Connecting rod bolts

[jbenajes]

Hello,

 

I am helping a friend here in Spain to refurbish a TR6 engine .

 

We doubt if it is necessary to use new connecting rod bolts, as the repair manual states, since according to responses from some well known UK dealers, "the bolts are just high tensile and normally you do not have to replace them".

 

What is your experience and recommendations?

 

Thanks in advance.

 

Jesús

[TR 2100]

We doubt if it is necessary to use new connecting rod bolts, as the repair manual states,

since according to responses from some well known UK dealers,

"the bolts are just high tensile and normally you do not have to replace them".

 

Con rod bolts for the 4-cylinder engines use locktabs and CAN

be re-used.

 

Con rod bolts for the 6-cylinder engines do NOT use locktabs.

Their design is different - they are designed to stretch once

torque is applied, do not use locktabs and cannot be relied on

to be re-used safely.

 

AlanR

[jmo]

hi

please use new ones a con-rod bolt failing can cause total engine failure ie a rod thruogh the side of the block .Save in cents could cost him many euros !

[88V8]

What is your experience and recommendations?

 

New bolts new nuts.

 

Ivor

[Alec Pringle]

Hi Jesus,

 

as already explained, the 4 and 6 cylinder engines use different types of bolts - too many folks wrongly assume that the 6 bolts nuts can be reused as per the 4.

 

Don't do it !! Buy new ones . . . .

 

Cheers,

 

Alec

[Tom Fremont]

 

Don't do it !! Buy new ones . . . .

 

 

 

Amen to that - it's not worth even the smallest chance.

 

However...theoretically a bolt which has not been over torqued should be able to be reused indefinitely. If there is something unusual about the TR5/6 conrod bolts which render them unusable after (1) tightening I'd like to be enlightened. And, every torqued bolt stretches in direct proportion to its tension - torque being an indirect route to this objective. Aftermarket rods often give a stretch figure in addition to a max torque

[Bald Rick]

Just an observation. I could be wrong but I though "stretch" bolt technology had only been in general use in the auto industry for about 30 years or so and was introduced to overcome problems with achieving uniform torque on cylinderhead bolts during automated assembly and to reduce costs (being made of lower strength steel). It's use later spread to other parts of the engine/car.

 

This does not mean that conrod and main bearing cap bolts should not be replaced. Even high tensile bolts can be (and often are) stretched.

 

Tony

[Tom Fremont]

Just to clarify, " stretch " in the healthy sense is elastic rather than plastic; i.e. it returns to its original length once loosened. When over torqued the stretch becomes plastic and doesn't return - this amounts to a failed bolt and certain failure if reused ( although they usually won't come up to full tightening torque again ).

[RogerH]

Hi Folks,

just had a sideways thought The big-end bolts are only under stress for the induction stroke (suck).

The other three strokes are (squeeze, bang, blow) have little affect on the bolts as the force is going up through the con-rod shaft.

I appreciate that stopping a piston travelling at 5000rpm and dragging it the other way produces a fair old force.

 

So why the hyp and panic about re-using the old bolts especially if they torque up OK second time around.

 

Roger

[Richard71]

Hi Roger,

 

It really depends on what the bolt manufacturer recommends. Some stretch bolts can be re-used up to three times, maybe more? You may always be able to gain the correct torque or stretch factor from any bolt, but the elasticity won't be as it should. Such is the ill-advised risk one may take with catastrphic results. Which reminds me of an old coleague of mine who was responsible for over £1million repair damage on a marine engine after it was proven he used a less than spotless big-end bolt. His mental health has never been the same, poor old fella!

 

My tuppence worth, shell out and buy new bolts, may as well get ARP bolts.

 

Richard.

[88V8]

may as well get ARP bolts.

 

I recall, very vaguely, that they don't fit the standard caps?

 

Ivor

[Richard71]

Really?

 

Then mine must have been doctored.

[Nick Jones]

Interesting thread. I'm pondering this one as well as I'll be building a 6 up soon.

 

I'd like to use new, but frankly I don't trust the new "standard" stuff available today to match OEM quality.

 

The route used to be to use Sierra Cosworth bolts - unfortunately NLA. You could hock a kidney and get some ARP ones but it is worth considering that the bolts screw into threads in the conrod, which you can't do anything about, and it is frequently these that let go first.... especially where fancy, extra-strong bolts have been torqued up tighter than standard.

 

The standard ones are not stretch bolts as such. It is true that they will stretch when tightened, and it's this that keeps them done up, but they should return to their original length when released, provided they've not been overtorqued. True stretch bolts, as used on many modern cars will be torqued and further tightened through a specified angle. These will be permanently stretched and are one-time use only.

 

Not all of the 4 cylinder Triumph engines use locking tabs. All but the earliest 1200,1300 and 1500 use the same arrangement as the 6. I've re-used the big end bolts on those several times, including an engine that was regularly revved to 7,000 rpm and got away with it - blind luck maybe.

