connecting rod clearance

Status
Not open for further replies.

John B in Bama

Joined
Jan 11, 2015
Messages
7
Country flag
I've got a set of new H beam connecting rods for my 850 BUT the cap and ARP bolts project further than stock actually scraping the inside of the crankcase at certain positions. It's been suggested to file the bolt heads to get sufficient clearance...but I don't appreciate the mod, possible loss of strength, nor the extra work on something expected to just be a drop in replacement for stock.

Then there is the possibility that crankshaft flex will make things worse and I have no way of measuring clearance under those conditions.

How to proceed? Return for refund and go with stock? Modify the bolts?
 
I have sometimes wondered about this problem. If you relieve the cases, you probably weaken them. If you grind the heads of the bolts or the nuts, they might crack.
 
I too have a set of H Beams ready to be installed once the crank is back from balancing, I'm very interested to hear how this pans out and will be checking the clearance myself as soon as the parts are back in my hands

Edit:

Just went out back and compared my H beam against a stock rod:

Length: stock = 7.875, H beam = 7.625
Width at big end: stock = 2.975, H beam = 2.938

So, it looks like I'm in the clear

connecting rod clearance
 
Last edited:
I have dealt with this issue.

I had a set of MAP H beam steel rods that wouldn't rotate without hitting the cases in a 750. I called MAP which told me that they would be happy to take them back, but pointed out that the Norton cases were not uniformly cast and suggested that I grind the interference point until I had at least .060" clearance. I used some machinist blue so that I could see where the interference was, used my Dremel to relieve the high point (while being conscious of the shape), which was on the drive side case, which had plenty of meat on it (more so than the timing side case). The engine now has a bit over 200 miles on it.

If you do this, be sure that you clean all parts thoroughly before final assembly and when you are removing material from the case follow the contour of the case rather than creating sharp edges.

Stock LF Harris aluminum beam rods sell for about the same money as the MAP steel rods and the Carillos are about $200 on top of that. I have fit Carillos and LF Harris without interference issues, and fit MAP steel beam rods in Triumphs without interference.
 
To the OP post - steel rods are overkill unless you are building and planning to do something extraordinary with the engine. The factory rods with steel caps are well designed and extremely durable for most Norton big twin applications.
 
I've encountered the same problem a couple of times, once with some Crower titanium rods in an 850, and once with some non-Carrillo steel rods and a longer stroke crankshaft. Normally, the Carrillo rods do not have interference problems in standard stroke crankshaft Nortons. But Carrillo uses 5/16" cap screws for these rods, and some of the aftermarket rods use 3/8" cap screws or studs and nuts, and they can have clearance issues. I've used a combination of light grinding on the inside of the cases and grinding a bevel on the cap screws or nuts after assembly and torqueing . That bothers some folks, but in the times I've had to do it, the amount of material removed and it's location were such that no strength was lost. The biggest concern was on the need to use new cap screws or nuts, and to grind them for clearance, each time the engine was disassembled. Not such a big deal on a street bike, and not even all that bad for a race bike, which gets torn down regularly. This is a picture of a longer stroke crankshaft with the heads of the cap screws ground for clearance.

connecting rod clearance


Ken
 
Ken
It would appear you would have to do the grinding after torquing (so that you know the place to grind).
How did you ensure grindings didn't get into bad places?
...and what did you grind them with (fair bit of hard material to be removed)?

Rob
 
Last edited:
Correct, ground after torqueing. I covered the crankshaft like this for grinding. I used a 4 1/2" disk sander with a grinding disk.

connecting rod clearance


Ken
 
connecting rod clearance
I tried to copy the image of the MAP "H" rod under the stock showing the added length/protrusion of the bolt head, but no success. (Even my computer guru/wife couldn't help.) And yes, it scrapes on the drive side only though the clearance on the timing side is minimal. I should have put in some Plastigauge just to see how close the cap bolts come on the side with clearance. I'd rather not butcher my crankcase and the amount of head bolt to be removed for clearance worries me. I think I'll punt and sent these back and use my stock ones. My racing days are over. I just wanted a dependable engine and, since it was apart to freshen, I thought I'd try these bullet proof ones. The end caps are beefier than stock and the ARP bolt heads also protrude more than the stock nuts (the axis is reversed on these MAP steel rods in comparison to stock). Thanks to all for ideas. I'm just not bold enough to grind off so much of the bolt heads.
My old rods are in good shape and highly polished and ready to go back to work.
 
