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Old 07-15-2011, 03:54 PM
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Rebuilt 236 Chevy six problem

I recently rebuilt a '55 235 stovebolt six and after only 6 hrs. of running, the No. 2 intake valve stuck open. Was I P____d when that happened. Here is the short history of the rebuild:
Had the head boiled, magnafluxed, decked, new bronze valve guides, ground the valve seats, reconditioned and mated the original valves to the guides and seats, replaced the valve seals, re-used the original valve springs (within orig. spec.), used Amsoil high ZDDP content break-in oil and did not over-rev the motor. We used accepted normal break-in procedures. We had the hed redone AGAIN and the second machine tech said all the exhaust valves were galled and he replaced the No. 2 intake valve. He also replaced all the valve guides with new bronze guides.
QUESTION: What would have caused the intake valve to stick? And why were all the exhaust valves galled?

After reassembling the motor for the second time, we noticed the oil feed to the rocker arms does not do a good job oiling the contact point between the rocker arm and top of valve stem. It seems the original design is not the best. True? Or are we missing something? Your points of view will be appreciated.
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Old 07-15-2011, 04:58 PM
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Quote:
Originally Posted by jpd37
I recently rebuilt a '55 235 stovebolt six and after only 6 hrs. of running, the No. 2 intake valve stuck open. Was I P____d when that happened. Here is the short history of the rebuild:
Had the head boiled, magnafluxed, decked, new bronze valve guides, ground the valve seats, reconditioned and mated the original valves to the guides and seats, replaced the valve seals, re-used the original valve springs (within orig. spec.), used Amsoil high ZDDP content break-in oil and did not over-rev the motor. We used accepted normal break-in procedures. We had the hed redone AGAIN and the second machine tech said all the exhaust valves were galled and he replaced the No. 2 intake valve. He also replaced all the valve guides with new bronze guides.
QUESTION: What would have caused the intake valve to stick? And why were all the exhaust valves galled?

After reassembling the motor for the second time, we noticed the oil feed to the rocker arms does not do a good job oiling the contact point between the rocker arm and top of valve stem. It seems the original design is not the best. True? Or are we missing something? Your points of view will be appreciated.
Bronze comes in several alloys from soft to quite hard, all require a high quality surface finish which is often hard to accomplish. This requires a sharp reamer and lots of cutting fluid to wash the chips out so they don't get caught up between reamer and guide. If done free hand they need to be a piloted design to get the alignment correct before starting the cut. The valves really should have a hard chrome or nitro-carb finish. Modern bronze alloys tend to be quite hard and they gall against unplated, unhardened, or stainless steel stems, so I certainly would never reuse old valves with stems having no surface treatment. For a street engine that doesn't see racing style RPMs a cast iron replacement guide is more forgiving of machining errors and lack of lubrication than bronze. Not that bronze is bad it just requires the person doing the work be good at it.

Bogie
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Old 07-16-2011, 09:29 AM
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hello,

Seems to me the valve stem clearence to guide contact was too tight.

The intake valves were less sensitive to heat from cool air fuel mix intake.. so they got away with the tight clearence.

But those exhaust valves got fried. They could have used a little oil seep but none was getting through.

What lube did you put on the stems before inserting them?
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Old 07-18-2011, 10:06 AM
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We had the head work completed by (what we thought was) a reputable machine shop familiar with old stovebolt motors. Guess some references aren't so good. We took the head to a second machinist and we are confident the job was done right this time. We will find out.
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Old 07-18-2011, 10:31 AM
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hello,

Just out of curiousity...

Does the 235 have an oil filter or cannister?

What is the cruising speed of the truck?
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Old 07-18-2011, 12:38 PM
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We installed a remote oil filter and dumped the canister type. It is working better than expected.

The cruising speed at the point of failure was 55-60 mph. The car will travel comfortably at 70 mph, but because we had just rebuilt it, we were being easy on it. We had also done the basic break-in procedure and had been driving it around on local streets at low speeds for about a week before the motor broke.
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Old 07-18-2011, 12:44 PM
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hello,

With stock gears and tranny.. I would have imagined 40-50 mph cruising speeds.

When your driving the higher speeds.. how much throttle are you using?

Like 25% or 50% or 75% or are you just pushing the gas pedal almost to the floor?

The oldest vehicle I ever owned and drove was a '59 chevy truck with the 235 one barrel carb...
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Old 07-18-2011, 07:35 PM
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It is a 55 Chevy Belaire with a 2 speed powerglide trans. I do not know the gear ratios for the trans or rear end.
At 55-60 mph, the motor does not sound like it is reving hard at all. I'm using maybe 2/5 of the loud pedal. There is plenty more throttle left to go at 55-60 mph.
Our machinist said the last machinist did the work wrong. It was not the rpm's that caused the problem. It was the quality of the workmanship.
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Old 07-18-2011, 07:46 PM
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yeah,

I'm guessing you are right about the workmanship.

If you were cruising at more than 50% throttle.. I would have thought the engine was getting over-wound.

But 2/5s or 40% throttle should not damage the engine.

Are you going to try cast iron guides or bronze guides again?
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Old 07-19-2011, 09:59 AM
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As I stated in my original post, the machinist replaced the valve guides with new bronze units. We have been driving the car for several days since the repair and so far everything is working fine. We are confident the repair will be most satisfactory.
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