Conrod big end clearance

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Dean

Dean

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Hi all

Can someone give me what the conrod big end to journal should be. I am about to get my crank ground, I have -010" under shells and with them installed in the rods and the caps torqued they measure up at 1.742".
If I grind the crank as per the book 1.740" - 1.7405" that will give me a clearance of 0.0015" - 0.002". The book says the "rod end clearance : less than 0.001".

Should I grind the crank to fit the rod with "less than 0.001" say 1.7415" that will give me 0.0005" clearance or am I missing something?

Cheers all,
D
 
Dean said:
Hi all

Can someone give me what the conrod big end to journal should be. I am about to get my crank ground, I have -010" under shells and with them installed in the rods and the caps torqued they measure up at 1.742".
If I grind the crank as per the book 1.740" - 1.7405" that will give me a clearance of 0.0015" - 0.002". The book says the "rod end clearance : less than 0.001".

Should I grind the crank to fit the rod with "less than 0.001" say 1.7415" that will give me 0.0005" clearance or am I missing something?

Cheers all,
D
Click to expand...

For street use I shoot for .001 clearance.
For racing I go .0015
They can run all the way up to around .003 but low oil pressure and cylinder wall flooding becomes an issue. jim
 
normal procedure when re grinding is to take the factory STD crank journal size
minus the required undersize and grind and polish the crankshaft to the required size

if the crankshaft is correct in size and the rod housing is not oval or worn the rod clearance is controlled by the shell bearings

Norton will possibly quote a max permissible wear before crank requires a re grind ( normally around minus one and a half thou )

it may be worth checking journal for wear with a good quality micro meter to check if it still in tolerance as you may escape with a polish
 
Thanks Jim

Hi kiwi

I have measured up the journals with a very good quality vernier micrometer ( measures 1/10ths ) and only worn about max 0.0017" and ovality max 0.0005" problem is it was "rode hard and put away wet" and left since 1986 and the oil against the jounal must have had a lot of moisture in it, and it has left surface corrosion on the journal for about 1.5 sq in around the oil hole.

As I have to get new shells and a crank grind is only about 30usd for both journals, I may as well get it done. The operator has a lot of experience on the machine and assures me the all important 0.090" radius is no problem. Also he is happy for me to stay and watch while he does it so what could possibly go wrong...

Thanks
D
 
Dean said:
Thanks Jim

Hi kiwi

I have measured up the journals with a very good quality vernier micrometer ( measures 1/10ths ) and only worn about max 0.0017" and ovality max 0.0005" problem is it was "rode hard and put away wet" and left since 1986 and the oil against the jounal must have had a lot of moisture in it, and it has left surface corrosion on the journal for about 1.5 sq in around the oil hole.

As I have to get new shells and a crank grind is only about 30usd for both journals, I may as well get it done. The operator has a lot of experience on the machine and assures me the all important 0.090" radius is no problem. Also he is happy for me to stay and watch while he does it so what could possibly go wrong...

Thanks
D
Click to expand...

I would fit the new shells in the rods and torque them up. Then bring them with the crank to your grinder so he can measure them and grind the crank to fit. Jim
 
The only place Norton factory used a torque wrench on were the Rod bolts, which ideally should be torqued to the lb-ft that gives ~0.006" to ~0.0065" stretch, depending on ones beliefs in the primacy of stretch over torque measures and faith in the quality of fasteners. i install to lessor torque 1st then turn crank for feel of drag and re-opening to check shell witness marks/high areas that get removed by judicous razor blade scraping.

If one were seeking ~.0015" clearance, how would that be gained if starting with 0.0010"?
 
Take the crank, both rods and shells to the grinders , leave the shells out and get him to check rods first, tunnel size and straight, if your rods aren't correct to start with you may as well let Hobot machine it for you with his razor blade and emery cloth
 
splatt said:
Take the crank, both rods and shells to the grinders , leave the shells out and get him to check rods first
Click to expand...
+1
Not that it seems to be a common issue with Norton rods, but it sure can't hurt.

splatt said:
...you may as well let Hobot machine it for you with his razor blade and emery cloth
Click to expand...
Easy, big fella; this is standard practice on many babbett bearings, including plain cam bearings in V8 engines. They even make a tool specifically for this http://www.goodson.com/Bearing-Scraper/

Nathan
 
Hehe, put down ole hobot's about exact parallel-normal to surface hand razoring but by golly no abrasives were used thankyou.
 
