13 replies to this topic

[Wiggy]

Hello guys.

 

My rocker shaft recently broke, so I replaced it with the heavier duty one just to get me back on the road.

 

My rocker bushes are clearly knackered though, and I'd like to replace them.

 

There's not a great deal of info that I can find on doing it, but I understand that they need reaming.

 

Is this something I could do at home if I bought a reamer? Any other tips, methodology, or info would be most welcome. 

 

Cheers.

 

Oh, the rockers are the JCW 1.5:1 ones. I'm assuming that they'll use the same bushes as standard?

[ACDodd]

It can be done with a reamer but you need to ensure they are machined square

I bore mine on a lathe.

Ac

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[Magneto]

Hello guys.

 

My rocker shaft recently broke, so I replaced it with the heavier duty one just to get me back on the road.

 

 

I'd be more worried about why the rocker shaft broke! Any automotive machine shop can ream the bushes after you press them in.

[DeadSquare]

Without in any way wishing to impune your ability, a reamer is a precision tool and the result is as bad as the way it is used.

 

As Magneto queried, I will add.  If the shaft broke, how long before a rocker is fatigued and brakes ?

[Pigeonto]

It can be done with a reamer but you need to ensure they are machined square

I bore mine on a lathe.

Ac

What AC says. It's a horrible job by hand and inaccurate.

I'm not even sure that a fixed reamer would do it in one pass, it's a while ago now but I used adjustable ream with several passes, each one with potential for mis-alignment

Oh, and yes, standard bushes, the greater offset is gained within the geometry of the rocker, not the bush

Edited by Pigeonto, 31 July 2019 - 07:04 AM.

[Wiggy]

Thanks for the replies.

 

As to why the shaft broke; I assume I wasn't careful enough when tightening down the rocker assembly on one occasion. Plus double valve springs, and then a higher lift cam, it had had enough. The shaft was absolutely shot regardless of the break.

 

Hmmm, sounds like I should employ the services of experts with the correct tools.

 

Do they require reaming to 9/16" exactly, or is there a clearance range depending on the exact diameter of the shaft?

[DeadSquare]

Thanks for the replies.

 

As to why the shaft broke; I assume I wasn't careful enough when tightening down the rocker assembly on one occasion. Plus double valve springs, and then a higher lift cam, it had had enough. The shaft was absolutely shot regardless of the break.

 

Hmmm, sounds like I should employ the services of experts with the correct tools.

 

Do they require reaming to 9/16" exactly, or is there a clearance range depending on the exact diameter of the shaft?

 

The BMC workshop manual says that they should be reamed to between  .5630" and .5635".   (14.30 and 14.312 mm)

[Spider]

The important aspect here is to be sure the Rocker Tip ends up parallel to the Bush Bore. You may or may not end up with this when reamed.

 

I do mine in the Mill.

[Wiggy]

Food for thought.

Cheers for the feedback guys.

[carbon]

Thanks for the replies.

 

As to why the shaft broke; I assume I wasn't careful enough when tightening down the rocker assembly on one occasion. Plus double valve springs, and then a higher lift cam, it had had enough. The shaft was absolutely shot regardless of the break.

 

Hmmm, sounds like I should employ the services of experts with the correct tools.

 

Do they require reaming to 9/16" exactly, or is there a clearance range depending on the exact diameter of the shaft?

I would suggest extreme caution using a high lift cam with high lift rockers. Don't rely on assumptions - measure, measure, measure

 

Why emphasise this point? If you assume the rocker ratio is 1.5 then you may be in for a surprise. I have found that both forged and roller rockers can give lift ratios of up to 1.65 when used with 'standard lift' cams. And that means valve lift of 10.5mm

 

If you then add in a high lift cam you could be seeing valve lift approaching 12.5mm. That's not excessive, but if you don't have the right sort of valve springs fitted you could go coil-bound. And when that happens things tend to break, bend, or wear out very fast.

 

Remind me again, what do you think happened to cause the broken rocker shaft. And had anything been changed recently?

[Wiggy]

The cam had been changed recently. It's only a 274i. JCW double valve springs and rockers.

 

I don't believe I was careful enough when tightening down or removing the rocker assembly (they've been off a few times), and thus created a stress fracture at the weakest point (oil feed hole), on an already rather worn shaft.

[1963MKI]

im looking at doing this at the moment. I have a mill and a lathe but am not sure how I can mount a pressed steel rocker with either machine to get them square.

[gaspen]

Hello

 

I am rebuilding a standard 850 engine which has the old pressed rockers. I will replace the bushes before the machine shop.

 

I have the 2A21 bushes from MiniSpares

 

I suppose this the correct position of the bush. The little groove is down to align with the oil holes on the shaft. Right ?

 

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[1963MKI]

at what angle do you insert the bushing into the rocker ? i.e. where does the split on the bush roughly line up with on the rocker ?

 

I have heard that the inner groove on the rocker should only align with the oil feed on the rocker shaft at a certain point so it pulses oil into the rocker as it opens and when the valve is closed, the oil hole shouldn't align with the internal groove.