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Discussion Starter #1
Everyone on here is so helpful and knowledgeable so I'm hoping someone can provide me with some assistance with this issue.
I am doing a complete motor rebuild for a 1970 CB350. I have the bottom end back together but when I attempt to assemble the top end I am having issues where I can not get the points housing or tach gear housing over the cam to attach the housings to the cam box. It almost seems like the cam chain is too short because the clearance is just not there. This even occurs when the a chain tensioner is completely removed from the motor (both the plunger housing and the tensioner itself) and all the rockers removed .

When I did the rebuild I bought the KA tensioner and the Tsubaki cam chain. I had the head machined for the needed clearance and the cam chain tunnel in the jug was also slightly opened up for the KA tensioner. I know the KA tensioner is not the issue because this issues still occurs when it is completely removed from the top end.

When this motor was disassembled it had a fat cam. This cam was past its prime so I had to buy a replacement cam along with the rockers (they were bad). I had to buy a skinny cam because a good fat cam seems to be impossible to find. This also means that I had to buy the skinny cam rockers, the skinny cam cam sprocket and the skinny cam cam box. So I have all the skinny cam (312) parts I think I need: cam, rockers, sprocket, cam box.

When I try to assemble the top end using these skinny cam parts I have the issue described above. If I assemble the top end using the fat cam parts (the ones that were originally on the motor before the rebuild) I don't have this issue even with the KA tensioner installed and the top end assembles without any issues. Can anyone tell me if I am missing something. Do I need to change any other parts due to the skinny cam? I disassembled the top end and assembled it back multiple times using the skinny cam parts with the same results. I even put spacers on top of the cam box and tighten it down to bring the top end together but I still can not get the two covers over the cam even with the rockers not installed on the cam box.

Any advice or knowledge about this would be greatly appreciated. Thanks.
 

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I'm not a 350 guy but I have thought. I'm guessing that you have dry assembled the head without the chain to make sure that everything fits that way correct? If the cam bearings, camshaft, rockers and sprocket all fit into the head fine then my bet is that the sprocket is too large which will cause the chain to be too tight. I would also figure that there should be no difference between the sizes of the sprockets between the skinny and fat cams as that would change the cam/crank revolution ratio if the cam sprocket was larger.

Smarter minds should be along soon, maybe one of these days I will run down a 350 to play with.

Welcome to the Forums by the way, feel free to post up in the Introduction section with some pictures of your bike(s). :D
 

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Have you test fit the housings into the cam box without the cam in place, just to see if they will slide into the bores of the cam box?
 

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pictures of the "skinny cam" parts would be helpful to make sure they are in fact the 312 parts. these can sometimes be finicky to install but if the fat cam goes in then the skinny cam will too (assuming you did get the correct rockers with the 312 embossed on them)

install the cam sprocket finger tight with just one of the bolts and get the timing marks lined up
when I install the cam I find it easier to install the left side first with the rockers (make sure the index marks on eccentric adjusters point up and away from the crank so they are at their loosest) and the right side without the rockers and the cam lobes pointing down.
then rotate the cam till all 4 cam lobes are not putting pressure on the rockers
then slide off the right cam box and install the rockers

if you still have difficulty remove the one bolt holding on the cam sprocket and that will take pressure off the rockers/cam
then reinstall the sprocket bolts and recheck timing

spin it through a few times to ensure timing is correct then locktite and torque the cam sprocket bolts
 

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Discussion Starter #5
Frogman79 – I have dry assembled the top end and everything seems to mate well. I am still having issues even when the rockers are not installed in the cam box. The skinny cam and fat cam sprockets have the same tooth count and O.D. (outside diameter). The centers of the sprockets are different to allow for the different sized lobes of the two cams. The fat cam has lobe with a 36.8mm rise and the skinny cam has lobes with a 39.7mm rise. Due to the larger lobes on the skinny cam the center cut out of the skinny cam sprocket is larger to accommodate the larger lobes. The only other difference with the sprockets is where the sprocket mounts to the cam itself the O.D. for the fat cam is 33mm and the skinny cam is 34mm. This is where the center cut out of the sprocket mounts to the shoulder of the cam when it gets bolted to the cam. I don’t see this being an issue as this 1mm difference is being absorbed by the larger cut out in the sprocket.

