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Old 05-05-13, 08:20 PM   #1
roxburd
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Default Bent push rod

Hiya,

Just pulled some more bits off my D. The push rod pushing the valve with the smaller diameter (the one nearer the carb/exhaust) is bent!

Exhaust valves are usually bigger than inlet valves so I'm guessing it's the inlet valve rod that's bent.

The engine ran fine with it like that - wonder how it got bent... the lower two inches of the rod were discoloured too, like it had got very hot at some point...

Are the rods identical? Can I just get a push rod or do I need to get one specifically for the inlet? Where can I get them?

Just hope I'm not gonna find it bent from a lubrication failure on the bottom end and that's all chewed to bits...

Cheers,
Dave
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Old 05-05-13, 08:29 PM   #2
Windrush
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Valves are the same diameter Dave, just that the inlet has a thicker head. The pushrods are the same as well, you should be able to straighten it but why not wait until you know of any other parts you may require and then post an add in the WANTED section, you may get everything in one go?

Pete.
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Old 05-05-13, 08:38 PM   #3
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Pete,

Straighten it!!?? Really?? I figured once-bent, it would never hold straight again... but if you're saying it will then I'll do that. I've seen straightness being checked just by rolling on a flat surface... is that OK? And what's the best way of bending - knocking with a mallet, protecting the rod with blocks of wood maybe?

Cheers!
Dave
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Old 05-05-13, 08:55 PM   #4
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It really depends on how bent it actually is.

This is one I've pulled recently from an engine which failed in service.



As you can see its a compound bend in this direction ......



..... and a compound bend in the other direction




I won't even be bothering to straighten this, it'll be in the scrap bin tomorrow. Luckily I have plenty push rods available.

Phillip
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Old 05-05-13, 09:00 PM   #5
steve-v12
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I have straightened several bent D pushrods and never had a problem with them.
Steve :)
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Old 05-05-13, 11:05 PM   #6
martinpaff
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Just a very minor point from the first post Dave; inlet are valves are usually bigger than exhaust valves, not the other way around. There is a very good reason - the exhaust leaves the cylinder under a lot of pressure so only needs a small valve, but the piston has to suck the fresh air in with no assistance, so a bigger valve makes it easier.

Martin.
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Old 06-05-13, 12:43 PM   #7
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Wow, yeah, I had a go and the bent one is now dead straight as far as I can tell by eye or by lining it up against a rule... don't suppose the discolouration is a problem. I guess I'll go with it as it's not gonna be running for thousands of hours.

air-cooled - that's bent rod! Flippin eck, mine was out by about a centimetre at the end and I thought that was bad!

Martin! That explains a few things!!! I was convinced that on my 1980's bike engines the exhaust valves were bigger to take account of the fact that far more gas has to pass through them each cycle... but I must have made that up!!! It seems inlet are always bigger - makes sense too.

Cheers,
Dave
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Old 06-05-13, 01:08 PM   #8
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The best way to check if a pushrod is straight is to roll it on a flat surface. If its even slightly bent you'll see the rise & fall of the bend easily.
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