18 replies to this topic

[Timrobins]

Hi, I am hoping someone on here can help with a problem that my son is having.

He has been rebuilding an A+ 1275 and as part of that rebuild he is adding a supercharger from Vmaxscart.

He has had a fast road cam fitted and due to the supercharger has had to fit a decompression plate.

The cylinder head and block have been skimmed slightly and the cylinder head has had new valves fitted with hardened exhaust seat.

He has now come to try to adjust the tappet clearance and he has run out of adjusting screw on the rocker. We have found some slight longer screw adjusters from Sommerford Minis, but even with these in, there is not enough thread to get the right clearance.

He has rung Stuart at Vmaxscart to ask if he know why, but he has said he has never had this problem before. So we are at a loss as to why this is happening.

Has anyone on here faced a similar issue or got any suggestions?

 

We have checked the length of the pushrods and they are all correct.

The decompression plate is 2mm plus the extra head gasket.

 

Thanks

Edited by Timrobins, 01 March 2024 - 01:01 PM.

[Midas Mk1]

Rocker pedestals shimmed correctly for head?

[Ethel]

As you have probably already figured out you've effectively made the block taller while the pushrods have remained the same length. There's no easy solution as you can't take thickness off the head or rocker posts without also making them shorter relative to the valve stem length.

 

If the cam is a regrind you'll also have lost some of its base radius in order to create more lift from the original lobe. Making the pushrods relatively even shorter than the block.

 

The best thing would be to get the extra volume you need from the pistons or head chambers, if possible.

 

A different cam and/or rocker assembly might make a difference. Higher ratio rockers give more clearance on the valve side from less adjustment on the tappet screw. It might also be possible to skim some of the excess height from the rocker posts, but you'll have to allow for it over the valves.

 

It does seem odd that you could run out of screw length for a few mm of block height - they're defo not small bore push rods 

[Timrobins]

Rocker pedestals shimmed correctly for head?

There are no shims under the rocker pedestals.

[imack]

Worn rockers and shaft?

[slidehammer]

It does seem strange that around 2mm of decompression plate has made that much difference. What about speaking to Avonbar as they used to do a bit of forced induction stuff

[Timrobins]

Thanks for all the replies.

We have measured the pushrods and they are 8 11/16, so a little under the listed 8 3/4

A visit to MiniSpares is going to establish if new ones are a little longer or not.

[bpirie1000]

Always the simple things.... always good to share the head scratching though..

[Ethel]

If any of  that 16th has been lost to wear, the rods could well need replacing anyway. The contact surfaces will be misshapen & a lot of the surface hardening will have worn away. 

 

Cam followers can also vary.

 

The rocker's effective length is from the centre of screw's ball to the centre of the rocker shaft. So you can imagine how much the screw length can reduce the rocker ratio and how the rocker not being perpendicular to the  valve can mess with the geometry where it matters most, which is coincidently where you adjust the tappet clearance.

[Timrobins]

Unfortunately new pushrods have not solved this problem! We are still running out of thread on the rocker adjusters.

The only other 2 places we could be losing height are either the cam followers or the valve stems?

As has been mentioned already 2/3 mm extra for the decompression plate and gasket should not be having this much of an impact on the valve clearance.

[Timrobins]

Just discovered there are 2 different valve lengths, 3.44” and 3.56”

That is a difference of 3mm and when you factor in the rocker ratio of 1:2.5 that multiplies up to 7.5mm at the screw adjuster!

So they need checking!

[sonscar]

Effectively your pushrods are now 2/3 mm too short.I can easily see running out of adjustment being easy.Has the cam been reprofiled?This reduces the effective length of the rods due to the base circle being ground away.Just thoughts,Steve..

[bpirie1000]

Are the rockers all on the right way round? Also some rocker seats have oil feed some do not. I do not know if there is a different in dimensions.... maybe a question for someone else...

Also, do they all sit on the same way. There is a locking tab that goes on top of one of the rocker bases. That will let you know which way is right..

Has this engine run on these rockers before?

[Timrobins]

need to fit shims under pedestals equal to head and block skim

and check rocker geo.

Putting shims underneath the rocker pedestals will make the problem worse.

[KTS]

 

Just discovered there are 2 different valve lengths, 3.44” and 3.56”

That is a difference of 3mm and when you factor in the rocker ratio of 1:2.5 that multiplies up to 7.5mm at the screw adjuster!

So they need checking!

 

 

not disputing it may be a factor, but the multiplier is the other way : movement at the rocker tip is 2.5 times that at the pushrod end ?