Which Pushrod Goes Where

I have been in process of re-ringing my 20 horsepower lawnmower engine and during the takedown process, the pushrods came out of position and I layed them on the workbench together.
I'm putting it all back together now and the rods are different in that one of them is all steel while the other one is made of an aluminum tube with steel ends.
Can anyone tell me which is the intake and which one is the exhaust rod? I suspect the steel rod is the exaust because it should be subjected to more pressure but I 'm not sure at this point.
My luck I'll install them that way and the aluminum rod will turn out be the exhaust rod.

a reply to: CharlesT

If it is a Briggs & Stratton then the aluminum one is for the intake.

On these OHV engines the cam has a ~0.010 lobe to keep one valve slightly open during the compression cycle for compression release and easy starter cranking. Since this is a lobe on the cam and not a "slide out of the way lever" design used on other engine brands, this means that one valve (on mine it is the intake valve) is slightly open for all or most of the compression cycle at all times. This seems like a less than efficient design but it apparently works. The open valve has an aluminum push rod to compensate for thermal expansion differences between the aluminum housing and head and a steel push rod. With a steel push rod at this location the valve gap would open on engine heat up and compression release on a hot engine (and easy cranking) may not occur.

a reply to: Lumenari
Yes, it's a Briggs. I have encountered this once befor on an engine I just tore down for parts but I didn't know why they made them different. I suspected just cheap but I didn't really know.
I understand the reasoning now. Thanks very much. very

a reply to: Lumenari

That's pretty good! I was thinking the same

You will also note that parts sites NOT B&S sell the aluminum rods for exhaust side. So the old debate is still alive judging by the parts sites.. A lot of racers that use these engines for racing (10k rpm) use aluminum rods for both intake and exhaust.

Aluminum Push Rod Specifications

This is a replacement push rod for your lawn tractor engine. It is made of aluminum and is the push rod used for the exhaust valve. Occasionally, the push rod in your engine can become damaged, and will need to be replaced before your engine runs at full capacity. This push rod is approximately 6 inches in length, and is an OEM part sourced directly from the manufacturer.

B&S has a nice page for your valve adjustments.
www.briggsandstratton.com...

Personally, I've switched them without any horrible effects like flying valves and secondary oil drainage holes.

originally posted by: Lumenari
If it is a Briggs & Stratton then the aluminum one is for the intake.

If it's a Stratton & Briggs, it's the other way round.

Sorry, just had to.

Happy mowing.

a reply to: billxam
I did notice the steel rod was around 3/16 inch longer than the aluminum one. I am reusing the original rods and hope I don't have to adjust valve lash but thanks for the reference material. I'll use it.

a reply to: CharlesT

hmm - unless you has mixed up the cam followers too - then putting the roccker arm on the head - with no pushrods - should indicate wich rod is for which valve a 5mm difference should be noticable

otherwise - this is a cautionary tail of :

always lay parts out in order - and keep " matched " parts - like con rods together

Plastic bags and a sharpie are your friends

a reply to: ignorant_ape