potential 420 problems
Discussion
Not sure if all 420's are the same, but I did some work on one that had a lot maybe 200 thou skimmed of the block. so it had to have 120 thou of rocker shims (this is very extreme and would and required 2 kits), this caused a lot of load on the tip of the rockers, and 2 had broken off.
maybe it was just this engine, but maybe all 420 was made like this due to the pistons used and to get the compression up.
but it's worth looking to see if your has a lot of shims, if so change to adjustable rods, rockers, or change the rockers regularly.
most tuners don't recommend any use of shims, let alone 120 thou (3mm ish for the young-ones)
maybe it was just this engine, but maybe all 420 was made like this due to the pistons used and to get the compression up.
but it's worth looking to see if your has a lot of shims, if so change to adjustable rods, rockers, or change the rockers regularly.
most tuners don't recommend any use of shims, let alone 120 thou (3mm ish for the young-ones)
Hi Leigh
When i rebuild my 420 there was no shims under the rockers for any preload or for any major head skims.
My heads are wire ringed, i was told it was done this way for higher compression engines in the eithties and coman practice for tunning engines for higher compression.
From what i have seen of my heads they were normal apart from staged 3 ss valves and heavely ported.
200 thou off the heads sounds like a lot and would cut into valves i would have thought, you will need to put valve pockets in the pistons to clear.
Would it not be easer to start with new standard heads.
Cambelt
When i rebuild my 420 there was no shims under the rockers for any preload or for any major head skims.
My heads are wire ringed, i was told it was done this way for higher compression engines in the eithties and coman practice for tunning engines for higher compression.
From what i have seen of my heads they were normal apart from staged 3 ss valves and heavely ported.
200 thou off the heads sounds like a lot and would cut into valves i would have thought, you will need to put valve pockets in the pistons to clear.
Would it not be easer to start with new standard heads.
Cambelt
Campbell said:
Hi Leigh
When i rebuild my 420 there was no shims under the rockers for any preload or for any major head skims.
My heads are wire ringed, i was told it was done this way for higher compression engines in the eithties and coman practice for tunning engines for higher compression.
From what i have seen of my heads they were normal apart from staged 3 ss valves and heavely ported.
200 thou off the heads sounds like a lot and would cut into valves i would have thought, you will need to put valve pockets in the pistons to clear.
Would it not be easer to start with new standard heads.
Cambelt
Hi Cambelt,When i rebuild my 420 there was no shims under the rockers for any preload or for any major head skims.
My heads are wire ringed, i was told it was done this way for higher compression engines in the eithties and coman practice for tunning engines for higher compression.
From what i have seen of my heads they were normal apart from staged 3 ss valves and heavely ported.
200 thou off the heads sounds like a lot and would cut into valves i would have thought, you will need to put valve pockets in the pistons to clear.
Would it not be easer to start with new standard heads.
Cambelt
I said it was taken off the block not the Heads.
The heads was not wire ringed, but was ported and had double springs (same as my 400SE)
The pistons was pocketed.
The block was stamped NCK after the skimming and matches the car, so I guess this was done from new.
The heads have been off previously and had been re-fitted with tin gaskets (so not going to help AFAIK TVR used a copper ringed composite gasket) I fitted Elring composite Gaskets, and Genuine composite valley gasket( had top open the bolt holes to get them to match).
Was you pre-load on when you rebuilt yours.
I had similar issues with a 420 caused by momentary sticking of the throttle cable and an over revving engine with no rev limiter. The steel pads on a couple of rockers broke-up caused, I assume, by valve bounce. Luckily the strainer caught all the bits, but when I first got the car the strainer had fallen off and was lying in the bottom of the sump! I was then told there was a period when there was a general issue with rockers breaking up, but I blame the revving in this case. Initially there were shims under the rocker shaft brackets, but once new hydraulic tappets and a composite valley gasket were fitted the shims had to be removed.
I habe hydraulic tappets so no need for anu preloading as the heads and decks have never been skimmed. If i were to change to solid tappets i would use ajustable push rods.
My gassgets are from je engineering and are the copper composite type with the twin valve springs, the rubber metal valley gassget and standard head bolts.
I would have thought there would have been a drop in CC of the engine with 3mm off the block and the compression would have gone up a bit with that work done. Not 4.2 more like 4.15ish.
Im running alloy rockers with the steel tips but i know you can get steel rockers. Proberly better for higher revving engines.
When i rebuilt mine i fould one of the tips had broken off and i put that down to the last owner over revving. Ive not had a problem since i changed them but i check every few thou miles.
My gassgets are from je engineering and are the copper composite type with the twin valve springs, the rubber metal valley gassget and standard head bolts.
I would have thought there would have been a drop in CC of the engine with 3mm off the block and the compression would have gone up a bit with that work done. Not 4.2 more like 4.15ish.
Im running alloy rockers with the steel tips but i know you can get steel rockers. Proberly better for higher revving engines.
When i rebuilt mine i fould one of the tips had broken off and i put that down to the last owner over revving. Ive not had a problem since i changed them but i check every few thou miles.
Even with comps I would have still had to use about 80 thou of shims, hence deciding to use adjustable rods, and even these was too short fully wound in, so I had to make them shorter.
I can understand decking the block, my 400SE has been so has my race engine, but no where near how much this 420 has been.
I can understand decking the block, my 400SE has been so has my race engine, but no where near how much this 420 has been.
Edited by TVRleigh_BBWR on Tuesday 19th April 13:44
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