32 replies to this topic

[m.angus0]

Hi guys right Im having a nightmare with my mini. I have a 998 A+ that I have put a 276 Kent cam in. Ive had the head skimmed to sort out the compression. I have put it all back together and I started it and there was a knocking coming from the rocker area then one of the rockers broke so I bought some rocker assembly spacers to raise it up as I thought maybe the pushrods were too long now with the cam and skim. Replaced the broken rocker and tried it again tonight and its broken a different rocker. Im lost? Maybe the machine shop have put too strong valve springs maybe Im just not sure. Any ideas? Thanks Matt

[m.angus0]

And a 12g295 head. Forgot to mention that

[Spider]

I'd suggest as a first thing, check for coil bind on the springs. You really want a minimum of 0.050" between coils. Might sound excessive, but if you saw in slow motion, how the springs behave in operation, you'd then see why, though, this results in spring failure.

 

Also check clearance from the top retainer to the valve guide.

 

Finally (for now!), if the cam has been fitted with too much advance, and with a shaved head, the Inlet Valves can kiss the pistons. I think if this had been happening, you'd possibly be seeing other issues too, but I don't discount it.

[Cooperman]

Yes, as Moke says, it sounds like coil binding of the valve springs. I always work on a minimum of 0.040" coil clearance at full spring compression. If you have double springs check the inners first, then finally assemble and check the outers.

 

It will not be the spring rate causing the problem, it will be an interference contact somewhere.

 

You will also need to replace the push rods in case one or more is bent now.

[DeadSquare]

What sort of rockers are you using?

 

it is unlikely that your springs are coil-bound unless you have a full race cam and more than 1.5 ratio rockers.

 

How much have you skimmed off the head?

 

As you have gathered, skimming the head requires shorter push rods.  This is compensated a little by your high lift cam, which doesn't actually lift any more than a standard cam because the increased lift is obtained by grinding metal from the back of the cam which lowers the cam follower.

 

To get the most out of the cam with the least stress, you have got to aim to get the angle that the rocker points down with the valve fully open to be equal to the angle that it points up when fully closed.

 

Shimming the height of the rocker shaft will increase the point down and reduce the point up, so you need to skim not shim the rocker pedestals, and this will mean that you need even shorter push rods.

 

I have seen push rods with 1/4" cut out of them, an inch from the bottom and sleeved together and also rods with the bottom cut off and a blob of hard weld applied and then ground to shape.

 

If you haven't skimmed too much off the head, you can effectively reduce the push rod length by 1/28" per turn of the tappet adjusting screws, and if you are using cast mini rockers, you can loose a bit more length by using a 90 degree countersink to screw the ball of the tappet adjuster further into the rocker, but this may need the top of the push rod grinding away to make sure it doesn't foul.

 

If you are using 1.5 roller rockers and they point from near level to very much downhill with the valve open, the roller has been known to come away from the top of the valve stem as the frame of the rocker pushes on the valve. 

 

THIS IS NOT GOOD.

 

It brakes the rockers and can bend the valve stem.

[m.angus0]

Hi thanks for the replys. I believe it was around 60 thou was taken off the head and yes Im using standard rockers. Thinking at whats happening the coil bind does sound right but surely the machine shop that did the overhaul should have checked that?! I took the rocker cover off and ran it to hear the knocking and it is coming from the valve area so I will measure the springs. Is the measurements done at full compression with the valve open? Im not sure Im understanding about countersinking the rockers? You mean opening the valve clearance?

[DeadSquare]

Do you know the lift of your Kent 276 cam?  so that we can work out how much more valve lift you are now getting.

 

.060" or 1/16" off the head should not be causing broken rockers, even if you are using pressed steel ones.

 

Where did the first rocker brake? and did the second brake in the same way?

[m.angus0]

Its .293 on the inlet and .315 on the exhaust. It broke along the crease above the rocker shaft on both rockers. They have both been exhaust rockers if that helps?

