Thermal Shock?

[melville]

So here's what happens:

I put my longboard in my 71 Kombi and drive 15 miles or so to my usual surt spot. The trip is almost all freeway, and on a hot sunny day the air temp will be 70 F or so. The last half mile of freeway is about a six or seven percent grade up, then I descend through the trees, where the temp in the shade might be 55 F or so, in first gear, engine spinning with no load. Then I shut it off.

Am I theoretically cooking/chilling/heat soaking the motor? Is this treatment one of the reasons my exhaust valves tighten to zero every 3000 miles? I'm not too concerned yet, as I've gotten 30,000 miles out of an $800 Bus, but when I build another motor for this one should I also get a dedicated surf beater?

Just curious.

Thanks

Mel

[raygreenwood]

Sounds like valves stretching. Bad ......watch for snapping a head off. If they were getting looser every 3k...I would think cam, lifter or adjuster screw wear...also not good. What kind of engine, valves and spring tension are you running?

Yes...you could be overheating the heads. That can stretch valves as well. Ray

[melville]

Ray:

It's a very stock appearing dual port T1. Still has all the throttle positioner stuff on it! Little vacuum lines running everywhere.

BUT:

PO said the local German guy (you know, from Germany) did a bunch of work on an old rebuild (early 80s? not many miles) to make it function without needing octane booster. I run it on 87 locally (not very hot) but fill with 91 (best available in CA) when I go inland to 100 F temps. My guess is that the old rebuild had higher compression than we can run today and German guy added a barrel shim or two.

Has external cooler (on the outside of bus, apparently very popular around here) plumbed into full flow filter with proper oil thermostat.

Has steel pushrods. I presume Berg or similar, so I adjust to .006 and about every 3000 the motor sounds like it has "pit lane speed limiter" for the first 1/4 mile until it gains warms up just a touch and gains clearance.

Valves, retainers, and springs look to be stock.

Runs about like a stocker should and has not lost any power in the time I've had it.

Later

Mel

[Piledriver]

Valves are stretching or the seats are receding, or keepers are eating their way up the keeper grooves.
(if the keepers have no gap, probable, easy to see)

I'd pop the heads and check it out before it grenades.
It's just headwork at that point, perhaps just a set of stock ex valves and properly ground keepers etc.

Wait too long and its a new motor, after an expensive and bloody inconvienent tow.