Ideal rod-bearing clearance.

Could I get an opinion on actual rod bearing clearance considered optimal for the commando? One of the shop manuals says "less than .001" That seems too vague. I need a shop to grind the rod throws and fit them to measured, torqued -up new bearings. Anybody got a shop that can do that? I can UPS in continental US. I've seen elsewhere that current Triumph rod shells yield too loose clearance if crank ground to shop manual specs and not to measured fitted rod bearing diameter

IIRC Norton vendors sell a bit thicker rod shell set for your situation to obtain the ideal .001" clearance that my still need a bit of razor blade scrapping so its only that close all over.

hobot said:

IIRC Norton vendors sell a bit thicker rod shell set for your situation to obtain the ideal .001" clearance that my still need a bit of razor blade scrapping so its only that close all over.

Click to expand...

?????? :shock: :shock: :shock:

SECTION C20

CRANKSHAFT INSPECTION AND REGRINDING

Wash each crank cheek in clean petrol and blow dry with an air line. Examine the big end journals for scoring and remove light score marks by the use of smooth emery tape. If there is excessive scoring or ovality and it exceeds 0.0015 in. ( .0381 mm) on the big end journals regrinding is necessary.

The regrind sizes are shown in Fig. C28. The revised big end journal sizes necessitate the use of connecting rod shell bearings of the correct undersize. The steel backed big end shells are finished to provide the correct diametrical clearance and must not, under any circumstances, be scraped. Shell bearings are available in undersizes of minus 0.010 in., minus 0.020 in., minus 0.030 in. and minus 0.040 in.

IIRC = If I remember correctly. I missed the -a- in may but otherwise U left same message, Ya buy off the shelf rod shells sold to take up the bigger gap of refinished journals. Journal refinishing and rod shell re-sizing is done by standard steps, like pistons & rebores. Also noted that rod shells are not are prefect made so a razor can remove hi spots, tested repeatedly til good all around. Oval-ness is what the machinist should be correcting before clearances can be set. Soft shell surfaces are designed to hold on to abrasives in case that influences ya on how to revive hi spots.

0.0005" clearance would be OK.If the rod will fall with its own weight,it's free enough.It will get looser when the engine is running and the rod warms up.

Ditto on the above, have your machinist grind the crank to the minimum next oversize bearing to remove any damage or oval shape. See the workshop manual for tolerances. Be sure and have him use a 90 degree radius on sides of the journals. Fit the correct oversize bearing, torque using old rod nuts and check clearance with Plasticguage. Should be .001 max. Fit new nuts and retorque.

.090" radius, NOT 90 degree!!!

Find somebody familiar with Norton cranks to do it. It's important. They need to know what they are doing. (Not just any automotive machine shop.)

Sorry, yes .090 radius.