Wear pattern of a Big Valve engine ?

[Canuck]

Not quite "how long is a piece of string ?" but what wears out first in a thrashed/hard driven engine ? Most engines follow a pattern of wear. One part more frequently wears or fails eg crank on a 356.

I am about to start a restoration on a 69' +2 with a Big Valve head fitted to the original engine. Nothing is known of it's history ( 48 thou. miles ? and I have not been able to drive it on the road nor under load - yet.)

The compression is excellent 10.6 : 1 measured. No smoke. The 'mechanical sound' has a hint of piston slap and maybe some little-end noise. Same general mechanical sound when any of the 4 plugs disconnected.

So what wears/breaks most frequently on these engines ? Any audible symptoms ?

i will be doing a full mechanical restoration but it would be nice to know from other Big Valve owners , what to expect in my first twin cam.

Cheers Canuck

[gordonlund]

Now that's a good one. It all depends on how hard you drive it ie what revs you use as to what will go first.

7,000 + and your big ends and mains will have a short life if your rods don't break first. Pistons tend to be fragile at consistant high end demands. Detonation can lead to top edge burn out leading to top piston ring distintegration.

High mileage willl see valve guides wear oval (no valve stem seals) and oil consumption will drop down to below 500 miles / pint. Exhaust valves will burn out eventually at between 50,000 - 75,000 miles. Modern fuels, lead replacement / octane booster additives and replacement guides / valves to run on unleaded fuels all add to valve component lextended ife.

Then there is the perenial water pump saga. Tight fan belt, low pump mileage. Same with the timing chain. Every time I take the head of I replace the water pump bearing and seals and fit a new timing chain. They are relatively cheap in contrast to everything else involved.

The secret is to keep the high rev stuff limited to short burst. For most of my road work I keep revs down to below 5,000.

I used to love hearing my twink at 7K but I have a drawer full of burnt out pistons and valves to remind me what that sound costs!!!

Gordon
68 +2 (Rhubarb)