Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in

Wiped Rod Bearings 2

Status
Not open for further replies.
FredRosse

FredRosse

Mechanical
Nov 3, 2004
131
0
0
US
I recently found some connecting rods for an engine that is out of production, and finding spares is very difficult. The rods are a cast aluminum alloy, without bearing inserts. The rods are OK, except the big end bearing surfaces are scored badly.

Is it possible to rebuild these rods, by machining the big ends and inserting new bearings? Is this ever practiced?

Thanke in advance for any help.
 
Replies continue below

Recommended for you

Sort by date Sort by votes
About a million years ago I used to repair worn Al rods on small engines by machining/resizing the big end. These were rods that did NOT have inserted bearing shells. Never had a problem, even on engines that were on exotic fuels and at hp ranging upwards of 40hp.

I see no reason why the rods cannot be resized or machined for modern inserts as long as there is enough material so as not to compromise the strength.

If these old Al rods have been abused, at least have them checked, NDT or Zyglo tested. Even then, it's a crap shoot.

Rod
 
Upvote 0 Downvote
Thanks Evelrod.

I assume you are saying that the new bearing surface might just be machined into the existing aluminum without any new insert.

This would be accomplished by milling a few thousanths off the rod to rod cap gap, giving enough metal for machining the new journal surface. Is that correct?
 
Upvote 0 Downvote
Brian,
My old Triumph motorcycle had aluminum rods. Caps were steel, though. Bolts were tightened to a stretch value, not torqued. This was a -67 model. Regular bearing inserts.
Dan
 
Upvote 0 Downvote
The V12 Lagonda of the late 1930's had aluminium rods acting directly (no bearing shells) onto hardened crankpins. Some SS, Mk. 4 and 5 Jaguar engines also had aluminium rods - but I don't know if they had bearing inserts or not.
 
Upvote 0 Downvote
I rebuilt the same engine you have for a customer about 3 years ago. I was able to get parts through a welding supply dealer. Same engine was used in Lincoln brand portable arc welders. Availability was good but very pricey. Yes you can resize them. Grind cap and rod parting surfaces and hone back to original size. The bore may not clean up compleatly at the parting line but that won't cause a problem.----Phil
 
Upvote 0 Downvote
I would be very wary of using these connecting rods in another engine even if they are from a low speed low power engine aluminium is notorious for suffering internal fatigue and whilst appearing to be ok externally will fail spectacularly in use - usually at higher speed/load combination.
You say that the bearing surface has suffered and your description suggests an overheating condition which must have had an effect on the rest of the rod. I would very much advise against remachining old aluminium under such circumstances. If the engine you are rebuilding is rare or expensive my advice would be to install steel rods either forged or cast, here in England there are a number of organisations who can make them for you.I have even heard of rods being converted from one application to that of another but please take great care to do sufficient calculations to ensure reliability.
 
Upvote 0 Downvote
So long as they are not to highly stressed aluminium rods can have a very long life.

They were certainly used in the Tiger Moth aircrat engine and I think in the 30/98 Vauxhall.

Many engine bearings are aluminium and many camshafts run directly in an aluminium head or crankcase. Almost every modern 4 valve DOHC engine springs to mind as does the VW beetle.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
 
Upvote 0 Downvote
The bodies of con-rods used in Triumph and Norton and BSA twins from the 50s and 60s were some aluminum alloy or another. The caps were steel on Triumph and Norton 500 and up, at least on the sporty ones. Some had wrist pin bushings, some did not.
I can remember boring the big ends of some smaller twins larger for inserts for some customers, or maybe to convert to more common inserts. It cut a long shallow scallop into each of the rod bolts, but those customers said it was a well proven, successful mod.

 
Upvote 0 Downvote
All '60's BSA's, Triumphs, and Nortons used shell bearings in their rods; can't say for earlier models. You can see the lock-tab cuts in the 3rd pic above.

Is the Onan engine even pressure-lubricated?
 
Upvote 0 Downvote
I would think the best would be to have the big ends of the rods remachined and honed to spec. Its what the engine was desighed for and would be the easy road on your part.
Only problem would be if the rods used studs or bolts to holt the cap on. As the studs will have to be removed before machining properly.
 
Upvote 0 Downvote
if you resize the big end, be sure to pay particular attention to the detail at the parting line.

there is generally a ramp cut into the upper half which directs the oil so it can effectively form the hydraulic wedge as the crankpin turns

if your resizing removes this ramp, you may not get adequate oil hydraulic wedge formation.

bob g

 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

enginesrus
  • Locked
  • Question
Connecting rod loading 1
Replies
17
Views
28
inline6
inline6
Tmoose
  • Locked
  • Question
Con rod threads for cap screws. 1
Replies
9
Views
10
tbuelna
tbuelna
yoshimitsuspeed
  • Locked
  • Question
Main/Rod bearing diameter and friction 1
Replies
11
Views
12
PackardV8
PackardV8
200v
  • Locked
  • Question
rod/stroke question
2
Replies
20
Views
26
inline6
inline6
racinvrxse
  • Locked
  • Question
Determining necessary oil flow volume for main and rod bearings 2
Replies
11
Views
12
BrianGar
BrianGar
Back
Top