Fxray – Yes I have fit the housings to the skinny cam box off the motor without the cam. They are a snug fit but as long as they are perfectly lined up they slide in without any persuasion. The ends of the cam also slide into the housings without any issues off the motor so there are no clearance issues there.

Outobie – Below are some pictures of all the skinny cam parts I have purchased. These are the parts I’m using during installation when I am experiencing the issue. I am pretty confident that I have all 312 parts as the rockers and sprocket have the stampings. The cam does not show the 312 markings and neither does the cam box so maybe I have an issue with one of those parts.
I am experiencing this issue even without the rockers installed. I figured maybe the rockers applying pressure on the cam was contributing to the issue so I removed the rockers all together and I still cannot get the housings on. I also install the left side first to minimize the likelihood of damaging the points seal in the housing. When I get the left housing on the cam and on the box it is so tight that it cocks the cam down on the right side. Then when I try to get the right housing on because the cam is angled down on the right side I can’t get the shoulder of the right housing in the cam box. The shoulder on the housing is noticeably too low to ender the cam box I would say a full mm.
The odd thing is once I go back to all fat cam parts (cam, rockers, sprocket and box) utilizing the KA tensioner and Tsubaki chain I have absolutely no issues completely assembling the top end rockers installed and all. With the fat cam parts and top end completely installed (with rockers installed and adjusted to spec clearance) the motor turns freely with any binding as a motor should.
I am thinking that I will have to start with everything assembled with fat cam parts and then change one skinny cam part over at a time until I find the part that is giving me the issue. Maybe the deck of the skinny cam box is taller removing some of my clearance I am not sure at this point.
I do appreciate the input everyone has and please keep the ideas coming.

cam 1.JPG cam 2.JPG cam 3.JPG rockers 1.jpg sprocket.jpg
 

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Discussion Starter #6

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your cam and rockers are correct together. next thing I would ask is...have you tried assembling cam box, end caps with rockers, cam and sprocket off the engine?

you know you can install these things with the cam sprocket off to the side of its mounting flange to get a little more slack
 

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What about your cam bearings I.e the cam box side covers with points on the left and tach drive on the right? Are you reusing those from the fat cam? I think those are different too for the skinny cams..
the cam bearing end caps are interchangeable for all years...same part number from Honda
 

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Jpbrusa, some others who have had this problem decided that the rubber (plastic?) silencer discs on the face of the sprocket were interfering with the Tsubaki chain and not letting it seat fully onto the sprocket. At least one person used a lathe to turn down the O.D. of those discs. The thought is that the Tsubaki chain, having straight-sided links, did not have the same clearance as the original Honda chain with the "figure eight" side plates.

I looked at that on my bike (1971 with skinny cam), and rejected that notion. If the center discs were too large to let the chain seat, it would move the chain far enough out from center that the pitch of the chain rollers versus the tooth spacing on the sprocket would be visibly affected. I wrapped the chain completely around the sprocket (more engagement than it would have when installed in the engine) and found it to be a perfect fit. I say this just FYI. You may look at how your Tsubaki chain fits when wrapped around both of your sprockets.

If it were me, the next thing I would try is what outobie suggested -- fit the housings while the sprocket is barely off its seat on the cam and see how that works out.

I'd also clean that gasket surface on the cam box some more before final assembly.

Good luck with it.

Ray
 

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Discussion Starter #12
Ninadm- as mentioned by outobie I believe all the cam bearing end caps are the same across the fat and skinny cam motors from looking at the Honda parts schematics I have examined.

Outobie- I have not attempted to install the cam, sprocket, rockers and end caps in the cam box off the motor. I will try that this weekend and post my results. If I can do that successfully I will then attempt to install the cam bearing end caps with the sprocket off the cam mounting flange. I know this should give me the needed clearance but then I have to see if I can get the sprocket over the cam mounting flange once the end caps are installed. I will also post my results from this attempt after I attempt this weekend.

Again thanks everyone for the questions and tips/techniques.
 

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Discussion Starter #13
Fxray- thank you for that info about the silencer discs on the sprocket and the Tsubaki chain, I did not think to check that possibility. I will examine how the chain seats in the sprocket, the concept makes sense as I did notice the difference the in link shape of a figure eight vs straight sided links. I will look into that possibility this weekend when I attempt outobie's suggestions.
 

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Discussion Starter #14
So I spent some time this weekend sorting through this issue.