John B in Bama said:
connecting rod clearance
I tried to copy the image of the MAP "H" rod under the stock showing the added length/protrusion of the bolt head, but no success. (Even my computer guru/wife couldn't help.) And yes, it scrapes on the drive side only though the clearance on the timing side is minimal. I should have put in some Plastigauge just to see how close the cap bolts come on the side with clearance. I'd rather not butcher my crankcase and the amount of head bolt to be removed for clearance worries me. I think I'll punt and sent these back and use my stock ones. My racing days are over. I just wanted a dependable engine and, since it was apart to freshen, I thought I'd try these bullet proof ones. The end caps are beefier than stock and the ARP bolt heads also protrude more than the stock nuts (the axis is reversed on these MAP steel rods in comparison to stock). Thanks to all for ideas. I'm just not bold enough to grind off so much of the bolt heads.
My old rods are in good shape and highly polished and ready to go back to work.
Click to expand...
My original old rods are fine and were polished up by the P.O.
 
3 of them said:
whats wrong with the standard con rods anyway ?
Click to expand...

Nothing, really. Depends on the use. I use stock rods in my street MK3 with moderately hopped up engine, and used them for years in 750 race bikes, but they will eventually fatigue and break. On the race bikes I inspected them every year, and replaced them when I saw cracks at the pin end. For 920 and 1006 engines, and for short stroke race 750s, and when I started using nitrous, I switched to steel for more insurance against failure. The biggest problem with stock rods is if you don't know their history. Back when you could buy a new set from your local Norton dealer, it was pretty common among the racers I knew to just replace the rods every season. Aluminum does fatigue with use, and I have seen a fair number of stock rods break, but again, that's mostly in race bikes. I've also had steel rods break in race bikes, so all that really says is that racing is hard on the rods.

Actually, I think the aluminum rod/steel cap is a great design, and considered making some similar billet rods with a higher strength alloy and machined steel caps, but it didn't seem worth the effort with high quality steel (or titanium, if you have the funds) rods available from Carrillo and others.

Ken
 
You guys might already know this but...
Aluminium is a shit metal from a fatigue point of view in that it does not have an endurance limit - meaning that, regardless of the stress levels you subject it to, it will fail one day. The higher the stress levels the earlier the failure.
Steel (carbon and low alloy), however, does have an endurance limit (usually, but not always, about 50% of the tensile strength) which, if you keep the stress below that number, will never fail.
So, if you subject your rods to high stress (high rpm) and want to keep them in one piece for a long time then steel is your best option.
It has been proven that, for relatively low rpm road use, that the stress levels in aluminium rods must be low enough for long service life.
I'm putting aluminium commando rods in my 650ss

I think this is relevant to the original post as some have questioned John B's selection of steel rods - I think science is on his side.
Cheers
Rob
 
Last edited:
How very dare you bring science into it?! That is most unbecoming of correct Internet forum decorum!

I’m not clever enough for science. All I know is that I broke a stock, low mileage, crack test, polished Commando rod once.

So, being of the ‘once bitten -twice shy’ type, and given the ‘cost of failure’ of a broken rod at high rpm, and with the avialablity of really superb steel rods, that’s what I use currently.

But my motor isn’t stock, and does get caned. I imagine that a stock motor in normal use would be perfectly fine with stock rods. After all, most are!
 
Last edited:
Acadian,
Curious where you got your steel rods. They definitely are different from the MAP ones with a less intrusive big end cap. I'd be surprised if you had any clearance problems.
And now, once again, I ponder the wisdom of reassembly with original rods. What are the chances of a broken rod with normal use.... Obviously minimal but still the nagging worry and "what if" intrudes. And if you're trying to build with an eye on dependability, the value of the steel rods cannot be denied.
Thanks again for all the ideas and comments.

John B. in Bama
 
BTW - I hope John B appreciates Ken's solution as it would seem to be his best path forward.
Let us know how you go here John.

Rob
 
Arcadians said - "I too have a set of H Beams ready to be installed once the crank is back from balancing"

Arcadian - the penny just dropped with me (slow, I know)
How are you getting your crank balanced (static or dynamic) without the balancer having your rods?
Would seem essential for him to have them.
Rob
 
Status
Not open for further replies.
Back
Top