Reaming cam bushes on a single

[Keith.owen]

Hi All,
Can anyone advise?

I have to ream the cam bushes, which I have replaced, on both my VH and my NH construction project.

Although I have a selection of adjustable reamers, the blades run lengthwise and would I imagine foul in the oil groove that has been cut in the bush, so, I imagine a fixed reamer with spiral flutes is the answer?

However, according to Waller the bushes should be reamed to 0.75 inches. My question is if I purchased a 3/4" reamer will I get a .75" hole within tolerance (which is not actually given) or will it be over-size?

I know these sort of questions probably seem naïve, but as a newbie I feel the need to ask.

Also on another subject, is there such a thing as a good text book that covers all of these engineering shop practises?

Thanks and BR,

Keith

[pappleton]

Hi Keith,
I think you will need a piloted adjustable reamer to go through from both sides, gradually creeping up on the finished size. Then the bores will be aligned. You wouldn't want a lot of clearance there, I'll stick my neck out and say .001" - .0015". An expert will be along soon. I fit the bushes by knocking in from both sides at the same time using a long stepped mandrill. Timing case fitted to the crankcase half and nicely heated up. You can see straight away if their going in square. A little judicious tapping from side to side as they get to fitted depth also helps seat the bushes and opens the bore size an appreciable amount too.
BR, Paul

[cmfalco]

Although I have a selection of adjustable reamers, the blades run lengthwise and would I imagine foul in the oil groove that has been cut in the bush, so, I imagine a fixed reamer with spiral flutes is the answer?

However, according to Waller the bushes should be reamed to 0.75 inches. My question is if I purchased a 3/4" reamer will I get a .75" hole within tolerance (which is not actually given) or will it be over-size?

The relevant principle is it is easier and cheaper to machine a shaft to any desired amount undersize than it is to make a hole an arbitrary amount oversize. So, if properly used, a 3/4" reamer will make a hole 0.750" in diameter. That said, I recently finished reaming the 3/4" camshaft bushes on my 1928 Model C with the cases bolted to my mill and they ended up 0.0006" oversize. A spiral reamer would be better, but unless your current ones have some odd construction there should be enough flutes in contact with the walls that the presence of the groove won't be a problem.

With the bore of the bushes determined by the standard-dimension reamer, whatever clearance the designer decided the camshaft needed would have been created by machining its spindles to the desired amount undersize. I can't give you that number because I haven't determined it yet for myself. The spindles of my cam are worn and pitted so shortly I will be hard chrome plating them and then grinding them back to the required diameter to give them the correct clearance. Obviously, I'll have to determine that "correct clearance" before I do the grinding.

[Keith.owen]

Hi,
Waller does actually specify the tolerance on the spindle ( I don't have the book with me at the moment but the spindle is about 2 thou undersize) but gives no mention of tolerance on the bush.

My adjustable reamers are long enough to go through both bushes if I assemble the timing case to the crank case half, so, if I take it in very easy steps I should be able to get good alignment. I think I shall measure across the blades of the reamer, even if slightly undersize, since although I have some telescopic bridges I'm not very confident in their accuracy.

Does anyone know if a 0.75" fixed reamer actually measures exactly 0.75 across the blades?

Thanks all!

Keith

[jj.palmer]

If using a hand reamer it should ideally have a left hand spiral and be right hand cutting and also have a slow lead into the material if it is a through hole, see attachment.
For size produced you could try it out on a scrap piece of similar material.

Regard,
John P.

[cmfalco]

Does anyone know if a 0.75" fixed reamer actually measures exactly 0.75 across the blades?

Depending on the manufacturer the tolerance of a reamer that size will be +0.0001" or so, and -0.0005" or so.

The issue you will face by doing this by hand isn't alignment of the holes when you're done, it's that without rigid and accurate support of the reamer when doing the first bush the hole will end up barrel shaped. BSA had a special tool for aligning the reamer for the timing side main shaft bush on A10s that consists of a thick piece of steel with a further steel bush in it, along with holes to bolt it to the timing case. This "outrigger" bush aligned the reamer so it entered the bush accurately aligned with the mainshaft (at least, as best as possible without having the cases on a mill) and kept the reamer from any off-axis movement to eliminate the barreling that will be inevitable if you do this without such a jig.

[Keith.owen]

Oh Dear,
This seems to be rather more involved than I would have at first imagined!

Keith

[Simon.Gardiner]

My 'bible' ('Tuning for Speed') advises that as long as the surfaces are good cam bushes can have up to 5 thou clearance, although if fitting new ones ream to 2 thou clearance.
I've got a spiral 3/4 reamer with a pretty good length of 3/4 shank above the blades. Where it's possible to ream a pair of bushes from each side, where they're not miles apart, ream the 'bottom' bush through the other, letting the shank be guided by the 'top' bush, leaving one of the old bushes in situe for the first pass.
With one bush done, replace the other old bush and ream that one from the other side, going through the new bush for a guide on the shank this time.
Need to make sure the shank is clean and with no imperfections that can plough up the inside of the new bush that you've just done.
Hope that's understandable!
I will now don my cowboy boots and stroll off to my cowboy horse......

SG

[cmfalco]

Not too far off topic is this photo of the A10 bush reaming jig I mentioned in an earlier post. They seem to be fairly uncommon. I don't even remember where I got mine but it must have been in a batch of parts. I photographed it at an angle so the three-dimensional character of the alignment bushings would be more apparent. It's made of 5/8" steel so is quite heavy.

[pappleton]

A bit more off topic, I just read an article in the VOC advising against the use of PB bearings in this location as they can tighten up, unscrew the cam spindle, force said spindle into the motor, break off the bottom of the cylinder liner and lock the motor up. Looking at other makes generally, it would seem oilite bushes, or its equivalent, are best for purpose. A bit worrying, as I just made a set of CA104 aluminium bronze bearings for this location..
The later timing chest also has two dowels to locate it to the crankcase. Did a different production method dictate this? Why the concern after 15 years or so of just using the cover screws to locate? Once the early cover is removed after reaming, just tap it about a bit when fitting the cam to re-alinge it?
So back to the drawing board? What are others using? Have I tripped on my own spurs?
BR, Paul