The first thing I did was take Outobie's advice and do an assembly of all the skinny cam parts (cam, rockers, cam sprocket, cam box) with the bearing end caps off the motor. All the parts went together with no issues, plenty of clearance. After that proved that all the skinny cam parts would assemble off the motor without the cam chain I then attempted to fit them to the motor again. I still got the same result, I could not get the bearing end caps over the ends of the cam. The cam chain again appeared too tight. Then I unbolted the sprocket from the cam with the chain engaged and moved it off the cam mounting shoulder. At this point I could get the bearing end caps on the end of the cam but everything was so tight I could not get the cam sprocket back on the mounting shoulder of the cam.

Then I moved on to what fxray had stated about someone experiencing that the rubber silencer discs on the face of the sprocket would not allow the Tsubaki chain to completely engage into the sprocket due to the difference in the shape of the links. This was very hard to determine just by looking but this is what I did see from examining the different parts.
When the stock DID chain is engaged in the skinny cam sprocket there is visible clearance between the bottom of the chain and the top of the rubber discs that are molded to the sprocket. When the Tsubaki chain is engaged in the skinny cam sprocket there is no clearance, the bottom of the chain is coming into contact with the top of the rubber discs on the sprocket. When I engage the old fat cam sprocket in both the stock and Tsubaki chain there is clearance on both. The rubber discs on the fat cam sprocket are noticeably shorter than they are on the skinny cam sprocket when looking at them from a side profile. The profile of the Tsubaki chain is also slightly taller than the profile of the stock DID chain.

So at this point I split the cases again and installed the stock DID chain just for testing to see if I could get the clearance I needed to install the bearing end caps on the cam. With the stock DID chain, KA slider and all the skinny cam parts I had no issue getting the bearing end caps over the cam with everything installed. The bearing end caps went on the cam as they should with no persuasion. The top end moved freely with no issues the KA slider was working correctly with the tensioner and everything was in tolerance.

From these findings I have to believe that it is a combination of the skinny cam sprocket and the Tsubaki chain. If you just look at them engaged from the top you wouldn't think anything of it because the teeth of the sprocket are mating with the chain. When you look at them it doesn't appear that the teeth are sitting abnormally low in the links. The only reason I looked further into this possibility was because of what Fxray stated. I think that this lack of clearance between the rubber discs and the chain is causing the chain to actually run on the rubber discs and not fully seat in the sprocket. This is resulting in the use the needed length of the chain to install the bearing end caps over the cam. This would also explain why I didn't experience this issue when I installed all the old fat cam parts because the rubber discs on the fat cam sprocket are lower profile and still maintained clearance between the bottom of the chain and the rubber discs when they were engaging.

So now I guess I have two options:
1. Take my skinny cam sprocket to a machine shop and have them take down the rubber discs on a lathe to match the same clearance it has with the stock chain.
2. Get a new DID stock chain and do not machine the skinny cam sprocket.

Can anyone tell me if there is a real necessity to run the Tsubaki chain with the understanding that this is a completely stock motor with the exception of the KA slider. There seems to be no clearance issues with the stock chain and the KA slider that I had read about other people experiencing.

I hope that if anyone else experiences these issues that this thread will assist them because it was starting to become quite frustrating.
 

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This issue has come up before, and someone made inquiry with Bore Tech. They replied that the KA Tensioner is designed to be used with the Tsubaki chain and will not work properly with the DID chain. If I can find that other thread, I'll post a link to it.

Ray

{Edit} Here's the link to the other thread where the member turned down the side discs Click HERE.

And here is the link where the member contacted Bore Tech about using a different chain Click HERE.
 

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you can just remove the rubber completely with no ill effects other than the possibility of a little more noise from the chain going over the gear wheel undamped.

the degreed race sprockets you buy don't have the rubber
 

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Discussion Starter #17
photo.JPG

Ok so I took the skinny cam sprocket to a machine shop and they put it on a lathe to remove some of the rubber disc material. As you can see from the picture I now have clearance between the cam chain and the rubber silencer discs. I can now assemble the top end and get the cam bearing end caps on the cam. As before when I did the assembly I used spacers and nuts on top of the cam box to tighten everything together. It is pretty tight but with slight tapping with a dead blow hammer I can get the cam bearing end caps on.

