View Full Version : TR4/4A Tr 3a & TR4 - connecting rod bolt problems?

09-02-2009, 04:43 PM
I am in the process of rebuilding a TR4 engine to put in my TR2 and have been using the "Haynes TR2-TR4a Workshop Manual" and an original factory "TR2 Service Manual".

All has gone well so far, but I may have hit a snag.

In the "How to Restore Triumph TR2,3,3a" by Roger Williams, he states there are two types of connecting rod bolts, which I knew. What is news to me is that the ones used without locking tab washers are called "stretch bolts" and should never be re-used.

My 2 manuals say nothing at all about not re-using the non lock tab bolts, and he doesn't explain why not but simply says
"obviously it's essential that the stretch bolts are never re-used". Am I to assume these type actually stretch, get longer and do not return to their original length but retain the stretch? So they can't be re-used even with locking tabs?

Why doesn't Haynes talk about this? Help!!!!!!


09-02-2009, 04:58 PM
At least with my Volvo head it used bolts that should never be reused because they do stretch. They have to be torqued down and are find until you take them out. Yes you could reuse but you will never get the same torque or proper setting again with these. If you really want to do it right then get new bolts.

09-02-2009, 08:32 PM
Normally, the TR3/4 rod bolts can be reused unless they have been damaged. The only difference between the "lock tab" and non-lock tab versions is the length.

In fact, most engine builders recommend leaving out the locktabs even on earlier engines that originally had them. It turns out that the tabs can actually allow the bolts to loosen, because the tabs themselves are soft enough to permanently deform under severe conditions. However, if you do this, check to be sure the bolt doesn't bottom out in the rod.

Since the stock bolts are not undercut, the weakest point (where they will stretch if overloaded) is the threads just outside the rod. One way to check is to spin a nut down the threads and see if it binds before reaching the bottom.

09-03-2009, 01:37 AM
I had the same dilemma when rebuilding my 4A engine. I called Moss and spoke to a very helpful, rather particular, guy in Tech Support who said what you read: Never re-use the bolts.

However, opinions vary widely, and having been on this forum for 5 years, I can say this is not a new topic.

Some here say they've frequently re-used the bolts, and some say they don't. I actually think the majority say it's OK to re-use them.

I fell into the overly cautious group and bought new bolts. And who knows? The new bolts might be inferior to the original ones. I just wasn't sure, so I took the most conservative advice figuring that if something went wrong I'd be able to say I tried and followed the written word.

There's also a kind of bolt called a "place bolt". Maybe these are the same. The idea is to avoid locking tabs and to withstand vibration. If you "Google" the subject, you'll see lots of talk and differing opinions, and still plenty of warnings against re-using them. Who knows.

Worrying is such a splendid way to pass the time.

09-03-2009, 11:16 AM
Does anyone out there have a Factory Service Manual for the TR4 that they could look into and see what the factory said about this bolt dilema? (to replace or not to replace)

Opinion seems to be that Haynes is "a bit" lacking in info, and I only have a TR2 Factory Manual. (TR2s used lock tabs)


09-03-2009, 11:19 AM
Actually that needs to be for a TR4a model not TR4.
The engine I am using is from a TR4a. (CT52879E)

thanks.... John

09-03-2009, 11:46 AM
I only have the Bentley "Complete Official TR4-4A" manual. The TR4 section on connecting rods was not modified and still shows the lock tabs. All I could find about the bolts was the general admonishment to replace any damaged ones.

Just curious, has anyone, ever, seen a rod bolt fail on a TRactor motor (except for race motors turning over 6000 rpm)?

09-03-2009, 12:02 PM
Let me ask this questions.
What are the cost of the bolts. What is the over all cost of the rebuild. Is it worth having to rebuild again over a couple of bucks?

09-03-2009, 12:12 PM
OTOH, do you have the equipment to test the new bolts, to be certain they are not defective? The old ones have already been proven to be good; and there are many defective new parts on the market these days.

And if you are going to replace the con rod bolts "just because", then why not the main bearing bolts, head studs, etc. It quickly runs into hundreds of dollars, even though the con rod bolts are only about $10 each.

09-03-2009, 12:21 PM
I am in the process of rebuilding a TR4 engine to put in my TR2 BTW, don't forget that the front plate is different, John. It's not obvious unless you take careful measurements or compare them side-by-side, but the TR4 mounting points are slightly farther out than the TR2-3 ones.

09-03-2009, 02:27 PM
Also the fan extension is different, both in length and diamitre.

09-03-2009, 02:45 PM
I just got off the phone with my machinist and he agrees completely with Randall's logic.

In addition he saw nothing wrong with the bolts when he inspected/worked-on the rods and would trust the old bolts over new aftermarket ones. He also has very little good to say regarding "stretch bolts" and questions if Triumph was really using them back in '65 as one author stated.

He goes on to say that he would trust the original ones, especially since I plan this TR2 to just be an occasional driver, and have absolutely no plans to stress the car in any way. We are talking after all about a 50+ year old car.

And Randall thanks for mentioning the mounting plate, I've already swapped for the correct TR2 size.