TheSamba.com :: View topic

Tue Dec 31, 2013 10:28 am

Hi all I've balanced my pistons to 0.1g between the lightest and heaviest (there was only 2.3 grams between the lightest and heaviest before balancing) but my conrods there is a massive difference between the lightest and heaviest. The lightest is 622.0 grams and the heaviest is 634.5 grams that's 12.5 grams difference. Is this normal to have such a large range in weight or not?
Plus where is the best place to remove all this weight?

Tue Dec 31, 2013 1:50 pm

Tom Wilson has an excellent book on rebuilding VW engines. You really should get a copy, it will help you do the task properly......... It's less than $20 and worth every penny.........

http://www.amazon.com/Rebuild-Volkswagen-air-Cooled-Engine-models/dp/0895862255

Anyway, a photo from his book.......... The arrows show where to reduce weight .........

You have a 10gram weight span between lightest and heaviest.

Dave

Tue Dec 31, 2013 2:05 pm

That is a large variation in weight, I would guess that they aren't an original set. It would be best to be able to balance them and for end, just guessing you can get them all equal in total weight but then check the ends and be off 5 grams or more.

Compare them and try to see where the heavier parts are! Then make them all the same. VW's guideline is ok to match ones that are already within 6g or so but you need to take off more than that!

Tue Dec 31, 2013 2:21 pm

Thanks for the photos mate.
I do have another set of rods from a different engine but can I mix and match rods from two different engines?
One set of rods are from a 1500 single port engine and the other set of rods are from a 1600 twin port or does it not matter what engine they are from
plus how do you jyst weigh the ends im not to sure what you mean sorry :-(

Tue Dec 31, 2013 4:37 pm

Yeah, you can mix and match.........if they match better that way.
No offense mate but but you should probably take them to the machine shop.
When rebuilding ANY engine the rods should be checked to for size at both ends and checked for alignment, and, if willing to pay the few dollars extra balanced too. This has been standard practice for all engines since the beginning of time, so there should be no trouble finding a business that does this.

You can also learn how and construct measuring instruments to do it yourself if you so desire, but that is a lot more work.

Tue Dec 31, 2013 4:42 pm

Pictures are almost better than words, follow the link......

http://www.angelfire.com/ca4/CorvAIRCRAFT/RodBalance.html

Dave

Tue Dec 31, 2013 4:51 pm

Yeah, that's it, but don;t forget to check the other more important things also.
The pin bushings are worn out more often than not, and after they are replaced some guys don't check the alignment..........don't underestimate how screwed up they can become :shock:

Tue Dec 31, 2013 4:59 pm

From my home-hack job grinding material off conrods, you won't get those 10 grams just from the areas shown in the photo above. You'll also need to grind off metal from the big end cap. The cap has two parallel "ridges" which are cast into the circumference. You grind those off/down as needed so that you reach the level of the base form. This is also how you modify rods for stroker clearance. If you have a bench grinder, hold the rod cap parallel to the grinding wheel, not across it. Test your weight frequently.

Here's a shot from theSamba gallery showing the two "ridges"-

Here's another gallery shot showing some rods (type 4) where the big end was ground, as well as the small end .

This method does not follow the "end for end" weight balancing method, but is simply for achieving the same lowest total weight as your lightest rod.

You could try to use other rods which have closer weights to begin with. You need to measure the length of the rod's body between the bore holes. Any variations from your original set, or among the rods, will result in variations of the piston deck height.

Wed Jan 01, 2014 11:58 am

As long as we are on the topic of balancing.......
Engine 1986 2.1 WBX Vanagon.
My pistons as a group and my rods as a group are all fairly close to being the same weight.

What I've done is pair the lighter pistons with the heavier rods thus resulting in four units that weigh in total within one gram of each other.

Is there a problem I'm over looking with this concept? More mass farther out from the journal type issue?
Is the few grams even an issue?
Specs are within 10 grams.

Here are my weights and how I paired them. The pistons are a reuse of my original and the rods are fresh remanufactured units utilizing the 1.9 rod cap bolts from Van Cafe.

Pistons:
1- 644 grams
2- 641 grams
3- 642 grams
4- 640 grams

Rods:
A- 470 grams
B- 471 grams
C- 472 grams
D- 473 grams

Even though all of these weights are well within tolerance for each component group, If I don't pair carefully I could end up with a fairly large weight spread!

