rod bearing clearance

Douglass Harroun · Feb 2, 2014

Is there a spec for rocking the rods side-to-side and determining clearance from that? I saw that referred to for Vincents. For example: 5 mm of wobble= .000x" bearing clearance. I've got a motor with the head and cylinder off.
Doug

concours · Feb 2, 2014

Douglass Harroun said:
Is there a spec for rocking the rods side-to-side and determining clearance from that? I saw that referred to for Vincents. For example: 5 mm of wobble= .000x" bearing clearance. I've got a motor with the head and cylinder off.
Doug

Only on 2strokes, AFAIK :idea:

Fast Eddie · Feb 2, 2014

concours said:
Douglass Harroun said:

Is there a spec for rocking the rods side-to-side and determining clearance from that? I saw that referred to for Vincents. For example: 5 mm of wobble= .000x" bearing clearance. I've got a motor with the head and cylinder off.
Doug

Click to expand...

Only on 2strokes, AFAIK :idea:

Why only on two strokes?

bwolfie · Feb 2, 2014

I'm sure there is a number somewhere.

I think a 4 stroke would get away with more clearance due to pressure fed oiling.

acotrel · Feb 2, 2014

If you are rebuilding the motor do the job properly. Check the big ends with plastigauge, preferably after getting the journals ground, and fit new big end bolts and nuts.

Rohan · Feb 2, 2014

Vincents and 2 strokes have ROLLER BEARING big ends.
Commandos have plain bearings = bearing shell inserts.
= Very different setup.

With no oil in there, Commando rods will have a mere trace of up-and-down movement.
The side-to-side doesn't matter, although there will be a small amount of sideways sliding possible.

X-file · Feb 3, 2014

acotrel said:
If you are rebuilding the motor do the job properly. Check the big ends with plastigauge, preferably after getting the journals ground, and fit new big end bolts and nuts.

Fit new rod bolts at your own risk,and the risk can be greater than using the old ones.
Where can you buy good quality forged replacements,as good as the original?I don't think ARP make any,although they do make them for Triumph twins.
If the old bolt passes stretch and torque tests during tightening,it's much safer than some of the currently available replacements.

Matt Spencer · Feb 4, 2014

IF you ROCK the rod , theres sideways movement at the little end . WITH the Big end held to one side . Two tengths of five eighs of S F A .

past 2 mm is naughty . Likely 3/4 mm .

Of course you CAN slide the Big End across the jouranal . But that wasnt the question . A fresh one its hard to pick , oiled .

5 mm and its shot .

Elementry trignometry would get you approximate clearance per distance side play .

Shot and youd hear it . Dang a danga . Old Falcons will spin a shell in the steel rods for thousands of miles sounding like noisey lifters . ( Both Actually

)

Alloy rods and caps arnt as forgiveing , at all .

concours · Feb 4, 2014

Rohan said:
Vincents and 2 strokes have ROLLER BEARING big ends.
Commandos have plain bearings = bearing shell inserts.
= Very different setup.

With no oil in there, Commando rods will have a mere trace of up-and-down movement.
The side-to-side doesn't matter, although there will be a small amount of sideways sliding possible.

This.

Hobot · Feb 4, 2014

Ugh, if you can hand detect ANY slack in the shells by up/dn motion w/o oil they are shot to shit. New shells scrapped to perfection so slight oi drag rotation by gravity will give maybe 1/16" total side motion at small end and be easy to slip big end sideways. My buddy got 40 yrs of regular use out of his big ends over a few re-ring jobs and re-bore, so the big ends ain't a high wear maintaince risk, but when Wes's did let go it seized engine with broken rod on hwy. Mostly boils down to how lazy vs risk taker vs disciplined time/money spender ya are this season.

toppy · Feb 4, 2014

What's wrong with genuine Andover norton rod bolts and crank studs or bolts???
Mick hemming fits them in his road engines and race engine and says he has never had any trouble with them.

concours · Feb 4, 2014

toppy said:
What's wrong with genuine Andover norton rod bolts and crank studs or bolts???
Mick hemming fits them in his road engines and race engine and says he has never had any trouble with them.

I think the OP was trying to determine rod bearing health, without splitting the cases. I plumbed a pressure gage instead.

rod bearing clearance

Similar threads