are working on the pistons, if you have access to a digital letter scale or such, spend an hour and bring all the pistons to equal weight.

Also, if your head is still stock, the least you can do is use a polisher and smoothen the transitition between the intake ports and the head a bit - takes only little time and has a great effect.

What else - oh yes, you should taper the sleeves before installing the pistons - otherwise getting the rings in would be a major PITA.

Good luck, and keep us posted how it comes out!

Axl
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fang
Posted: Sat Jul 22, 2006 8:36 pm Reply with quote
Junior member Junior member
Joined: 19 Oct 2005 Posts: 23 Location: Austin, Texas, USA
Thanks Axl I sure appreciate your input. I have been benefiting from your extensive knowledge here on this website (the old one) for quite a number of years now. Thank you!

I painted the cases, head and cylinders yesterday (aluminum engine paint with black cylinders). It all is looking really good.

My new primary chain arrives in a few days. It was not all that expensive and I decided that it was worth $90 to protect my several thousands already invested in this project. Plus I was not sure I was ready to trust a 30+ year old chain from a motor that I have never personally run.

The primary rubbers... I cannot move the twin gears by hand, but I can with a screwdriver wedged between them. I think they are OK, besides they are a real pain to replace and I spent that money on the primary chain set. I am broke for another two weeks, and I am hoping to start assembling the motor before then.

I never thought about the valve to piston clearance. I'll have to carefully check that.

My sleeves are beveled on the inside for ease of piston installation, but the outside is not tapered. Yeah, I can't even imagine the horror of trying to get the rings in there otherwise!!! I remembered th have the machinist do that when he cut them for me.

peace and grease
-fang
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