Centre Web

Mike 30 · Post by **Mike 30** » Sun May 08, 2011 6:46 am

Hi to all. Can some one help me with some technical information. I have a 1955 G9 500 Twin. I want to no what the Centre Web tunnel size is. ie The size of the tunnel when the centre Web cap is torqued up with no shell bearings fitted. The work shop manual supplies a size for the conrods but no information on the Centre Web.

petert120r · Post by **petert120r** » Sun May 08, 2011 10:33 am

I assume you mean the internal diameter between the cap and base without shells. Just measured one and it's 1.75" although I'm mystified as to why you want to know. Standard and o/s shells are available from JSL.

zwarts · Post by **zwarts** » Sun May 08, 2011 5:11 pm

I cannot find a dimension for this in any of my handbooks, but my own 650cc M31 centre main bearing housing was 1.771 inches dia. where the shell bearings fit. It was also truly round (i.e. no ovality) to within 0.001". This would be the same for the G9 and G11 as the crank bearings were the same on all these models ,except for the very early twins ,which had a combined centre main shell and thrust bearing arrangement ( I think that these were also the same dia. as the later bearings with the seperate thrust washers, but differed in width). Hope this helps,
Bob.

Mike 30 · Post by **Mike 30** » Mon May 09, 2011 10:02 am

Thank you to peter120r and zwarts for the reply about the centre web tunnel size on my 1955 G9 500 Twin. The reason I wanted this info was to check the tunnel size, as I am rebuilding the bottom end. This tunnel size needs to be perfect to achieve good oil pressure.

Groily · Post by **Groily** » Mon May 09, 2011 12:49 pm

A web I have on the shelf tallies exactly with Bob's measurement, at 1.771" and 0.001" ovality. Nuts done up per book. So either our measuring sticks have the same inbuilt errors, or that is what the size is. (I used calipers and a 2" mike, and then a known-good slide gauge with clock on it, and (surprisingly) got the same answers to under half a thou.)Edited by - Groily on 09 May 2011 11:53:14 AM

petert120r · Post by **petert120r** » Mon May 09, 2011 2:05 pm

Whoops! Tried it again with a dial caliper and guess what? 1.771".

zwarts · Post by **zwarts** » Mon May 09, 2011 2:46 pm

While you are doing the job properly, don't forget to check the con rod big end housings in the same way. The correct dimension here is 1.7710 to 1.7715 inches. I have seen many rods where this dimension is way out, mainly ovality, due to the nature of the loadings in the big end bearings.
Bob.

Mike 30 · Post by **Mike 30** » Thu May 12, 2011 7:30 am

Thanks to all who helped reguarding the Centre Web tunnel size for my 1955 G9 500 Twin Matchless. I can now move foreward. Take care Mike.