conrod weights

[tdafforn]

Just going through the parts I am going to use to build up my new engine (711m long block).
Have just weighed the rods and got the following
708g, 708g, 711g, 684g...
I guess the last is way out, and the others will have to be machined to match?
Cheers
Tim

[elanman999]

Tim,
I think they should also be matched end for end. They used to be balanced by hanging them up on two spring balances (one at either end)but I assume it would be a bit more hi-tech now. You could try two sets of kitchen scales.
There is a lot of metal to come off a 711M rod.
Cheers
John

[rgh0]

Tim

I would look for a new rod to replace the 684 gm rod that is closer to the the other 3 in weight otherwise you have a lot of metal to remove off the 3 heavier ones to get them to match the one very light one. I would be suspicious of why the one rod is so much lighter ( 3%) it may have been modified previously or have some other forging problem.


The first 3 rods are within the sort of tolerances for Ford production rods that I have seen and its relatively easy to get them to match within a gram both in total and end weights using a set of cheap digital kitchen scales. Be careful where and how you remove metal so you dont introduce potential cracking locations in the rod. It you dont know or have a book that shows what to do ask someone else who knows.

regards
Rohan

[tdafforn]

Doh!
Had another look at the rods...
Weight difference is due one of the shells not being in the lightest rod!!
have removed all the shells and now they range from 600g to 605g, I guess this is more like it (<1% difference).
Is it worth working on them further bearing in mind it is for a Road +2..?
Cheers
Tim

[msd1107]

If you have the time and capability, balance them as closely as you can, not only total weight but each end.

The engine will be smoother, and you will have less problems with vibration induced problems.

The mechanic who did my engine did this, and the difference was quite noticable.

David
1968 36/7988

[rgh0]

Tim

Glad to see you found the problem with the light rod

Even for a road engine I would balance everything and for the rods in particualr aim for less than 1 gram difference in both total and end weights. I would do the pistons to the same tolerance and have the crank, flywheel and clutch balanced professionally both individualy and as an assembly.The cost is low when doing a rebuild yourself and the performance and smoothness benefits signficant.

cheers
Rohan

[tdafforn]

Hi Rohan,
I can work out how to setup two balance's to measure conrod balance (In fact we have 5 figure balances in my lab so we could get to within a 1/10 mg!)
Do you have any advice (before I dive into the books I have) on where to remove mass from the rods?
Cheers
Tim

[rgh0]

It depends a little on how much you have to remove and the design of the rods themelves. But given the design of the Ford rods and the small amounts you need to remove the normal advice should apply in your case.

Typically you want to remove it from the very top of the small end and the very bottom of the big end. Do it very gently with a fine file over about a 30 to 60 degree arc. How big the arc is depends on how much material needs to be removed as you want to keep the thickness of metal removed as small as possible. Stay well away from the sides of the big end where the bolts are located or sides of small end where the round section blends into the straight section of the rod. Ensure you avoid creation of any sharp cuts in the rod and remove the material consistently across the area blending in smoothly at each end. Then polish the filed area progressily going down to at least 1200 grit paper to remove any file marks and have a near mirror finish. For racing purposes where you are typically using shot peened rods you may also want to re-peen the filed area.

You can do it with a power sander /linisher if your very very careful rather than a file if you wish but it is harder to control and you risk putting too much heat into the metal removal area and causing microscopic surface cracks that can lead to later failure so I prefer it is done manually and slowly with a fine file. Never use a grinding wheel on a rod to remove metal as this guarrantees too much heat input and subsequent cracking risk is high

Weigh each rod first figure out which is lightest both in total weight and end weights and then file off the heavier rods until you get to your target numbers. Take it slowly as its easy to remove a gram very quickly even with a fine hand file. If you take to much of you end up having to then take weight of the other 3 and you can end up chasing your tail.

cheers
Rohan

[tdafforn]

Hi Rohan,
Thanks for the info, sounds like a perfect job for the dark winter evenings in front of the TV. I am having the crank assembly balanced as you suggest
Cheers
Tim

[r.agnew]

Tim - You either have A) A TV set in the garage or B) A very understanding wife who doesn't mind metal filings in the living room carpet !!

[tdafforn]

Definately the second!
garage is too small for a TV due to garage size being traded off for a nicer house (Makes wife happy!)
As a result bits of the car are allowed in the house!
(Was weighing conrods on the kitchen scales last night in kitchen while she made supper on the worktop next to me... )
Cheers
Tim
(PS Plan in progress to double garage size in the summer!)

[denicholls2]

Bear with an ignorant old dog for a moment:

What's a good method of establishing "top" and "bottom" weights? Or are we speaking only of the rod with and without its big-end cap?

Thanks!

[rgh0]

Doug

I weigh each end of the rod by suspending the rod horizontally with one end on the scale and the other end hanging from a wire. I weigh the rod assembled with the bolts in place and no bearing shells.

Do it after the small end bush has been reamed to size if your replacing them.

cheers
Rohan

[tdafforn]

Hi Rohan,
also being a bit thick here
Do you take the weigh off the square shaped boss on the bottom of the big end in order to balance? Ditto with the boss on the small end?
tim

[rgh0]

Tim

Yes take it off the bosses. If you really want to get enthusiastic you can remove the bosses completely and lighten all the rods a fair bit. I have attached some photos of various rods to show what can be done.

Also have a look at the Wilkins twin cam engines book page 105 has some photos and description of the process there.

People use the lightened and shot peened 125E rods in racing to 7500 rpm but I believe they are marginal at these revs and I have seen them end up through the side of blocks if used to the limit regularly

cheers
Rohan