Bristol pushrod problems

[Ron Hollis]

Last year the horizontal pushrod on number 1 exhaust fell out? and got bent. On inspection with a replacement rod, I noticed that the rod on the exhaust side, was virtually in contact with the aluminium sleeve and there was a witness mark on the bent rod at this point (which obviously could have occurred after it fell out). The rocker sat central on the valve stem, so that was in the correct position. I did have the head rebuilt about 5,000 miles ago with new valves, guides etc; one job was swaging the sleeves firmly into the rocker covers, so this could have brought the sleeve nearer the rod. Looking at the other rods most are not central in the sleeve due to casting tolerances. Because of the head rebuild I thought a sticking valve unlikely and compression is fine; so made a shim washer for the rocker shaft to move the rocker further from the sleeve (20 thou. thick). Sorted I thought.
Today it happened again, same valve. Before I take the head off is there any useful experience out there?

[TTM]

Ron,

Your other post mentions using Weber carbs, which suggests to me your engine is not quite standard spec anymore. What about the camshaft and valve springs?

With an excessively high tappet clearance, the pushrod may not be held securely enough between its two rockers (N340760 & N341240), moreso perhaps before the engine is fully warmed up, thus causing the pushrod to drop. This is just theoretical speculation on my side though as I have not heard before of the problem you are facing.

[Ron Hollis]

Thanks for the interest and speculation.
Carburettors are the only modification  from standard 100D2 spec.
Camshaft was checked and fine at time of head overhaul and springs were tested. Inspection now shows springs ok. Problem happened at a modest 3500 rpm. so bounce should be relevant
Valve clearances were checked less than 1000 miles ago and are standard at 2 thou. cold. My Bristol Engine Manual states that this will result in a running clearance of 8 thou. So more likely to drop out when hot.
Nick Finberg from Classic Autos has never heard of it either. 

[10kph]

Hi Ron,
           You have given a few clues. Rebuilt head, pushrod dropping out, occurs when engine hot, c/r ok, Leads me to think that the valve stem clearance is not large enough for an exhaust valve in bronze guides or stem seals have been used .both will lead to a temporary sticking valve. Do not wait until it is hit by a piston.
  Hope this helps
      Tony ( bristol Aceca )

[Ron Hollis]

Hi Tony,
Thanks for your interest and warnings. No stem seals are fitted. New standard valves and guides were fitted, but this doesn't preclude insufficient clearance. On removing the pushrod an oily/aluminium deposit was apparent around the pushrod head at the valve end. This could be the result of the rod rubbing on the sleeve over time. I don't think it would have occurred after the rod fell, as the rod would immediately stop moving. I am presently investigating the sleeve.         

[Ron Hollis]

For the record; I removed the head and sure enough it was a sticking valve. No. 1 exhaust guide was cracked at it's tapered end; it looks as though products of poor combustion had been accumulating on the guide, occasionally stopping the valve closing and cracking the guide. A check showed the same on number 4. Based on advice from historic racer, new guides fitted, shorter and without taper and ignition being checked to explain poor combustion.

[Flyinghorse]

Ron-Might be worth looking at iridium spark plugs if not already running them. Theres a thread on the forum regading the benefits seen, but no compensation for the correct timing.
Graham