[Spider]

 so I will measure the springs. Is the measurements done at full compression with the valve open?

 

Measured at full valve lift between each coil, similar to this;-

 

 

Don't assume the machine shop checked anything, unless they assembled it.

[Spider]

One other thought here that sometimes occurs with these heads on some block, in particular if bigger valves have been fitted and with the lifts you have.

 

With an accumulation of normal manufacturing tolerances, on some builds, the valves can contact the block right on the side of the bore. This can occur even with stock valve sizes in 12G295 heads, not often, but definitely can occur.

 

<Edit: you might want to remove your rocker gear and re-fit the pillars & shaft only, and the head nuts (just do them up firm), then open each valve in turn to it's full lift + 0.040" (1.0 mm). Do this with a hand operated lever under the rocker shaft, so you can feel if the valve is making contact with anything. You can set up your DTI on each valve cap to measure how far you are opening each valve. >

Edited by Moke Spider, 05 May 2018 - 09:01 AM.

[whistler]

I had this problem once back in the stone ages. I had fitted the wrong length valve into a Mk3 S head and started snapping rockers. Fitting the correct inlet valves sorted the problem.

[DeadSquare]

I think I have found the cam lift for your Kent 276, but because I don't know what cam it replaced, this is a bit of guess work. 

 

As you have given the skim as 60 thou, all these figures are in inches.  Kent 276 inlet = .292" and exhaust = .314"

 

50 years ago the coopers and even the 1/2 hot 731 had .250", but if your 998A+ was still using the standard mini cam with a lift of .221", so much the better.

 

.292" less .250" = .042"  and .314" less .250" = .064" ground off the back of the cam .

 

The exhaust cam follower is now .064" lower in the block which makes diddly-squatt difference if you have skimmed .060" off the head, and I really can't see that .018" on the inlet can be causing your trouble as this will be taken out when adjusting the tappets, so skimming the head is neither here nor there and as there is in theory a .004" benefit on the exhaust, I am baffled as to why it is the exhaust rockers that have broken.

 

As a full race cam with even more cam lift (.065" more) can be fitted without the springs becoming coil bound, this shouldn't happen.

 

people used to regrind the valve seats too deep, but this cant happen with hardened inserts.

Edited by Moke Spider, 05 May 2018 - 10:33 AM.

[DeadSquare]

Hi thanks for the replies. I believe it was around 60 thou was taken off the head and yes I'm using standard rockers.  I'm not sure that I'm understanding about countersinking the rockers? You mean opening the valve clearance?

 

As you mentioned, skimming the head upsets the valve geometry as the push rod length doesn't change, this can be compensated by withdrawing the tappet adjuster into the rocker, but this is limited by the ball on the end of the screw.

 

By taking a counter sink and machining metal from the underside of the rocker, the ball can be screwed further in, thus effectively shortening the push rod a bit more which allowes that bit more to be skimmed from the head.

 

With a .060" skim, this is not relevant but I used it as a cunning dodge on a full race engine.

[DeadSquare]

Hi guys right Im having a nightmare with my mini. I have a 998 A+ that I have put a 276 Kent cam in. Ive had the head skimmed to sort out the compression. I have put it all back together and I started it and there was a knocking coming from the rocker area then one of the rockers broke so I bought some rocker assembly spacers to raise it up as I thought maybe the pushrods were too long now with the cam and skim. Replaced the broken rocker and tried it again tonight and its broken a different rocker. Im lost? Maybe the machine shop have put too strong valve springs maybe Im just not sure. Any ideas? Thanks Matt

 

 

With the rocker cover off and the car in top gear on a smooth flat surface, pull it forward and watch what happens.

 

P.S.  Pull the plug leads off, in case you have left the ignition on.

[carbon]

+1 for Moke Spider's suggestion that the valves may be touching the block. Were both the rocker breakages on exhaust side?

 

This is a very distinct possibility if the 295 head been fitted with non-standard exhaust valves and there are no cut-outs in the block.