The question I have now is that the cam chain is extremely tight. It is so tight it is as if I don't even need a tensioner (obviously i am going to run one but just for description purpose). The KA slider is tight up against the chain and when you push on the KA slider from the opening in the back of the jug it has very little to NO play. Is it normal to have the cam chain this tight? If I use a DID chain there is more slack in the chain and the KA slider has some movement which would be tightened by the tensioner. I know Bore Tech says that the KA slider is built to work with a Tsubaki chain but I don't have clearance issues with the links of the DID chain against the walls of the KA slider. Am I better off running the Tsubaki chain with a very tight cam chain or running a DID chain?
 

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Discussion Starter #18
Also just for reference so maybe it helps others in the future here is a picture of the skinny cam sprocket and the Tsubaki chain. From the picture you can see that the chain actually rides on the rubber silencer discs and there is no clearance between them. This is before I had the rubber silence discs taken down on a lathe and the picture above in the previous post is after to give them clearance.
before.JPG
 

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The question I have now is that the cam chain is extremely tight. It is so tight it is as if I don't even need a tensioner (obviously i am going to run one but just for description purpose). The KA slider is tight up against the chain and when you push on the KA slider from the opening in the back of the jug it has very little to NO play. Is it normal to have the cam chain this tight? If I use a DID chain there is more slack in the chain and the KA slider has some movement which would be tightened by the tensioner. I know Bore Tech says that the KA slider is built to work with a Tsubaki chain but I don't have clearance issues with the links of the DID chain against the walls of the KA slider. Am I better off running the Tsubaki chain with a very tight cam chain or running a DID chain?
I wish I could help, but all I can do is tell you what I found on my engine. I have the skinny cam, with the discs. I did not turn down the rubber side plates. My Tsubaki chain is rather snug compared to the original Honda chain, but there is free rotation and I could easily make it tighter by using my finger against the KA tensioner before I had reinstalled the tensioner push bar and housing.

When I was installing the KA tensioner, I had to clearance the channel in the cylinder head, the new head gasket, and the cylinder block. I removed the most material from the head, and only a slight amount (how's that for a finite definition?) from the cylinder block and head gasket. I made sure that my KA tensioner could move through its entire path fore and aft without rubbing the sides. Its rearmost surface, the push-pad if you will, would come out about flush with the surface where the tensioner push-bar housing mounts to the back of the jugs. Until I installed the push bar, my KA tensioner was not tight up against the chain.

I have the bike mostly together, but got delayed waiting for a good day to paint the starter motor and the LH side covers. Then when I got that done, I discovered that I did not have a new O-ring for the nose of the starter. I am waiting on that to arrive before I finish up. So, I can't give you any more feedback at this point. I'm wondering:

  • Do you really have full motion of your tensioner? Does it fit like I described for mine?
  • Is there undue mfg variation in the Tsubaki chain and/or the KA tensioners? I would doubt this, but it is possible. Mine and some others on here went together without much issue. On the other hand, you are not the only one to have the difficulty you are describing.
Ray
 

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Discussion Starter #20
Ray – I do have free motion when I install the slider without the chain. When I started this project I took my head and jug to a machine shop. They machined the bottom of the head in the cam chain tunnel to allow the slider to enter the tunnel in the head and move freely. They also took some material out of the cam chain tunnel in the jug mostly just extra casting to ensure the slider does not drag. When I assemble everything without the chain the slider has enough motion that the back of the slider actually rests against the back of the jug where the tensioner mounts to the jug.
I called Bore Tech this morning and they told me that using the DID chain would work but it is not advised because the shape of the links (figure 8) would cause excess wear in the slider. He stated that because the links in the Tsubaki are flat it eliminates the chain from wearing into the slider. He also told me that he has seen motors go together where the cam chain was tight like I described. He said he didn’t know if it was variations in the cases or other parts but he said he has also witnessed what I was describing. He said that as long as the motor turns freely (which it does) it should not be an issue. He said that between having the head torque to spec (vs just the spacer and tightening it down) and the motor ran for the first time the chain will have some initial stretch and I should gain some slack.
Here are two pictures to show how much movement I have in the slider with everything installed including rockers and cam bearing end caps. One picture is how it naturally sits and the other is me applying good force with my thumb. From the picture you can see that I am getting a little bit of movement but not a lot and I am using a good amount of force to get this movement.

Is this about the amount of movement you are used to seeing and does it take a good amount of force to get this level of movement?
ten 1.JPG
ten 2.JPG
 
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