Valve springs with 428s? (damaged engine)

[Baal]

Well long story short, after modifying a rally clio with a worked head and some 428s ive bent 2 sets of valves (had to change piston once) and i just cant figure out why since it they broke at high and very low rpms.

Now that it has a new vvt pulley in and they broke again im running out of options, i bough the car used as a group N Clio and im thinking the head was previously milled (cheating), and now we have overlap issues, the strange thing is that it doesnt happen right away, it once did it at idle, then it happened in after 15 minutes of intense testing in a tarmac stage.

AFAIK the 428 run on stock valve springs, retainers etc right?

 

[Gray]

stock springs and retainers for 428's yeah

 

[sideways danny]

Sounds like timing is slipping one engine is started.

 

[Baal]

Sounds like timing is slipping one engine is started.

And how could that happen?

The belt was changed the 1st time this happened.


Can it slip without the belt actually jumping teeth? like when the screws of adjustable pulleys slip (i have the stock VVT pulley tho)

 

[sideways danny]

yes easily, all the pulleys are free floating (non keyed) if the correct timing procedure wasn't used and the correct tightening torques and angles adhered to it will do exactly what you've experienced

 

[Baal]

yes easily, all the pulleys are free floating (non keyed) if the correct timing procedure wasn't used and the correct tightening torques and angles adhered to it will do exactly what you've experienced

Well we just opened the engine and it was the same valve: exhaust valve of the same cylinder, the valve guides are also new, the valve springs on that piston are also new... why is the same valve breaking over and over again?

If it were the time slipping why its not hitting the other valves?... what could be causing this?

 

[sideways danny]

sorry, you said a "set" of valves. took that to mean all cylinders

check the piston in that cylinder is fitted the right way round. If the head has been skimmed it could well be enough to make contact if one piston is fitted the wrong way

 

[Baal]

Fitted in what way wrong?

Also if there is overlap shouldnt it happen right at starting or the first time i rev it high at high temperatures, not so randomly? (it did it once at idle, it did it another time after 15 minutes of hard tarmac testing)

We already changed 1 piston (it damaged it the first time), and my mechanic just called me telling me the piston is damaged again...

 

[sideways danny]

where abouts are you? can you get any pictures of the failures?

 

[Baal]

Mexico.. i guess i can take some pictures but it wont help you much, just a broken valve probably bending the rest in that cylinder and a very dented piston.


They measured the head and its milled 0.020" which shouldnt cause overlap issues afaik, we are going to measure the cam to see if there is some manufacturing mistake or something.

 

[Baal]

Jesus christ, the head is done for, the combustion chamber of that (the same) piston is destroyed and the piston is even worse... ill upload pics later

 

[Baal]

Exhaust valve broke at high rpm and destroyed everything, im installing the head of my street clio (including all pulleys), ill get measurements to see if the head was over-milled.

Is there a possibility that it could be the crankshaft without hearing noise or feeling vibrations, seems unlikely but quite frankly we still have no f**king idea what is happening.

 

[sideways danny]

ahh, i see what's happened.

Looks like the fairly well known fault of valve heads falling off. They're made in 2 pieces and they seem to just fall apart sometimes. There's also a theory about damaged valve guides (from bent valves) increasing the likelyhood of valve head failure

http://catalogue.bgautomotive.co.uk/technical_bulletins/TB0702.pdf

 

[Baal]

Oh well its not much use now to know why the head was ruined lol, the valve guides and valves were new, the head was inspected for damage but it didnt show any, the hidraulic lifters werent, maybe they werent aligned perfectly or something, the head was also milled which doesnt give much margin of error.

We are going to use the destroyed head to test the intake ports for maximium tolerances to porting (finding the water jackets), how susceptible is the Clio to torque loss when opening intake ports?

 

[Baal]

should piston rods be replaced always?

 

[Laine_16v]

Nah you wouldnt need to change the conrods.

 

[Turu]

Why dont you put the other head, tighten it with a little bit of modeling clay mixed with oil in the piston pockets and rotate the camshaft to see if there's enough clearance in all the valves? You should have at least 2mm in the intake valves and 2.5mm in the exhaust valves. If not, you have to grind the valve pockets until you have that clearance.

Check also clearances in the main and rod bearings, and be careful the head is not milled in an angle, as that could cause that the valves in one of the pistons would hit and the others wouldnt

 

[sideways danny]

something is VERY wrong if the clearance isn't enough on a 428 exhaust cam.....it's standard

 

[luis_villarruel]

which part of mexico are you?

i think Turu is right, try to do it!! good luck!!