I usually buy shells of the next size and grind the crank to specification, then check the clearance with Plastigauge rather than use a micrometer. The temptation if you have too much clearance, is to increase the crush on the shells by polishing the mating faces of the rod and cap. If you do this it is irreversible.
 
Nater_Potater said:
splatt said:
Take the crank, both rods and shells to the grinders , leave the shells out and get him to check rods first
Click to expand...
+1
Not that it seems to be a common issue with Norton rods, but it sure can't hurt.

splatt said:
...you may as well let Hobot machine it for you with his razor blade and emery cloth
Click to expand...
Easy, big fella; this is standard practice on many babbett bearings, including plain cam bearings in V8 engines. They even make a tool specifically for this http://www.goodson.com/Bearing-Scraper/

Nathan
Click to expand...

I agree with the above, but leave hobot alone. The machinist will do the job properly for you if you give him all the stuff.
Dereck
 
A fellow traveler in these woods. I had a crank ground undersize recently by one of the big names. The rod throws were not equal in height by .006' (measured on a lathe and at the pistons in the cylinder) and at an angle on both- shown by using a known straight set of rods. I've measured used rod pairs on a known straight crank and found the pistons up at angles: one side of each piston up to .004" higher than the other, including a rod pair I had re-sized at a respected machinist. I settled on a good used crank and rod pair, assembled, both pistons came up equal and not tilted. Caveat emptor.
 
Douglass Harroun said:
A fellow traveler in these woods. I had a crank ground undersize recently by one of the big names. The rod throws were not equal in height by .006' (measured on a lathe and at the pistons in the cylinder) and at an angle on both- shown by using a known straight set of rods. I've measured used rod pairs on a known straight crank and found the pistons up at angles: one side of each piston up to .004" higher than the other, including a rod pair I had re-sized at a respected machinist. I settled on a good used crank and rod pair, assembled, both pistons came up equal and not tilted. Caveat emptor.
Click to expand...

And that is why I don't recommend reconning Norton rods. They seldom really need it and people who can and will do them right are few and far between.
If they are out of round I throw them away as they have likely used up there safe life. Jim
 
Never had this issue. The Valve guides wear out so- much -way before such situation. Only dirty oil or lack of it, or engine disasters could cause this. :roll:
 
Conrods measured up with nuts torqued and are bang on book measurements. Put both rods on one piston pin and turned up a piece of thick walled pipe to slide fit in big end and all square. Then polished up and moment weighed small end within 1g and overall weight less than 1g.

Now to get the crankshaft grind done.... Fingers crossed.

Thanks for the feedback.
D :)
 
splatt said:
...you may as well let Hobot machine it for you with his razor blade and emery cloth
Click to expand...
Easy, big fella; this is standard practice on many babbett bearings, including plain cam bearings in V8 engines. They even make a tool specifically for this http://www.goodson.com/Bearing-Scraper/

Nathan[/quote]
Stop advocating butchery, Tri metal bearing as in big end inserts and Bi metal cam bearings are not the same thing, the overlay is not designed to be readjusted, if the big end bearing is tight and rubs the problem will not be the insert( if it's the correct insert), distorted rod and cap shift are the most common cause. The big end bearings in your bike ARE a high performance piece, show them ''some '' respect
 
When I check big end clearances, I worry about putting the nuts on and off too many times, and I've never checked the stretch in the bolts after torquing them up. I've never had a failure, however it could be expensive if it happens.
 
You should worry because the special con rod nuts are one-time use only. Take a close look at them and note that the top cuts are to spread out for retaining forces security on a one use basis. :|
 
Amend to new nuts each time apart but if rods have been through a trauma better replace the rod bolts too or at least have magfluxed and cryo tempered before reuse and be aware/search up to avoid the counterfeit rod bolts. Rod bolts are the easiest to measure stretch and interesting to record torque values vs stretch to see why I don't bother with t-wrench any where on my Cdo anymore. Check rod seats have clean flat clearance too of course. W/o cryo tempering shoot for .006" but .0005" more if chilled, which ends up about 27.5 lb ft on lubed clean threads if ya must T-wrench measure for mental health issues.
 
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