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MG TD TF 1500 - MG TD Connecting Rod Needed

I'm in the process of rebuilding my engine (TD/C 7254, XPAG 7427) and my machinist suggested I try to find a replacement rod for one which weighs much less than the others.

This engine was "rebuilt" sometime between 1960 and 2005, and I've found many problems, this being one of the fatal flaws in the earlier rebuild. I weighed the rods before I took them to the machinist, the light rod was 11 grams to 14 grams less than the others.

The machinist said the three heavier rods cannot be cut sufficiently to bring the set into balance with out compromising their integrity. These three rods weigh between 655 and 658 grams complete -- including cap and the 3 bolts.

Does anyone have a connecting rod with a total weight within or close to this weight range, which I might purchase?

Charlie
Charlie Adams

Charley, I have one that weighs 668g with all three bolts and nuts. It hasn't been resize yet and this is with the standard hex head small end bolt.

If your interested, email me,
Richard Cameron

Richard,

Thankyou!

I'll check with my machinist on a rod of that weight, I'm thinking with some resizing, it just might do the trick.

Charlie
Charlie Adams



Here's a picture of what I weighed it with.

Richard Cameron


Another photo of numbers on rod

Richard Cameron

I think I would first opt for matching numbers, then matching weight in balancing.
Larry Brown

Hi Charlie,
There are lots of different numbers on rods. Sets usually have matching numbers and that probably means they are close in weight .... However ... the important thing is that they are similar weights. Ideally they should be similar little end and similar Big end. If you can get a rod that is close to the same overall weight, a crank/rod/piston balancing shop should be able to balance them accurately. Balancing experts don't just balance overall weight. They also balance weight distribution. If rods just went up and down, overall weight would be all that mattered. BUT the little end goes up and down with the piston and the big end also goes across! ... hence the experts balance each end.

Also check the length of the rods. They should all be the same within a few thou. (not sure of tolerance ... would guess about 10 thou max?) I assembled an engine 40 years ago and found that two pistons protruded a few thou ..one protruded an EXTRA 20 thou and one was short of the block face by about 10 thou. The engine must have had some trauma! I found two other rods and balance them to match so they were all close to the same length.
Bob Schapel
R L Schapel

Richard, I have not yet heard from the machinist.

Bob, The light rod among them is 645 grams, but the big end is about 17 grams lighter than the others. I will tell the machinist to check the lengths, its interesting about those differences.

The numbers on my rods are
24005 *50 which weighs 645 grams
24005 *24 which weighs 655 grams, and
24005 *25 (2 rods) which weigh 657 and 658 grams


Charlie Adams
Charlie Adams

I have concerns about buying a 70 year-old used con rod.

About 5 years ago, I helped a friend resurrect a TC engine that required a rod. He found one and I suggested he Magnaflux it, and check it for twist and length; and found it had stretched by about ½ a millimeter. We ended up making a special small end bushing and machining it off-centre to get the correct length ... certainly not something I would recommend to anybody, but in this case it worked.

I don't believe that the original MOWOG rods are available (pls correct if someone knows to the contrary). And I understand that the only new rods available are Carillo which fetch a King's ransom!

14 grams is excessive. In my present TF engine, I got each rod assembly down to basically zero, buy playing the old "shell game" ie: mixing and matching rods, bolts, shell bearings and by grinding a wee bit off the bearing cap heel of one rod.

And Charlie, if you want to reduce the big end weight of your rod assembly, replace the crown nut, cotter and washer, with a Nylock nut only. Not a big difference I know, but nonetheless, a bit lighter. It all helps!

Gord Clark
Rockburn, Qué.
Gord Clark

I have three that still have pistons on them that only had about 10,000 miles on them. If the others don't work out, let me know and I will get mine out and take the pistons off and get weights on them.
Bruce TD4139 Cunha

Hi Charlie,
I am a bit of a "back-yarder" so don't know what tolerances are allowable. When I used to balance my own rods (I was 18 in 1970) I got them to within a "rubber band weight" ... I used my dad's bank coin balance (Yes that was a long time ago) but later I graduated to using the university's electronic balance (Had to close the windows as a gentle breeze would make the readings waver! Mid 80s?) ... but I did not balance individual ends. I have since (mid 90s to now) had them done professionally. I am usually aiming at 7000 rpm racing specs but a road motor doesn't really need such accuracy. Overall weight equality is probably all that is needed.

I would suggest that any XPAG rods can be balanced. The heaviest rod can have metal taken off the gudgeon end by filing off areas which are not needed and chamfering the top "face" near the threaded hole. The big end can be filed outside the bolt boss and the whole rod can be be filed down along its whole length. Remember that filing will NOT weaken the rod. It will strengthen it until all nicks and blemishes are filed away. Rod prep for racing involves filing all the nicks out. The rod in the photo obviously did not need much lightening but it was filed and polished to remove nicks and blemishes which would be stress points. I have dotted white marks on the areas where metal can be safely removed. Always file or grind ALONG the rod. Scratches ALONG the rod are not potential crack points but scratches ACROSS the rod are!

I know it seems strange to file metal away along the rod beam but it is actually doing GOOD! I would not consider racing an engine unless the rods have been filed/ground smooth. The big end cap can also be lightened underneath. I will send a second photo with white dotted areas to lighten.

Bob Schapel

R L Schapel

Second photo.
There might be a simpler way of sending more than one photo at a time .... but I am too old to learn how to do it!
Bob

R L Schapel

Bob thanks that picture made all the difference. Now I understand. Thanks for taking the time to do that.

Joe
JWP Policastro

Bob

Instead of filing would there be any detrimental effects if a linishing belt was used to remove defects and weight.

Graeme
G Evans

I would think that any method would be ok. However the important things are .... The rod should be smooth along its length anywhere that could break (don't grind a narrow, weak spot). If any marks/scratches are not polished out, they should run along the rod, NOT across it. There should be no nicks or indentations anywhere the rod could break. The surfaces where the numbers are cast are not really a problem because they are not in a stress area where a crack would start. In fact they could be ground back to lose weight in the balancing process. (Those areas are not ground in the images I posted.)

It is a bit hard to explain the idea in a paragraph or two. However, if you have ever tried unsuccessfully to break a piece of wire by bending it .... and then put a nick in it and retried ... you will see the point of the exercise.

Remember standard rods have lots of blemishes. Ground/filed and polished rods are usually only used for racing. The reason for mentioning all this is that a lot of weight can be removed without weakening (in fact strengthening) the rod.

Bob Schapel
R L Schapel

Bob,
Very helpful information!

I now have 4 rods on their way to my home which are in within 7 grams of the trio which weigh about the same. I'm hopeful the machinist will find one of them acceptable to him.

I'm also going to show him your photos and have a chat about how you strengthen the rods and reduce the weight.

Thanks,

Charlie
Charlie Adams

This thread was discussed between 09/12/2015 and 14/12/2015

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