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Rocker shaft bolt snapped


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Hey guys, Just got to the end of my road, lost power and had a clunk.

 

Poked under the bonnet and the Rocker shaft bolt nearest to the windscreen had snapped. The 3 others are ok.

 

What is the likely cause of this? Its a rebuilt engine with 1000 miles. The head is skimmed a fair bit and I had the

 

rocker pedestals on shims to compensate. I suspect it could be this biting me? I couldn't see any bent pushrods

 

but will know tomorrow when I have the rocker shaft off. I think I shall order some shorter pushrods and ditch the shims.

 

I hope there's no other damage :(

 

Colt

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Hi Colt,

there is a cyclic load on all the studs so they are trying to be stretched rapidly for 10,000' miles. by the action of the push rods and springs.

The thread run out on these studs is not the finest so possible stress raisers here and there.

 

Can you try and post a pic of the two fracture faces so we can see how/where the crack started etc.

 

In focus and high a resolution as possible 100Kb or better.

 

Roger

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I'll look at these things tomorrow thanks.

 

Having read another similar thread I saw a comment by Stuart about pulling the head off with these bolts, I'm guilty of this I'm afraid

 

as I had no lifting eyes on the head so I may have stretched the bolts as the head was well stuck in place?

 

Where can I get lifting eyes? arrgghh!

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Hi Alan,

it may have been a USA problem and possibly a tufrided one at that.

Just be careful what you get.

 

A slight bend is not a major problem as the four pillars will pull it straight.

 

There was also a problem about 10 years ago where the shafts were a good 0.001" over size. Nothing would fit.

 

Roger

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Thankfully no bent pushrods.

 

When springs are fully compressed theres no gap which is normal right?

 

Picture attached. Notice shard of metal from either the head or bolt hmmm

 

Next steps dismantle rocker shaft (it is a new one from Rimmers when engine built)

 

Order shorter pushrods.

 

I should hopefully be able to remove the broken bolt from the head, groan.

post-13416-0-09573900-1515504191_thumb.jpg

Edited by Britishbest
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Thankfully no bent pushrods.

 

When springs are fully compressed theres no gap which is normal right?

 

NO See reply 5

Posted Yesterday, 08:24 PM

I agree, i would suspect coil binding. Check that at full lift you can lever the valve open another 3mm. Check all valves.

 

Iain

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post-7533-0-11431900-1502364734_thumb.jp

 

Picture attached. Notice shard of metal from either the head or bolt hmmm

 

Next steps dismantle rocker shaft (it is a new one from Rimmers when engine built)

 

Order shorter pushrods.

 

I should hopefully be able to remove the broken bolt from the head, groan.

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That breakage looks like coil binding to me.....been there, not with the TR but an "A" series with a wild cam...........it pulled the studs out of the head. :o You have been lucky, the head looks salvageable

 

Iain

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Hi Colt,

the fracture face looks nice and clean. Something powerful snapped it.

 

When the valve is fully down there MUST be gaps in the spring coils.

 

Before ordering shorter rods try and find out why the coils were tight together.

 

Roger

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Lest take a double check on this.

 

The 3mm I refer to in post 5 is an extra 3mm of valve movement above that created by the rocker shaft actuation.

 

So with the rocker shaft in position and bolted down correctly, when number one valve is fully open, springs compressed, can you,with a lever between the bottom of the rocker shaft and the head of the valve, push that valve down another 3mm?. Repeat for all 8 valves. to check

 

Iain

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ok checked, theres 4mm extra travel on each one when fully compressed. The end 2 where the bolts snapped aren't completely down for that reason but I still feel there's 4mm on those also.

 

My gut feel is I weakened the bolts when I last pulled the head (we had a very long gas pipe on them) so I'm going to replace all 4 and see how that goes.

Edited by Britishbest
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The rocker pedestal bolts do not get torqued to the same as the head.....that would break them. Head is 100-105lbft. Rocker pedestal 24-26lbft..........

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My gut feel is I weakened the bolts when I last pulled the head (we had a very long gas pipe on them)

so I'm going to replace all 4 and see how that goes.

 

Sorry, but that is idiotic. You are lucky if you can

just replace the studs and suffer no other damage.

 

AlanR

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Sorry, but that is idiotic. You are lucky if you can

just replace the studs and suffer no other damage.

 

AlanR

Hi Colt,

this is a good warning. The stud broke for what is an unknown reason.

Replace the studs as normal. I would go for short push rods just yet.

When all assembled rotate the crank and make sure the rocker arms/push rods/Valves and springs are all the right thing and have plenty of room for more travel.

 

If this appears to be working then run the engine.

 

Only do further mods once it all appears to be OK.

 

As for stressing the stud by lifting the head - did the front wheels come off the ground?

If not then I don't think you did anything wrong there.

 

Things can break when you least expect it.

 

Roger

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