TRIUMPH CONNECTING ROD BEARINGS...BiMetal vs. TriMetal!

Rebuilding a Triumph engine usually involves a lot of cleaning and identification, measurement, machining, and sometimes cursing. Some of the parts you need to replace, like connecting rod bearings, can be confusing to pick out.

Here are a few things that may help in your selection and success with getting your rods back on your crankshaft.

First of all, make sure the connecting rod big end bore is round!

I know that sounds really basic, but you would be surprised at how many people do not measure this very important part.

Triumph rod bearings were designed to be installed in a machined rod big end, if you replaced the rod bolts (or accidentally switched rod caps), this is something that has to be checked, and corrected if out of round. 

The rod can be measured with a bore gauge while the rod bolts are torqued to the specified figure, it should measure 1.7700" and a plus or minus of .0005" is acceptable. Any more and the rod will have to be replaced or resized.

But I heard Triumph rods can't be resized!

Triumph 650 rods can be resized. I have done it in my shop for a decade, and not one of them has failed. However, not all rods should be resized. Only rods that are in good shape other than the big end journal being out of round are solid candidates. The small end bushings should also be replaced and honed before the rods are reinstalled.

If you replace your rod bolts and the cap does not line up, it would be foolish to throw them away. Most can be resized using Sunnen rod equipment and careful technique. If your rod is stretched or the small end is damaged you would be best off finding another rod or set.

both of these high quality bearings are in stock in 5 sizes

now... Back to "England vs. USA"

Bi-Metal and Tri-Metal refer to the construction and materials of the bearing inserts.

Because this is a complicated subject I will not get into the advantages and disadvantages of these materials, but will say that both types will be excellent for almost every Triumph engine, but the Tri-Metal is superior in high load and high stress racing applications.

BiMetal bearings have one layer of aluminum tin (Al-Sn) on a steel backing. They are also lead free and more economical to manufacture.

TriMetal bearings have a babbitt alloy layer on a copper-lead alloy, also adhered to a steel backing. 

Both of these bearing types are great for most Triumph applications.

They replace Triumph part number 70-3586 or E3586, Glacier B2026, and Vandervell VP56.

Hepolite is manufacturing the bi metal bearings in England, and the TriMetal bearings are being made in the USA by Mahle/Clevite. Because they have different advantages and different pricing, both types are available and in stock.

Triumph 650 750 connecting rod bearings

Which is a better fit? What is the clearance?

Measuring your connecting rod journal clearance is tricky unless you have the right equipment. The crankshaft journal must be measured with a 1"-2" micrometer that reads in tenths (.0001"). Then the bearing shells have to be installed in the rod, torqued to spec, and then carefully measured vertically (top to bottom) with a bore gauge.

Be careful when measuring as the bearing material is extremely soft and is easily scratched!

Plastigage may be used to check the clearance If there is no bore gauge present, or as an alternative. There is a lot of information on how to use Plastigage but the range you want to use is .001-.003 inch, (Mahle Clevite part number MPG-1). 

 Then the size of the crankshaft journal is subtracted from the size of the bearing journal, and the result is your operating clearance.

Mahle Plastigage box MPG-1

Plastigage® MPG-1 can measure .001" to .003" clearance

Generally, the traditional rule of thumb for rod bearing clearance is .0010" for every inch of journal diameter.

This would make a Triumph 1.6235" journal ideally have .0016" rod to journal clearance. These bearings have both been designed with this formula in mind, and because crankshafts, rods, and bearings are not all the same, it is best to always check.

Sometimes the actual installed clearance is a little more or a little less than the "rule of thumb". There are a lot of factors that  can affect bearing clearance. Some engine builders prefer specific clearances for different reasons. Rod clearance also plays a huge part in selecting oil viscosity, and with Triumph twins most people seem to agree that .0015" to .0020" is good for almost all engines. 

the actual results...

 Using a Triumph 650 rod with a journal size of 1.7700", both sets were installed at the the same temperature and conditions. A Sunnen AG-300 gauge was used to measure. Using a -.010" crankshaft journal measuring 1.6135" as the target, the two bearings sets gave the following clearances:

Hepolite (bi-metal) bearings             .0016" rod clearance 

Mahle/Clevite (tri-metal) bearings   .0019" rod clearance

If the crankshaft was ground on the high side of the specification (.0005" bigger), the clearances would have been .0011" and .0014" respectively. 

(For Triumph crankshaft journal sizes see the tech article by clicking here)

There are a lot of things that can affect your bearing clearance, from rod journal size, connecting rod bore size, rod bolt torque, and the list goes on. 

The bottom line is that the Hepolite and the Clevite/Mahle bearings are great, and both of them will provide the right clearance and last for tens of thousands of miles.

To purchase 650 / 750 Triumph connecting rod bearings click here


Older Post