Weighing rods Etc

[AllAlloyCup]

Just wondering how critical the comparitive weight
of connecting rods are?

Weights in gm. are:
No 1- 359
No 2- 355
No 3- 350
No 4- 342

Given the format of the A7 crank

1+ 4 = 701gm
2+ 3 = 705gm

Should I refrain from removing any metal from these rods?

They will be fitted to a new steel crank and still debating about doing some modest
Lightening of the flywheel before getting the whole assembly balanced.

Not planning on doing any racing or serious trialling

Still plugging away on the alloy wings and tailcover so mechanicals are a pleasant relief!

Regards

Bill G

[Peter Clayton]

Obviously the originals were "balanced" during production but I suspect they were not produced as matched sets, although were stamped with engine ID. The ones fitted to my '33 RP were not original to the engine or each other,so I thought I would match lighten polish and balance them. I took a fair amount of metal off just getting rid of casting feathers and grinding scours. They were balanced by removing metal mainly from the big end areas,which now enables them to just slide down the bores, thus making assembly (without splash plates fitted) easier. I would recommend balancing as much as possible to reduce vibration and improve reliability. Take a look on you-tube , under "balancing con-rods" some good DiY balancing jigs that you could make.

[Speedex750]

Hi Bill,

Adding to Peter's advice - those weights sound like overall totals. You need to get end weights, particularly the little end. It is possible to juggle pistons and gudgeon's around to get things closer but I'd suggest not removing anything from the little end of the rods or piston, match the heaviest little end with the lightest piston assembly. I was taught to balance to the second lightest and try and add a little with a washer etc on the lightest. A cheap set of digital 1Kg kitchen scales does the measuring very accurately. The big ends can be adjusted by removing the two knobs either side of the bearing or removing an unused turn of thread or two on the end of the big end bolts. It is possible to get everything within a few grams. I doubt they were made in matched sets, the production line was probably just fairly consistent in batches(?) so there does seem to be variations between odd rods so beware if you're combining an odd set of rods. Of course assembly becomes very specific after you've done all this matching, something to remember if you do a partial strip and rebuild in the future!

Dave

[AllAlloyCup]

Thanks for the initial advice.
I’ve not weighed the pistons yet so there may be some room for
equalising the weight of assemblies.

Three pistons have the same engine no, factory marks?
Which matched the block, both crank and old pistons were all at std.
Before being changed.

[Bob Culver]

I have burrowed through many other workshop manuals over the decades and the new tolerances cover a wide range. Typical of the more stringent is Ford 10 (astronomically high revving in the Fordson) which quote 4 grams for pistons and the same for conrods.

[Ian Williams]

The more accurately tolerances are maintained the better any engine will run, that is the principle of blueprinting, 4 grams in my opinion is way too much, but of course each to their own.

[AllAlloyCup]

Does the fact that the A7 engine crank has two up and two down
Mean I can simply equalise the weight in between
Pairs of pistons/rods?

Given the differences between the steel webs on the crank are
likely to have larger weight differences?

Then there’s the balance in the flywheel and clutch
to accommodate...

Bill G

[Stuart Giles]

There's been a thread on here previously that covers balancing.
http://www.austinsevenfriends.co.uk/forum/showthread.php?tid=460&highlight=balancing

[jansens]

I find the limiting factor for me when trying to blueprint something is the accuracy of whatever I am using to measure things. I usually end up measuring things multiple times and finding I get to a point where every measurement is slightly different but eventually they are come down into a range that doesn't vary if that makes sense. I figure at that point I am as good as I can get things with the tools I have.

I cheated when balancing my rods/pistons. I was getting the flywheel, clutch and crank dynamically balanced so I let them do everything else as well. Everything came back with a note saying don't mix up the pistons, rings and gudgeon pins as they were all balanced as units.

I've been reading an interesting book (from the 50s) all about sports car engines and they talk about inline 4s and how you can never get them perfectly balanced anyway. Secondary harmonics and other such things apparently. But it still makes sense to get everything as good as you can as Ian says I think. I was very careful when I did my MGB engine to get everything as close to specs as I could and that really runs nice and smoothly so was worth the effort.

Simon

[Bob Culver]

Hi Simon

If the book you are alluding to is the once very popular The Sports Car by Campbell, at least in the 3rd Ed gives wrong formula for piston accel (and does not make clear the increase and decrease at top and bottom). And the chart of big end  loads at firing at high rpm  does not clearly distinguish net up (long stroke) from down.

On repeatable measurements, micrometers are a challenge. It is difficult to repeatedly acheive the claimed accuracy. The anvils have to be free of oil film. The oil film in the thread changes with repeated closures. The static setting differs from measurement of an object. Errrors accumulate when using separate internal and external.