Broken Rocker

[greenmeanie]

No its not a bad 80's heavy metal band.

I am having diffculty with my newly rebuilt RV8.

The engine spec:
Rover 3.5L V8 from a 1976 101. Heads have been skimmed for composite gasket, 9.3 comp pistons installed, RP4 cam. All rockers, tappets and pushrods have been replaced. Valve seats were recut and original valves lapped in.

I have been having a problem which I discovered was manifesting itself as low compression on cyl #7 and zero compression on cyl #8. AFter some thought and investigation with a straight edge it seems that the valve seats were all cut to different depths meaning that the valve tips all sit at different heights. As I was gettign zero compression on cyl #1 I thought that one of the valves may not be allowed to close. I checked this by removing the rocker on that bank which, after testing, showed good compression on all four cylinders. In order to provide more clearance for opening I added shims under the rocker pedestals. The thinnest didn't help so I tried the next size up . On turnin g the engine over with the starter to get a compression reading the exhaust valve (2nd from end) rocker on cyl #1 snapped. I take it that this is not a good sign. Swearing at it didn't sem to fix anything either.

The question is what is wrong? The valve was moving before it was shimmed so I don't think that it is sticking. Any suggestions before I tear the head off and reinstall from scratch.

Cheers
Gregor

[john 215]

Hi Gregor,just a quick thought on this problem,are you 100% sure the valves that are giving you problems are not slightly bent,so sticking in the guides?Giving the impression of different hights.If a valve was to stick open then i have seen on other engines (i must admit not on Rover V8)that rockers getting broken,as when a cam belt breaks and the valve timing goes varible!! Sorry to hear of your problems hope this may help,happy new year John.

[katanaman]

I don't think you can solve your problem with shims. If the installed heights are so far out from each other then putting shims in to sort one preload is probably just going to screw up another one. Did you fit the same shims under all pedestals on one side? If you didn't then you have to otherwise you will bend the shaft and have stiff rockers.

Your best options in your case I think is to either fit adjustable push rods which are easy to get and reasonable price or fit adjustable rockers which aren't easy to get and can cost a small fortune.

If you didn't fit the same shims all over then this might be why you broke a rocker (you probably need a new rocker shaft too now) if you did then you need to check your valves are free. Could also be you put it back together wrong or even you used too big a shim and you hit the piston with the valve which would also break a rocker. Don't guess the size, remove the inlet and measure the preload at the valve lifter.

[greenmeanie]

Matching shims were used under all pedestals on the same bank so I don't think a bent rocker shaft was the issue.

"Could also be you put it back together wrong or even you used too big a shim and you hit the piston with the valve which would also break a rocker."

Unfortunately this could be it. I was under the impression that adding shims would move the rocker upwards from the valve thus allowing it to shut properly. Please educate me on how adding shims would cause the valve to hit a piston.

I did check the preload on assembly and things looked good. As stated above when the rockers were in place I was getting zero compression on #1. With the rockers removed I am getting good compression so it seemed to be an issue with the rocker/tappet assemblies.

Sounds that either way I am going to be pulling the head to check for damage.

Cheers
Gregor

[katanaman]

sorry your right with the shim it wont cause a problem my mistake still on holiday mode.

If you checked the peload properly and it was ok then something has to have changed as they don't suddenly change on their own. I still say you need to check it again if its went tight then I would have a good look for why the clearances have went tight. I also still stand by shims not being the best for you and would recommend you get adjustable pushrods. As for the rocker breaking I can only think that for whatever reason the valve has hit the piston. Why I don't know maybe the ball of the pushrod wasn't in properly but I really don't know. Could just be a coincidence that it broke and you were lucky it wasn't running but I doubt it. Before you turned the engine on the starter did you turn it over by hand and if you did, did it feel ok?

[ian.stewart]

How much have you had skimed off the heads, and did you do a valve clearance test before the final assy of the engine,
have you checked the pushrods for being bent since you broke the rocker arm, rockers will break if the pushrods have not been installed correctly, meaning the pushrodd is not installed in the cup of the lifter, but on the rim of the lifter, I have done it so I know it is possible,

[HairbearTE]

What springs/retainers are you using with your nutty cam Ian?

[greenmeanie]

so valve clearance isn't an issue with skimmed heads. I DID check that by hand before start up.

As the valve was moving before I broke the rocker I am starting to think that I just screwed the pooch and didn't install the push rod correctly as you state.

In many ways I was beginning to look at this as the perfect excuse to buy a set of '97 heads on eblag and get the bigger valves. Now I just feel like a ponker. Trust me to go cro-magnon on a precision assembly.

As a matter of interest, in case I feel rich before the auction ends, were the sticky valve issues sorted by '97 (Discovery I believe) and will they fit my 76 block and carb manifold without issues.

Cheers
Gregor

[ian.stewart]

What springs/retainers are you using with your nutty cam Ian?

Sounds strange, I cant remember, I think they are RPI doubles with matching caps

[greenmeanie]

I put a straight edge over the valve stem tips at the weekend and measured the variation over each valve. Well the #1 and #7 have two high valves and the rest of them are all about .025thou lower. I suppose that explains why I was not getting compression. All other things being equal, is this something I should be able to shim out or is it heads off and back to the machine shop for a bit of a chat about quality control.

Cheers
Gregor

[katanaman]

use adjustable pushrods and job done, far better than shims.

[greenmeanie]

I agree they would be better but.....

Adjustable push rods from V8 tuner =
135 GBP plus IIRC removing heads for machining of push rod holes.

Shim kit =
Free as they came with the build kit and no machining required.

Bear in mind this is not a performance application. I just want the beastie to run and shove my truck down the road at a comparitively sedate pace and pass emissions.

So the question is whether this differnetial can be shimmed out or do I need to go the more expensive path.

Cheers
Gregor

[katanaman]

lets put it this way if you had open valves before I would say no you wont be able to shim it and get decent results. Will you be able to shim it so it runs then yes you probably can but I cant recommend it with what you appear to have. All that said and done I don't understand why a valve sunk by 25thou has ran out of lifter adjustment. You said you measured the preload originally and set it properly which should be about 65 thou there is probably about another 120-160 thou after that so your sunk valves shouldn't be anywhere near bottomed. Either you got the preload wrong originally or something else is going on somewhere.

[GreenV8S]

Not seen it myself, but I've heard of cases where the valve seats have recessed into the head giving symptoms like that.

[katanaman]

yes agreed but not when the preloads were supposed to have been set correctly and the odd valves are only 25thou out.