Rods and big ends

[Chris01573]

Hi All,

Measuring some conrods (I have 16 to get through) Using a bore gauge and micrometer I am seeing on the big ends measurments of 1.897 side to side (just shy of the join with the cab.) and then 1.895 top to bottom. So 2 thou out of round.

I can't find any tolerances in the ROM for these measurements, any advice from you chaps ?

[jbuckl]

Believe Faversham classics have re-conditioned con rods.

Theres quite a lot to get done to make them as new.

I had a lot of trouble when a prototype machine shop failed to spot a twisted & out of round conrod….. took them (plus spares) there to get them all measured & balanced into 1 good set of 8.
Subsequently caused bearings to fail.

Typical values for a similar conrod:-
Roundness 5 microns ; cylindricity 7 microns; perpendicularity dia. 40 microns.

pin to big end parallelism 50 microns on a 100mm bar gauge; bend and twist 100 microns on a 100mm bar gauge.

The R.O.M. Does cover the twist using a gauge iirc.

[Chris01573]

So spent some time taking measurements... Suggests at most there is a thou out of round on the big end diameter and then all the small ends are .875 so towards the upper limit according to the ROM (.8755)...

I think these are OK but would apprecate a second opinion :-)

Screenshot 2024-03-09 121040.png

[jakesmig]

Well done for being able to measure down to i thou.....

[jbuckl]

Well done for being able to measure down to i thou.....

try measuring a roundness of 5 microns… (0.2 of one thou!) that’s what’s on a modern conrod. The equipment to do that would buy about 10 nice triumph stags!!!!

it’s amazing how a conrod is ‘just’ a piece of metal with 4 holes in it…. But a lot more to it when the surface is ’scratched’… literally or so to speak.

A roundness of 5 microns is a very tricky thing to measure… especially at home.

[flying farmer]

I intend buying a bore gauge before I rebuild my next engine so that I can measure the con rods, particulaly the big ends.

Can't say I have ever tried measuring the big ends on any of the engines I have rebuilt, but none of the old big end shells have ever shown abnormal wear patterns which would be a big red light.

This is probably because the Stag engine normally wears the main bearings before the ends get damaged. I have had a stretched big end on a 1300 Triumph engine that started knocking, but they normally knocked out the big ends before the mains.

I have had to have one set of Stag rods rebushed on the small ends as it was possible to feel movement between the new gudgeon pin and the old small end bush, didn't need a gauge to measure that.

Neil

[jbuckl]

I intend buying a bore gauge before I rebuild my next engine so that I can measure the con rods, particulaly the big ends.

Can't say I have ever tried measuring the big ends on any of the engines I have rebuilt, but none of the old big end shells have ever shown abnormal wear patterns which would be a big red light.

This is probably because the Stag engine normally wears the main bearings before the ends get damaged. I have had a stretched big end on a 1300 Triumph engine that started knocking, but they normally knocked out the big ends before the mains.

I have had to have one set of Stag rods rebushed on the small ends as it was possible to feel movement between the new gudgeon pin and the old small end bush, didn't need a gauge to measure that.

Neil

My stag engine had little end noise….

imagined that I could press or cut out the bush and replace it with a new bush at home….

did manage to change the bush easily… however…

hoped to be able to hone the size with an adjustable reamer afterwards…. No Chance…. The size cannot be achieved that way.

Needs to be done on a special honing machine’sunnen’ or similar…

cost was £45 + vat to properly hone one small end bush. (N.o.s. Piston pin had to be supplied)

cylinder 6 & 8 conrods little ends seem to get more little end wear than other conrods … due to the engine installation angle and direction of crank rotation throwing up more sump debris to those two pistons.. probably.

[flying farmer]

My stag engine had little end noise….

imagined that I could press or cut out the bush and replace it with a new bush at home….

did manage to change the bush easily… however…

hoped to be able to hone the size with an adjustable reamer afterwards…. No Chance…. The size cannot be achieved that way.

Needs to be done on a special honing machine’sunnen’ or similar…

cost was £45 + vat to properly hone one small end bush. (N.o.s. Piston pin had to be supplied)

cylinder 6 & 8 conrods little ends seem to get more little end wear than other conrods … due to the engine installation angle and direction of crank rotation throwing up more sump debris to those two pistons.. probably.

I swapped all 8 bushes myself then had to take the rods to my local machine shop to get them sized. IIRC he had to bore them first as there was too much material to just hone to size.

Can't remember what it cost, but it was cheaper than £45 per rod or I would have remembered!

Probably because the set up time of the tooling is the same for 1 rod or all 8

Neil

[Chris01573]

Rimmers sell refurb rods for £21 however I can't vouch for the quality. They have had new little ends and have been checked and machined apparently.