Bent push rod

roxburd · 05-05-13, 08:20 PM

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

**Windrush** · 05-05-13, 08:29 PM

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.

roxburd · 05-05-13, 08:38 PM

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

air-cooled · 05-05-13, 08:55 PM

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

**steve-v12** · 05-05-13, 09:00 PM

I have straightened several bent D pushrods and never had a problem with them.
Steve :)

**martinpaff** · 05-05-13, 11:05 PM

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.

roxburd · 06-05-13, 12:43 PM

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

novo · 06-05-13, 01:08 PM

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.

05-05-13, 08:20 PM	#1
roxburd Member Join Date: Oct 2012 Location: Suffolk Posts: 39	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

05-05-13, 09:00 PM	#5
steve-v12 Forum Supporter 2011 Join Date: Oct 2009 Location: I live in sunny norfolk. Posts: 2,319	I have straightened several bent D pushrods and never had a problem with them. Steve :) __________________ 1949 Lister B. 1941 Bamford EV3. 1941 + 1949 lister d. Sectioned lister d. Carera saw bench, Petter A. Petter A1.

06-05-13, 01:08 PM	#8
novo Senior Member Join Date: Jul 2011 Location: Essex Posts: 6,134	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. __________________ Growing old is mandatory,Growing up is optional

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05-05-13, 08:29 PM	#2
Windrush Forum Supporter 2011 Join Date: Apr 2008 Location: Matlock, Derbyshire. Posts: 6,949	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.

05-05-13, 08:38 PM	#3
roxburd Member Join Date: Oct 2012 Location: Suffolk Posts: 39	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

05-05-13, 08:55 PM	#4
air-cooled Senior Member Join Date: Sep 2007 Posts: 5,825	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

05-05-13, 11:05 PM	#6
martinpaff Admin Team Forum Supporter Join Date: Mar 2007 Location: Lincoln, UK Posts: 11,675	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.

06-05-13, 12:43 PM	#7
roxburd Member Join Date: Oct 2012 Location: Suffolk Posts: 39	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