Does size (of a mainspring) matter?

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Of course it does, but how precise must it be?

I had to replace the mainspring of a cal. 342. Ranfft says it should be 250mm, but the one I purchased was 300mm (I measured it after I failed to get it in the barrel). I got this one on ebay but I've seen them on other websites for this caliber, clearly advertised as 300mm. After breaking it trying to rewind it by hand, I bought one from another seller on ebay, and though it's currently in the barrel, it does seems smaller. The opening of the shipping ring was significantly smaller too, which made me a little nervous about it unwinding quickly as I pushed it into the barrel, but I got it in.

So does 50mm difference matter? Is there another source of information to confirm what Ranfft says?
Edited:
 
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Of course it does, but how precise must it be?

I had to replace the mainspring of a cal. 342. Ranfft says it should be 250mm, but the one I purchased was 300mm (I measured it after I failed to get it in the barrel). I got this one on ebay but I've seen them on other websites for this caliber, clearly advertised as 300mm. After breaking it trying to rewind it by hand, I bought one from another seller on ebay, and though it's currently in the barrel, it does seems smaller. The opening of the shipping ring was significantly smaller too, which made me a little nervous about it unwinding quickly as I pushed it into the barrel, but I got it in.

So does 50mm difference matter? Is there another source of information to confirm what Ranfft says?

Base caliber for the 342 is the 330. AF list the dimensions in their cattle dog, length as 300mm.

 
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Bestfit calls for 110 mm x 0.10 mm x 10 3/4”. Bestfit appears to agree with Ranfft.
 
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The confusion continues, then. I suppose either one works just fine.