Jump to content

Triumph Connecting Rods with Stress Relief Hole


Recommended Posts

Ok here's one for the experts

 

Which Triumphs were fitted with a connecting rod with stress relief hole just above the big end bearing?

 

as in

 

http://www.bpnorthwest.com/connecting-rod-reman-tr6-spit-gt6-3086.html

 

 

I have done a huge amount of internet trawling and can find very little except a few hints that they were used on GT6's but when, why appears vague to say the least.

 

Every part reference I can find says that All TR6's, 2.5 PI's, all large crank 2.0 6's and 1500 Spitfires all used the same connecting rod part number 146454. I find some references that the TR5 had a slightly better connecting rod, but no information why.

 

So where were these rods with a stress relief hole fitted, and why?

 

Are they any good? better?

 

I did find one reference that said you could expect to find either in a 6, and it didn't matter which, as long as they were all the same. I have also found a couple of adverts for connecting rods that said specifically "without stress relieving hole" so its got to be a known option, but not one that anyone seems to have had any opinions on, which give that Triumph nuts will argue till the cows come home about the advantage of one specific bolt over another surprises me. Or are my Google Skills getting worse?

Link to post
Share on other sites

Hmmm that's a brain teaser.

 

The forces in a con rod are mostly compressive.

So ideally you need the forces running down the outer I (eye) section of the rod.

This pushes it down at the 9 & 3-o-clock position or just a little north.

This would happen with the stress relief hole.

 

With no hole the forces also press on the 12-o-clock position. Could this deform the big-end section and cause what problem the relieving is trying to reduce.

 

Roger

Link to post
Share on other sites

I thought the only sense was these rods are lighter.

I bought SCAT H-beam rods that did not have that hole

but also bought a ultralight version from them that had same hole.

 

Maybe it was easier to drill the hole with thread for rod bolt

with the hole for easier thread cutting.

I found them in my 1969 TR6.

 

Anyway if aiming for a better rod these steps are small

compared to a Maxspeeding rod that is still affordable

instead of the Carillos.

I use these in my 2.7 litre TR6 with the VW pistons.

Edited by TriumphV8
Link to post
Share on other sites

I found them in my 1969 TR6.

 

Anyway if aiming for a better rod these steps are small

compared to a Maxspeeding rod that is still affordable

instead of the Carillos.

I use these in my 2.7 litre TR6 with the VW pistons.

 

OK am I clear that you found con rods with the stress relieving hole in your 69 TR6?

 

Oh and on the Maxspeeding rods, given that all reports so far that I have read have only been speculation that they are or are not not any good, will or will not fit, your the first person I have seen admit publically that they are using them :-)

 

How long have you had them fitted? what sort of RPM are you pulling regularly? did they fit down down the bores (or only because you are over bored to 2.7? Sorry lots of questions, but nobody else has admitted to using them and I have been digging for a while.

 

Alan

Link to post
Share on other sites

+1, someone used this method to lighten and balance the rods. As it was found unfit for sale as is, a smart Alec labeled it "stress relief" for resale purpose. Re-branding trick, etc... Note that "this is a rebuilt connecting rod..."

 

I thought the only sense was these rods are lighter.

Edited by Geko
Link to post
Share on other sites

Good morning Alan

I did read those negative reports, too.

 

The new rods are cheap, the stock ones are coarse and heavy

and will never come in the weight range of them so I tried!

 

When they came in they looked like all the others I bought for the Rover V8:

 

http://www.scatcrankshafts.com/scat-rod-anatomy.html

 

Look at the 4th picture, there is the one with the hole!

 

SCAT says about that hole:"Formula “1” style lightening hole

which reduces weight, but does not compromise strength"

 

You can get a set of 8 from 200USD to 600USD which look similar.

So pricing is in the range and only Carillo is in another world.

 

The rods are properly balanced what I checked and come with ARP bolts

and I had to change the upper small end bearing to 17mm bore

for the VW pistons.

Upper and lower bores are in limits, width also, nothing to complain.

 

The engine revs daily beyond 6000, rev limiter is set to 6500

but I tried more when engine was new.

Rods are new because I had to wait for a good offer and after that

took my time to modify them at the small end.

Edited by TriumphV8
Link to post
Share on other sites

+1, someone used this method to lighten and balance the rods. As it was found unfit for sale as is, a smart Alec labeled it "stress relief" for resale purpose. Re-branding trick, etc... Note that "this is a rebuilt connecting rod..."

 

 

Not sure I agree about that diagnosis, in fact make that I don't agree with that diagnosis.

 

The reason I started digging wasn't because I found that advert but because I have some, and wondered where they came from.

 

An aquaintance who knew I was about to start open heart surgery on the bottom end of my engine, gave me the bottom end of a MK3 GT6 engine that he had had sat around for about 10 years just in case it might be useful. It has 6 con rods in it with those holes! Having seen the car it came from about 2 decades ago, I'm fairly certain that the closest that car ever got to having any performance enhancements applied to it, was having a pair of furry dice hung from the rear view mirror. So we are talking about a standard (well standard triumph anyway) car that I think had these from the factory.

 

I have also found a few more photo's of rods with holes, not being described as anything special, and in fact not even mentioning the holes. I have attached a couple.

