52 replies to this topic

[AndyMiniMad.]

Sounds about right yes! while you have this one off maybe you could post back on here and let us know what went wrong..

[Sprocket]

Bending all push rods with 1.5:1 rockers and then subsequently breking two different sets of sintered rockers in the same place tells me that there is some binding going on somewhere. If its not the springs binding, the spring retainers binding on the valve guides, or the valves hitting the block because of the increase in lift and a seriousy skimmed head, then it has to be the fact that the rocker posts have not been propperly spaced to correct the rocker angle, due to the previously mentioned serious skim!

[Sprocket]

well......... there was plenty of gap between the spring coils

what do you mean by this? The springs should have a minimum 0.014" gap between the spring coils at full lift. Standard springs will bind with a high lift cam.

[tiger99]

I guess he misunderstood my first comment and assessed the spring gap in the valve closed position, not at peak valve lift! From subsequent information, I am even more convinced that the springs are coil bound and/or the valve collets are hitting the guides.

I personally would do the plasticene test, which costs almost nothing, to get some facts before parting with money for a fix that might not be needed. I would also think about having the springs off, one at a time, to check that the valves move freely, just in case it is a bent valve which is jamming.

A Mini head is very simple, no overhead camshaft for instance, and comes off in about 10 minutes, with practice. The true cause of the problem ought to be visible. If things have been hitting each other, there will be marks.

One more thought. If there has been too much skimming, so that the rockers are at an unsatisfactory angle, could the radiused end of the rocker be so badly misaligned to the end of the valve stem that it comes off it at peak lift? That would jam the tip of the rocker against the valve stem, and something would have to break.

[rover96]

Just a quick question...Are there any shims between the rocker pedestals and the head. They may be necessary as stated above to achieve the proper rocker geometry with the skimmed head.

http://www.minisport...o_C-AEG392.html

[giz]

no there are no shims, i'm asking around at the moment to find out if anyone has a more suitable head locally.
the spring gap was 0.010 average between all valves, some more some less.

[rover96]

Just for information, I put a stage 3 kit on my 1275..with a 12G940 head that had been skimmed an unknown number of times. Without the pedestal spacers, I was unable properly set the valve clearances or spring compression. I´m sure that some of the experts here can give you more information.

[tiger99]

Rocker pedestal spacers would actually make the geometry much worse, which is why I asked if the rockers were actually missing the valve stems. The only time they would be correct would be if some fancy head mods had caused the valves to be resessed more deeply than normal into their seats, but that would be rather contrary to normal practice.

Always, if you think you need pedestal shims, you actually need shortened push rods instead. The relationship between rocker pivot axis and valve needs to be maintained, and that has nothing to do with head thickness, only rocker shaft height, valve stem length, and valve depth in its seat.

I am not saying that it has happened here, but in the past far too many people have caused a more serious problem by trying to fix a simple one the wrong way. Not that obtaining shorter push rods is a simple fix, as you have to obtain them first. As both ends are thisker than the middle, you can't just cut the old ones down, and there is not usually sufficient metal at the tappet end to be able to regrind the spherical profile again.

[giz]

so a new head is the way to go? i'm yet to find any evidence of this cam and head combo before and it's proving impossible to find a more suitable head within walking distance. i don't fancy taking the risk with another cam4810 in case it happens again.

[Sprocket]

Wa

no there are no shims, i'm asking around at the moment to find out if anyone has a more suitable head locally.
the spring gap was 0.010 average between all valves, some more some less.

Was that with the 1.5:1 rockers? if not, this is your problem.

I'll say it again. Standard springs with high lift cams do NOT work, the coils WILL bind.


Take things into perspective. The increase in valve lift with a 276 cam from the standard cam is 0.96mm on the inlet and 2.13mm on the exhaust. with 1.5 rockers, that equates to 1.15mm on the inlet and 2.51mm on the exhaust

To bend all pushrods with the 1.5 rockers, it is surely valve spring binding. were the brocken sintered rockers on the exhaust valves?

The fix might be as simple as fitting a double valve spring set.

[giz]

yes it was the 3rd cylinder exhaust valve both times.

i've been out and measured all the heads i could find before you posted,
the current head from the top of the block to the top of the front right head bolt hole is (vernier set to in.) 2.723
the head on the old engine which has never been skimmed measured in the same place is 2.776
and another cam4810 measured in the same place except it was lay on a gasket measured 2.781
so it's safe to assume that the new head has been roughly skimmed by 50 thou give or take

[Sprocket]

0.5mm skim is hardly anything to write home about really. Your problem is the valve springs! buy some dual springs and retainers.

[Sprocket]

http://minispares.co...id=33655&title=

there you go, you don't even need to change the retainers

[giz]

not a bad price

[giz]

with the double valve springs fitted should the 1.5 rockers be fitted or the standard again or try and find a set of 1.3:1's?