Cam bushes

Hi, in the workshop manual specs they say the cam bushes have one and a half thou clearance to cam. I would like to know at how much clearance would it be time to replace them? Mine has three thou clearance. Thanks Dano.

When you can't find out where the rattle is coming from ?

If its open and you can change them - do so, it makes sense for what they cost.

Dano said:

Hi, in the workshop manual specs they say the cam bushes have one and a half thou clearance to cam. I would like to know at how much clearance would it be time to replace them? Mine has three thou clearance. Thanks Dano.

Click to expand...

Good question. don't know the answer, but an Engine Reconditioner would know. I guess if its done over 100,000 miles they may need changing.

Dereck

I thought you were a reasonably intelligent man Mr acotrel but your response proves me wrong, Dano

.003 clearance is about average for an engine with some miles on it. It is not enough to cause a problem.

I would replace them if I was replacing the cam. Jim

Thanks a lot for the info Jim, Dano.

comnoz said:

.003 clearance is about average for an engine with some miles on it. It is not enough to cause a problem.
I would replace them if I was replacing the cam. Jim

Click to expand...

You make it sound like an easy job :wink: however you need some special tools to do the job right. First tool to remove the blind bush, second tool to line ream the (blind) bushes to avoid point load for long durability.

I'm about to find out how difficult this job is. The chocolate cam in my 850 has also eaten the drive side bush (of course it has to be that one!) so there is something like 0.030" clearance - a rattling good fit ;-)

I'm planning to use a blind bush puller + heat to get the little bugger out, and then in with the new in the same operation. I'm really hoping that line boring will not be required though. I do notice that the later bushes are shown as "do not ream" on A-N page at http://www.andover-norton.co.uk/SI%20Ca ... bushes.htm

Hope you can post some pics Steve

Sorry, no pictures, but I did get the drive side bush out by heating the crankcase in an oven and then using a blind bearing puller. I also ground & cut 2 grooves in the old bush with a Dremel and a short length of hacksaw blade.

I see some reports that the new bush can be just pushed in - not on my crankcases! I'm going to have to make a suitable piloted drift, heat the cases once more and bang it in gently.

I tried t remove the TS bush without slitting it, and it was too tight for the amount of force that I was willing to use, so I'll be slitting it. Always a challenge, these Nortons. Hopefully though they will line up when finished.

Duplicate post deleted by Steve.

Progress update:
After putting the crankcase halves into the oven & leaving them there for a while, it was possible to remove the old bushes using a blind bearing slide hammer. I'm surprised that they should be so difficult to remove, the main bearings drop out under their own weight at this temp (around 180 degrees C).

Then the fun starts. The new bushes (genuine Andover Norton items) were in the freezer, but still needed some good whacks with a hammer & drift to get them in (and yes, the bores were clean and free from any visible dirt etc.). Then after the cases cooled off, I find that the bushes have now shrunk down to around 0.863 or so (difficult to measure with a normal vernier since they sit so deep). The manual states a fitted ID of 0.875, so they are more than 0.01" undersize, which is a hell of a lot to remove without going skew or ending up with a taper.

I'm thinking about what to do now. As I see it these are my options, bearing in mind that I don't have access to a machine shop and getting this sort of one-off work done here in DK can be very expensive:

1. Turn down the new PW3 cam to fit the bushes. Probably the easiest & quickest method, but it seems so wrong. On the other hand, it's possibly the most certain way to preserve the concentricity of the 2 bushes. Assuming that they were concentric to start with of course...

2. Extract the bushes (may require buying 2 new ones, depending on how much struggle they put up) then machining them to fit. Nice, does not offend my engineering senses, but it's quite difficult to machine the ID (and the OD) of such a short, thin bush. ID could be honed, but taking a tad off the OD (which would be a Good Idea to help fitting and reduce the ID shrinkage) is tricky. Expensive in setting up time.

3. Make up 2 new bushes. Fairly straightforward machining job, but requires finding someone who will do this correctly with the correct lump of phospher bronze. Not cheap either.

4. Ream or hone in situ. Not so easy, requires a special reamer, or at least making up a special pilot bush for the drive side to do the timing side, not certain how to do the drive side though.

5. Mill the bushes to size in situ. Could mount the case halves on a mill, clock the mating faces to ensure they are horizontal and then mill, but again, a tricky operation probably requiring a lot of expensive setup time.

I'll speak to my supplier when I get a chance to hear what he says, but at the moment I'm a bit stuck. I'm just very surprised that the fit is so tight as to cause such shrinkage on the ID though.

Never a dull moment with Nortons....

Bad habit this, replying to one's own posts, but anyway...

I've spoken to Mick Hemmings who supplied the bushes and had an email back from Andover Norton (good service that, a fast reply). Both say that these bushes have to be sized to fit after installing. A-N will be changing their web page which I quoted above to remove the statement about no reaming required.

Having given the matter some thought, I'm still inclined to go for my option no.1 above as the least likely to give a poor result.

/Steve.

Option 1 ? you're nutz
I have acouple of questions did the bush fit the cam before fitting, what was the interference before fitting
As you say it doesn't appeal to your sense of engineering, guarantee if you ream with a 7/8 reamer you will be back to 3 to4 thou clearance
I would have said about 0.003'' interference and the same cam clearance, and reckon you would end up about 0.0015 -0.002'' clearance, one would have thought that this should have been a pre finished bush unless the tunnels were so badly machined that they had to finish the bush afterwards to get the correct fit, ask anyone who has fitted cam bearings to a vauhxill viva

splatt said:

Option 1 ? you're nutz
I have acouple of questions did the bush fit the cam before fitting, what was the interference before fitting
As you say it doesn't appeal to your sense of engineering, guarantee if you ream with a 7/8 reamer you will be back to 3 to4 thou clearance
I would have said about 0.003'' interference and the same cam clearance, and reckon you would end up about 0.0015 -0.002'' clearance, one would have thought that this should have been a pre finished bush unless the tunnels were so badly machined that they had to finish the bush afterwards to get the correct fit, ask anyone who has fitted cam bearings to a vauhxill viva

Click to expand...

Yes, the bush was a loose clearance fit on the cam prior to fitting, and no I did not measure it. They are (I think) sintered bronze and thus compress quite a lot on fitting, I just did not think they would compress so much.
I suspect the tight interference fit is specified to ensure that the bush does not rotate in the crankcase, but I'm puzzled as to why it should be so undersize on fitting - I would have thought that the finished size would have been a lot closer.

Yes, a 7/8 reamer in a mill would probably solve the issue, the problem is getting it done. Sometimes best has to yield to available. Logically though, there should not be a problem in taking the 12 thou from the cam, and I'm rather unlikely to be wanting to sell the cam on to someone else.

Hmm, still thinking.

/Steve.

Best to ream the bushes with pilot as just grinding the cam smaller assumes the bushes are in line. I did mine with a hand reamer but the first bush was already to size so it did not cut, then I put a spacer over the shank so it was using this first bush as a datum to cut the second.

Interesting that the timing side was to size in your case. Do you recall which bush you used? On mine, both bloody bushes are undersize, so a hand reamer is not going to do a very good job methinks...

My timing bush was the original, I only needed the driveside renewed, interestingly the driveside journal on the new cam was oversize and needed grinding but it came in a box and no idea who made it.

In Phill Irvings book he states that 5thou would be time for replacement