Crank Pin and Rod Bearing Shells

[smdl]

Hi, folks.

I'm in the process of rebuilding my 850-T engine, and have run into something confusing.  By all accounts that I can find, it seems that rod bearings for all big blocks are the same from 1967-1997.  You can see this here, on the MG Cycle website:

http://www.mgcycle.com/index.php?main_page=product_info&cPath=68_126&products_id=1159

However, when I look at the Guzzi manuals, the crank pin measurements are different.  Because of the wear on my crank, I went to third undersize, and I now see that the crank pin measurements are different across three separate manuals:

V700/750 manual (page 26)   1.7028"-1.7036"
850-T manual (page 20)   1.700"-1.702"
850-T3 manual (page 89)   1.7025"-1.7030"

Problem for me is that I didn't notice this before, and must have gotten the measurements for the V700 manual (the shop that did the work has marked it as 1.7028-1.7036), and it seems that the fit is too tight.  When I test-fit the rod to the crank pin (lubricated, of course), the rod is tight on the journal and will not turn.  I'm trying to locate my Plastiguage to check clearance, but it is eluding me, at present.  When I mic the journal, I get 1.7028, but I don't completely trust my measurements as I don't do this every day.

The question is, if the bearings are all the same, how is it possible that the crank pin specifications in the manuals are different?  Has anyone run into this before?

I suppose I could go back and have the crank ground a little further, but that is a pain as it has already been balanced. 

Any input would be appreciated.

Cheers,
Shaun

[JoeW]

Welcome to the world of Guzzi, where the only thing consistent is inconsistency! I just ran into this same thing on a T3. I don't have my notes here at home. The best I can tell you is, the shop manual specs are wrong. My engine guy used the specs from the Ambo manual and his experience to get me the proper fit. I believe the clearance I ended up with was .018". We both questioned the manual because it makes no sense also, there are two different sized rods and crank pins, designated by a blue or white paint mark but, only one size standard bearing. Take your crank, rods and bearings to a good engine machine shop and have them fit your rods. BTW, you always check bearing clearance dry.

[nc43bsa]

Welcome to the world of Guzzi, where the only thing consistent is inconsistency! I just ran into this same thing on a T3. I don't have my notes here at home. The best I can tell you is, the shop manual specs are wrong. My engine guy used the specs from the Ambo manual and his experience to get me the proper fit. I believe the clearance I ended up with was .018". We both questioned the manual because it makes no sense also, there are two different sized rods and crank pins, designated by a blue or white paint mark but, only one size standard bearing. Take your crank, rods and bearings to a good engine machine shop and have them fit your rods. BTW, you always check bearing clearance dry.

Not to nitpick, but I think your clearance figure is off by a factor of 10.

[acogoff]

    One to two thousandths of an inch clearance on the big ends normally.

[JoeW]

Not to nitpick, but I think your clearance figure is off by a factor of 10.

Yup, that's what I get for trying to think after 11 pm, .0018!

[canuck750]

WOW  I had no idea Guzzi could be that inconsistent!

That really sucks Shaun, any chance you can try swapping some shells and measuring with plastigauge to find a suitable tolerance fit?

I have had three cranks ground and in each case I took the new bearings, shells and rods in for the work. I think I got off real lucky!!

Good luck with the crank

Cheers

Jim

[smdl]

Hi, folks.

Yep, lesson learned.  Will always bring all the parts in together, in future.  The only reason I didn't is that the third undersize bearings were not readily available, and had to come out of Italy.  By the time they finally arrived, I had already had the crank ground.  Oh, well, not the end of the world, by any means.

Thanks for the info, Joe.  Will take everything back into the crank guys and get it ground to size.  .0018" sounds appropriate.

Now the only question is if the crank will need to be rebalanced as a result of this change.  I guess I'll give those guys a call, as well, and see what they say.

Cheers,
Shaun

Edit:  I guess I should count myself lucky that it didn't go the other way so I ended up with too much clearance!!  Small victories!  Also, thanks for the reminder on testing clearances dry, Joe.  I'm aware -- I was just trying to indicate that the reason the rod didn't rotate wasn't for lack of lubrication.  I would definitely always check clearances dry.  Now if I could just find that Plastigauge!!).