# Length between 2 centers excenter rod and expansion link

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#### Mechanicboy

##### Well-Known Member
Hi folks..

I am not sure there is right length between the excenter rod and expansion link to the Stuart compound twin steam engine. The measure is 2 1/4" in the drawning of expansion link if use reversibel Stuart compound twin steam engine. Also the excenter rod has 2" length between centers and the expansion link from fastening hole for excenter rod to the bow hole is 1/4 = Total length is 2 1/4". Is it fault measure at 1/4" in the drawning of expansion link? Would it be 1/8" from fastening hole to bow hole in center of measure? See at the drawing of expansion link: R 2 1/4" and R 2"

My Stuart compound steam engine who are not reversible has excenter rod with the total length between centers 2 1/8" as this drawing..

#### Charles Lamont

##### Well-Known Member
I don't have the original drawings to check but I can't see anything wrong with your expansion link drawing. Perhaps the valve rod has to be shorter with reversing gear?

#### Mechanicboy

##### Well-Known Member
Charles, let us say the excenter connecting rod without reversing gear has length at 2 1/8" from big end to little end versus the excenter connecting rod with reversing gear has length at 2" from big end to little end and the fastening hole for little end to the bow hole is 1/8" where give the total lenght at 2 1/8". Then at drawings of reverse gear expansion link has a distance at 2 1/4", also minus 1/8" too short? In metric 3.175 mm to short.

#### Charles Lamont

##### Well-Known Member
Jens, I agree that there is a 1/8" difference between the reversing and non reversing designs.

There is nothing wrong with the expansion link drawing. If you tried to make the eccentric rod holes 1/8" from the centre line of the curved slot, they would break into the slot. The 1/4" is necessary to allow the eccentric rod ends to move past the valve rod knuckle when reversing.

So the logical conclusion is that the extra 1/8" must be allowed for somewhere else, for example by moving the valve rod knuckle 1/8" closer to the valve chest.

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