 

This is an interesting article..... sort of related

 

http://www.enginebuildermag.com/Article/1126/perfect_engine_sealing_starts_with_proper_head_bolt_use.aspx

 

And one more thing - how much do you trust your torque wrench? I thought I'd check the calibration on mine a few days ago (crude method using a spring balance) and not only was it miles out, but completely non-linear as well. Bin food.

 

Nick

[Alec Pringle]

My concern with reutilising the bolts is from past observations - having seen several times in the 70s and 80s the disatrous result of them letting go. A pal of mine drove police 2.5PIs in the early 70s, the bobbies used to over-rev them for a pastime, and lost several rebuilt engines to bolts letting go second time around - then they stopped reusing them. Why they should be a potential failure issue I know not, they aren't stretch bolts as such.

 

Cheers,

 

Alec

[rpurchon]

Hello,

 

I am helping a friend here in Spain to refurbish a TR6 engine .

 

We doubt if it is necessary to use new connecting rod bolts, as the repair manual states, since according to responses from some well known UK dealers, "the bolts are just high tensile and normally you do not have to replace them".

 

What is your experience and recommendations?

 

Thanks in advance.

 

Jesús

 

i have always re used old con rod bolts with no problem.

no mention of using new bolts in my triumph factory 2000 manual. or more importantly good old haynes manual.

and lets face it more people with have rebiult 2litre and 2.5 engines over the years relying on a hayes manual.

and ive never heard any tales of 6 cylinder triumphs been prone to throw conrods because new bolts werent fitted.

save your money .that is unless your building that RACE engine that most of the forum members on here seem to have fitted

richard

Edited September 24, 2010 by rpurchon

[Guest ntc]

First of all what is the journal size? + 10 reuse + 20 replace any more than that scrap the lot including rods and get the block checked

[poolboy]

What about the way the big ends are divided ? At the angle at which it's split in half, it would seem like there's isn't as much stress on the threads from compression and momentum as if the split was in the horizonal plane.

Edited September 25, 2010 by poolboy

[Tony Millward]

I read the link that Nick gave and found this clip interesting:

 

"As a rule, the threads and underside of the head on most standard automotive head bolts should be lubricated with motor oil before the bolts are installed. The torque values specified by the engine manufacturer are typically based on oiled threads and fasteners - not dry fasteners. Most service manuals recommend using straight 30W oil or 10W-30 multi-viscosity oil. Though 10W-30 is obviously a thinner oil than straight 30W oil, one gasket engineer we interviewed said the difference is negligible and has almost no measurable effect on bolt loading".

 

I wonder if this applies to conrod bolts too.

 

Cheers

 

Tony

Edited September 25, 2010 by Tonymill

[88V8]

Then mine must have been doctored.

 

I must have misremembered. That's all right then.

 

"As a rule, the threads and underside of the head on most standard automotive head bolts should be lubricated with motor oil before the bolts are installed. The torque values specified by the engine manufacturer are typically based on oiled threads and fasteners - not dry fasteners. Most service manuals recommend using straight 30W oil or 10W-30 multi-viscosity oil. Though 10W-30 is obviously a thinner oil than straight 30W oil, one gasket engineer we interviewed said the difference is negligible and has almost no measurable effect on bolt loading".

 

I wonder if this applies to conrod bolts too.

 

Some shop manuals are more specific than others.

The rebuild manual I have for Rover V8 not only specifies that torqued fasteners should be 'lightly oiled', although it doesn't say with what, it specifies manufacturer-specific products for lubricating certain high-torque fasteners.

 

ARP supply their own lube with the bolts.

 

I would always oil lightly. Actually, I think this has come up before.....

 

Ivor

[Steve Knight]

You can re-use conrod bolts with confidence. Failures are not proven; it is usually some other factor - like not torquing them properly in the first place, or mixing up caps. As said they are not stretch bolts - did the word even exist in the 60's. New bolts or studs are often very poor quality, I've seen rb just strip the thread or break in half. Old stuff is far superior so you dont need to spend £120 on some ARP bolts. Never over oil into a blind thread, as hydrolocking is an issue.

[Britcarfan]

 

Interesting thread. I'm pondering this one as well as I'll be building a 6 up soon.

 

I'd like to use new, but frankly I don't trust the new "standard" stuff available today to match OEM quality.

 

The route used to be to use Sierra Cosworth bolts - unfortunately NLA. You could hock a kidney and get some ARP ones but it is worth considering that the bolts screw into threads in the conrod, which you can't do anything about, and it is frequently these that let go first.... especially where fancy, extra-strong bolts have been torqued up tighter than standard.

 

The standard ones are not stretch bolts as such. It is true that they will stretch when tightened, and it's this that keeps them done up, but they should return to their original length when released, provided they've not been overtorqued. True stretch bolts, as used on many modern cars will be torqued and further tightened through a specified angle. These will be permanently stretched and are one-time use only.

 

Not all of the 4 cylinder Triumph engines use locking tabs. All but the earliest 1200,1300 and 1500 use the same arrangement as the 6. I've re-used the big end bolts on those several times, including an engine that was regularly revved to 7,000 rpm and got away with it - blind luck maybe.

