Tappet Clearance

[peter t]

HELP!

I have recently obtained my pride and joy a TR6. New owner, nosiy tappets, obvious job - adjust them!

Taking great care set to 0.010" using method in Haynes.

Start engine, nice and quiet until - BANG - number 8 rocker breaks ******

Don't know for sure, but think it has a non standard cam - silly question could this require larger gap?

Engine number on block removed and now only reads 'MV5' anybody any clues?

[Richard Crawley]

Welcome to the forum Peter.

Sorry it’s a bad start for you! A different cam may well need slightly different tappet clearances but it shouldn’t make a lot of difference to immediate running as long as you actually have a gap & the gap doesn’t totally close up when the engine is at operating temperature, what was the original gap? Even setting with no gap at all should not give the result you experienced, it should just miss-fire when the valve is no longer closing properly.

 

It may be just coincidence & you’ve been the victim of a poor quality aftermarket rocker arm; there is some total rubbish around but personally I’ve never seen one break! Only other possibility is that there is something seriously amiss with the cam/valve combination & somehow the piston has hit the valve – which could be more serious I’m afraid! You need to do some basic checks to establish exactly what has happened.

 

Haven’t got a clue what that engine number means but it seems a bit sus! Matching numbers (or at least ones that make sense) can mean everything to the Pedigree of these cars, did you not ask any questions when you bought it? If you got it from a dealer I would certainly go back & knock on their door!

[IanR]

Peter,

Setting the wrong clearance should not have caused this to happen. Breakage of a rocker smacks of either a faulty rocker or collision between the piston ond the open valve, even then I would expect the valve to bend and /or the push rod to bend and not the rocker to breakl. If the engine has hitherto been running OK it is unlikely to be a case of valve timing error but I'd still check it, I would suspect simply a poor rocker as Richard suggests. Could be a siezed No 8 Valve but why?? Have you checked your rocker shaft oil feed maybe the rocker has siezed on the shaft and subsequently broken..

Dont have any musch other ideas

 

Hope this helps

 

Cheers

[JohnC]

I remember when asking about cam timing a year or two ago that I was told that the TR6 valves & pistons cannot collide, assuming standard rockers (unless, I suppose, something is broken already). So Richard's assessment of a faulty rocker arm sounds most likely.

 

Tom - I think the info about valve/piston clearance came from you, but I'm not sure - any comment?

 

Cheers,

John

[Tom Fremont]

I remember when asking about cam timing a year or two ago that I was told that the TR6 valves & pistons cannot collide, assuming standard rockers

 

That is correct for the all factory cam profiles, as the lift does not exceed 0.380". The depth of the seated valve in the head is greater than 0.400" ( this taken from a 10.7:1 c/r head, so the original depth would be greater ). Hence valve timing is not a factor as they never go down as far as the piston top @ TDC.

 

Note there are some high lift cams out there; against my own advice I've just installed one with 0.508" lift. Even this would require drastically wrong valve timing to collide - more than would likely enable the engine to run. Of course I'm depending on the timing chain never to break, and the cam sprocket bolts likewise

 

On the topic of rockers, it may come as a surprise to some that I found one to be about 0.100" shorter than its fellows, revealing a slight bit of the valve stem top. ( The rocker shaft rebuilder promptly sent a replacement. ) This was the first I'd ever heard of it.

[Bob Stubbs]

If its any help, mine has an aftermarket (fairly elderly) Fentune cam in it and the correct valve clearances for it are 16 thou inlet and 18 thou exhaust. So significantly different than the standard set up.

[jonlar]

Hi all, it can also make a difference whether the clearances are for engine cold or hot, I can't remember whether the clearance opens up or closes when hot.

[peter t]

Thanks to all who have replied - what an excellent place to come for help.

I will be removing the head this weekend to see if it has caused any other damage (fingers crossed it has not!) and I will be checking all the above to ensure it does not happen again.

As to the source of the car, yes it came from a dealer and I will be speaking with them (no names --- yet).

I hope in time to be able to return the advice

Many thanks

Peter

[Rhodri]

If you have a very high lift cam then it is possible for the valve springs to become coil bound if they are not compatible with the cam. When this happens the weak point, the rocker , will snap. Perhaps the tappet gaps were a bit excessive before and prevented the coils binding. Before removing the head I would rotate the engine by hand and check , with a feeler gauge, what clearance there is between the coils on each spring when they are fully compressed. I can not remember what the clearance should be, but one of the TR specialists should know. I would push down hard on top of the valve, with the broken tappet, with a hammer handle or similar to see if it slides up and down, as it may not be damaged.