So, pairing rods with pistons......
1 & A : 1114 grams
2 & C : 1113 grams
3 & B : 1113 grams
4 & D : 1113 grams

Dave

Wed Jan 01, 2014 1:08 pm

there are many places to remove weight,but be sure to do each end so the balanced rods are balanced, not just the same weight.....and leave no sharp edges,dont get the cap or any portion too thin or too hot,I just buy new rods for less than $100 & dont wory about anything like bushing coming out,or dead rod bolt.I have a sunnen rod machine,just no reason to waste time on stock rods.but Ive seen they do make a nice chair when welded togeather nicely.

so what did you pay for reworked rods that need reworking???

Thu Jan 02, 2014 1:44 pm

mark tucker wrote: there are many places to remove weight,but be sure to do each end so the balanced rods are balanced, not just the same weight.....and leave no sharp edges,dont get the cap or any portion too thin or too hot,I just buy new rods for less than $100 & dont wory about anything like bushing coming out,or dead rod bolt.I have a sunnen rod machine,just no reason to waste time on stock rods.but Ive seen they do make a nice chair when welded togeather nicely.

so what did you pay for reworked rods that need reworking???

Golly........ Why are you saying my rods need reworking?
Specs are for a 10gram spread..... I've got 4grams.
They are rebushed, trued and new rod bolts.

Please explain the reasoning behind your comments and tell us what is wrong with my rods ............ that you've never seen.

Oh...... $110
http://www.van-cafe.com/home/van/page_679_947/connecting-rods-set-of-4-see-description.html

Dave

Thu Jan 02, 2014 2:19 pm

a lot depends on what your "spec" is. Stock spec is one thing, race spec is quite another. You have to be specific or you'll assume you are saying one thing and people will assume you are saying something else entirely.

Many people feel a 5g tolerance on pistons is acceptable, even VW thought that was ok. We balance to .5g, and if you are really anal you'll go to a .1g tolerance.

Thu Jan 02, 2014 2:36 pm

Thanks John.

I should have mentioned that mine is a stock 2.1 WBX, no racing, no hard use.

Dave

Thu Jan 02, 2014 3:01 pm

it can still get ugly.

You build an engine with "stock spec" components, and wind up with a 5g heavier piston, with a 10g heavier rod, etc etc. This is one of those engines that never runs "smooth" despite all parts being "within spec".

When you look at a balance sheet for a rotating assembly, sometimes you'll be shocked at the #s of imbalance in the "BEFORE" spin. We are talking hundreds of pounds of imbalance at speed.

Thu Jan 02, 2014 3:59 pm

Hi all here's an update good news all four pistons are now within 0.1 gram from each other and the conrods with bearings and nuts on all weigh the same :-)
BAD NEW I've just broke about 1/3 of a cooling fin of number 4 cylinder the closest one to the head is that the end of that cylinder or can I still use it?

Thu Jan 02, 2014 5:27 pm

Unless the bore is disturbed then it'll be fine to run it, what happened? did you drop it? if so measure it up before making a decision

Thu Jan 02, 2014 9:37 pm

For practical purposes,1 gm difference will cause an imbalance force equal to the weight of 1kg at around 4000 rpm.That's not very much,but at 8000 rpm it would be 4 times that (still not very much).

You can easily have 5-10 gms of carbon on top of a piston.

The big end mass of the rod is doing different things to what the small end mass is doing,so it pays to make all big ends equal and all small ends equal (not just the overall rod mass).

If you put a shaft (or even a gudgeon pin with a drop of Loctite) through the small ends of 2 rods with the rods facing 180 degrees away from each other,you'll soon tell which big end is heavier.Put your pin or shaft on level straight edges and the heavier big end will fall. Even good vice jaws can work as straight edges.

Thu Jan 02, 2014 10:32 pm

So John, are you saying that the rod sets you ship are balanced to 0.5 g?

Thu Jan 02, 2014 10:34 pm

Just a FYI, Scat i beams come balanced from factory. The last 4 sets of AA rods I bought required every rod but one in one set to be balanced. I have my own jig and I can tell you it takes quite a bit of time to shave off 10grams from a big end. You have to take it off where you can at that point. Every stock set ive balanced have been with-in 5 grams from start. Some sets even less. Scats are the best to buy if you want them already rtg balanced. Every H beam set ive come across or used: Bugpack, CB, and Scats have been tits on balancing.

Thu Jan 02, 2014 10:46 pm

No. We balance pistons, but are not set up for balancing rods, or anything else. We do offer balancing of full assemblies, but we have a specialty shop do that for us.

But we do sell a lot of rods already balanced. All the SCAT rods are, as are the Bugpack ones. So are DPR.

Frankly, it really doesn't matter if they are balanced or not most of the time, because you SHOULD get the whole assembly balanced anyways.

ap_sand wrote: So John, are you saying that the rod sets you ship are balanced to 0.5 g?