I couldn't agree more -

To me the bolts used in my 1500 engine do not look like they were neant to permanently stretch. When an engineer designs an item such as a stretch bolt he needs to control exactly where the stretch is going to take place, the bolt being necked down in that area smaller than the thread. This not the case with either the big end nor the main bearing bolts. The stems of the big end bolts are equal to the minor diameter of the thread, so if it were to stretch it would equally stretch in the threaded area, destroying the female thread. In the case of the main bearing bolts the stem is actually equal to the MAJOR diameter of the thread! so the thread in the block would strip before the bolt even thought of stretching.

 

My experiences with after-market parts have left me with little confidence in them. The original bolts were made by Guest Keene & Nettlefolds from a specific high tensile steel to a British spec and given a specific heat treatment. I've currently got my Spitfire 1500 engine stripped down and have examined and measured every big end & main bolt - Firstly the mains: the BL manual has the length wrong at 3" - they actually measure 2 7/8" (unless they've shrunk) and I can see no evidence of plastic (permanent) deformation. The big end bolts are given as 1.65" but all come out about 15 thou less than that, so perhaps they were meant to be 1 5/8" with an elgation of 10 or 12 thou, which I would be more inclined to put down to 'creep', where a material changes shape as a result of long term stress (eg sagging car springs).

 

Anyone who did a tensile test in college (our test piece was copper) will know that most metals will stretch in a linear manor within thier 'elastic limit' - within this range if the load is relaxed the specimen will return to its original length. Eventually the test piece will reach its elastic limit, with a sudden increase in length for no increase in load. At this point the material has necked visibly in one place and this area work hardens. The graph of stress over strain then continues to rise a short way until the piece fails with a resounding bang.

 

Thus for permanent deformation to occur the steel must exceed its elastic limit, as with the modern stretch bolts which are visibly designed to stretch.

 

Perhaps there's a metal fatigue or microscopic cracking issue but I wouldn't have thought so given that the stress in one direction only.

 

Having said all that, the BL manual suggests changing the big end bolts but I can't understand why, and there is no mention of the main bearing bolts.

 

As rule classic cars don't do many miles and unless they're high performance engines, thier owners are unlikely to over rev' or cane them.

Unless the engine refurbisher can convince me otherwise I'll be refitting the originals - a known quantity.

 

Disclaimer - the above are just my own opinions and obervations. Whether or not you change the bolts is your choice and your responsibility.

Edited October 30, 2015 by Britcarfan

[RogerH]

Hi Britcarfan (have you a name),

welcome to the forum and this old topic.

 

I tend to agree, as mentioned in my previous post, but am open to persuasion.

 

I am about to rebuild my 4A engine with about 170,000 miles on it and about 250,000 on the big end/main bearing bolts.

Unless they show signs of wear (MPI test to be carried out) they will be re-used. Is this wise?

 

We have had various comments about changing them and possible disasters - but very little scientific proof one way or the other.

 

The original bolts on the 4A were a 'one use only' type but this was new technology at the time - this came with liability and spares sales issues.

 

Using this type of bolt also limits any trial fitting as once a full torque trial is done the bolt can't be removed and re-used - a bit of a conundrum.

If an 'in service' inspection of the big-end shells is carried out are the bolts replaced - probably not (we were only looking at the shells and not doing an engine rebuild, mister)

 

Roger

[Britcarfan]

Hi Roger,

I was watching some YouTube videos the other day to do with checking conrods. In both cases the bolts were fully torqued up to test for twist & bend - Does this mean that the bolts are then scrap? - ok I suppose if money's no object and the customer's paying....

I've no experience with TR 4A bolts and have never seen them so am unable to comment. Perhaps some 4A owners may have an opinion. If the originals were definitely stated as one use only, perhaps best to play safe.

 

Dave

[peejay4A]

I've reused mine in the 4a and it seems to be OK.

Edited November 1, 2015 by peejay4A

[TriumphV8]

In the early days we reused the bolts unless they

are stretch bolts indicated that the are torqued easy

and than more by turning a defined angle, maybe 90 degrees.

 

Later we decided to take the rod bolts new when refurbishing the engine.

I worked on the BMW four cylinder race engine that was opened every year

and we collected a box of bolts over the years.

 

Opposite to that I continued doing so and bought new bolts from Rimmer

for my TR6. I had to learn that there are different bolts with different weights

in the market and they are sometimes somewhat rusty.

So I preferred to refit the bolts I found in the engine being a lower risk.

 

Some years ago I learned from my expensive SCAT rods with

ARP bolts that there are some things between heaven and hell

I did not meet earlier. They provided a nice shedule where every

rod bolt is listed and has to measured before first fitting.

Next refurbish length has to be measured again to decide by use

of a template if reuse is possible without a risk.

To be honest I lost the shedule :-)

 

But I think it is a good idea anyway and as I tend to shorten the

rod bolts a little bit for the weight gain it would be clever to make them

precisely all same length. So if the engine has to be stripped later

one has to measure all the bolts and if one became longer it is

suspect to fail in the future....