 

 

 

post-12405-0-32250100-1415616632_thumb.jpg

 

So as the engine I have and the photo's are of english cars, unless someone in the UK also had the idea that it would be a good way to lighten con rods (which I would think would be very risky unless you really knew what you are doing) And if someone was making these as specials in the UK I'm certain that they would have come in for review and/or derrision, and I can find nothing. My conjecture is that these are factory rods which maybe the factory tried in late GT6 MK3's maybe not long before production of the 2.0 6 cyl engine stopped .

 

Wow I may have just answered my own question, look at the picture of the 146454 GT6 connecting rod on the Rimmer site!

 

http://www.rimmerbros.co.uk/Item--i-GRID008192

 

So I think these are standard factory rods, I'm just amazed that I have never heard of them before, and that nobody has ever commented on them, and that they shared the same part number.

 

Of course the big question is are they any better or worse than the common rods, I can't believe the factory would have used them without good reason or serious testing (but maybe I can!)

 

Alan

 

Edited by oldtuckunder
Link to post
Share on other sites

I have seen many original Triumph 6 cyl engines stripped, both had conrod sets with or without hole.

 

Always thought it was the factory taking weight out by a simple and single machining process. I can not definitely say the sports cars were the only ones to have the 'holed' rods nor what period they were used. Any one out there with a Vanguard 6 catalogue? What rods did that use and how are they pictured?

 

Just be sure you rebuild with a set of rods that weigh the same. Small kitchen electronic scale is ideal and costs under 10 quid in Lidl. Ideal for the pistons too.

 

Cheers

Peter W

Edited by BlueTR3A-5EKT
Link to post
Share on other sites

There's currently a set of NOS rods on fleabay complete with holes!

Link to post
Share on other sites

Maybe the hole is to open up the end of the threaded drilling for the cap bolt?... to prevent cracking at the end of a blind drilling? The hole intersects the drilling in the photo in Alans first post.

Link to post
Share on other sites

Redline on the late TR6 engine ( Federal version anyway ) was increased to 5800 from 5500. These carried the heavier flywheel and perhaps the lighter rods as well. Do these changes make the rationale for higher redline?

 

At least one noted specialist in the UK won't touch the holy ones ( " I'd bin them! " ) :mellow:.

 

I've got balanced originals in one '250 engine and Carillos in the other. My next engine will definitely have the latter which make the engine feel like a short stroker by comparison. 450 grams vs. 750 and I've not heard of breakage. That's a 4 lb savings on the crank throws where it really counts. Made in the USA too :D.

 

I've got a set of un-holy TR6 rods if anyone's interested. For forumites I'll guarantee against defects.

Link to post
Share on other sites

Maybe the hole is to open up the end of the threaded drilling for the cap bolt?... to prevent cracking at the end of a blind drilling? The hole intersects the drilling in the photo in Alans first post.

 

Now that's the best theory I have heard so far, it would also ensure no swarf left from tapping, and also prevent a hydraulic lock when torquing that bolt.

 

So the interseting question is still when they were used, and also if they are regarded as inferior why is there an almost complete absence of info on them, or at least people reporting that they have had problems with them, if anyone has?

Link to post
Share on other sites

The hole appeared on later engines. Not sure when exactly, but suspect '73 ish based on engines I have had apart. The weight is more or less the same as rods without the hole. Used on all vehicles that used that rod size (Spit, Dolly, Toledo 1300 & 1500, and all the 6 cylinder cars).

 

As already suggested I believe it is there purely to ease manufacture - it's easier to drill and tap a through hole than a blind hole.

 

Carillo and Pauter rods are very lovely......... but also very expensive.

 

Nick

Link to post
Share on other sites

The hole appeared on later engines. Not sure when exactly, but suspect '73 ish based on engines I have had apart. The weight is more or less the same as rods without the hole. Used on all vehicles that used that rod size (Spit, Dolly, Toledo 1300 & 1500, and all the 6 cylinder cars).

 

As already suggested I believe it is there purely to ease manufacture - it's easier to drill and tap a through hole than a blind hole.

 

 

Nick

 

Thanks for the info, explains why there was no change of part number. So given that nobody ever seems to have complained about "hole" connecting rods breaking it appears that the rod sets are interchangeable (as complete sets).

 

I guess all we do is wait for someone to explain why they think they shouldn't be used :-)

 

Alan

Link to post
Share on other sites

Tom referred earlier to standard 250 rods at 750gms . . . . . which sounds awful heavy to me.

 

When I cleared out the last couple of sets of 6-pot rods that I had here, one set was 640 and new and the other used set 590, give or take a very few grams.

 

When I had the engine of MRV 302G rebuilt in 1979, prior to that untouched from new, the rods were all well under 600gms ex-factory . . . . along with a few other tweaks, but then it was a road test car.

 

As for the polo rods, as far back as I can remember it was pretty much pot luck what you found keeping the pistons at an appropriate distance from the crank, solid rods or the ones with the hole. Can't say I've ever noticed any particular logic to fitment . . . . nor have I noticed any difference in performance either, rods don't often break anyway in road use.

 

Cheers

 

Alec

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.