Conrods weight difference

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thenomadcat
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Conrods weight difference

Post by thenomadcat »

Hi,

I'm rebuilding from scratch a 1.7d KY engine and just received a NOS set of conrods. I was surprised to find a weight difference of 10g between the lightest and heaviest conrods.
I wasn't expecting racing grade balancing (0.1 to 0.5g diff) but from my experience with VW 1300 and 1600 engines the maximum acceptable weight difference is 5g.

I wasn't able to find anywhere this information. Can anyone shed some light?

Thanks,
Andrei

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937carrera
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Re: Conrods weight difference

Post by 937carrera »

I dug out my old Haynes for the 411/412, older books being more technical. (type 4 engine was developed for the 411 originally)

There are two weights given for con rods;

746-752 grams
769 - 775 grams

So I would say rods should be within 6 g, pretty much what you originally thought. T25 will probably be different, but within the same range

Would you mind posting your weights so other people can compare against their own measurements. I'm in the middle of getting ready to rebuild one and have weighed the entire rotating assemblies, but not the individual components yet.

I'm assuming diesel will be stronger / heavier ?
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
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thenomadcat
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Re: Conrods weight difference

Post by thenomadcat »

That is interesting. The current set I have is: 684g, 689g, 691g and 694g.
I have two more sets I can't use because each one has one conrod either bent or cracked. One set is around 700g, all are between +/- 3g. The other set is in the 680g range, all +/-5g.

I keep finding mixed information, none from proper sources. The guy who did my engine block and crankshaft (fantastic fellow, the tolerances are right in the middle for all measurements) told me he found a catalogue saying the piston together with the conrod and bearing should be within 10g of each other. I can achieve 8g but I'm still not comfortable. Others told me for diesel it doesn't matter while some say the difference matters only in racing cars.

The Bentley manual states "only replace conrods in set of four" while another manual from H.R. Etzold (I suspect is based on the Bentley) says nothing at all. Heynes does not cover diesel engines, right.

Does anyone has this data for the Golf SB engine? I believe it's similar if not identical with JX which has the same conrods and crankshaft of my KY.

Thanks,
Andrei

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937carrera
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Re: Conrods weight difference

Post by 937carrera »

You threw me when you talked about Beetle engines, was still thinking flat 4.

This should be a good read for you https://www.thesamba.com/vw/forum/viewt ... 06&start=0

I've read advice about only replacing rods in sets........ and known good engine rebuilders replace a single rod in an engine. If the weights are good, then that's the measure i would use.
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

thenomadcat
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Re: Conrods weight difference

Post by thenomadcat »

Very interesting topic on TheSamba, thanks for pointing it out.
But I'm even more confused now because I don't know what information should I take from there.
I saw they talk about 0.1g which is what I would expect from an engine running constantly at high rpm. But for these engines you have a ton of work to do balancing everything, not just the conrods or pistons. If your crankshaft is way off it really makes no difference if you have 5g or 10g between the conrods.

Someone else mentions 10g as acceptable and points out carbon deposited on the piston can easily exceed 5g which is so so true. Under these conditions, there is absolutely no point in having the conrods perfectly balances.

But again, the biggest question mark is how do I translate this information to a diesel engine? The difference in RPM is huge when you think about it over a long period of time.

I have two more engines and I'll try to take the conrods off and weight them. I was really looking forward to use the NOS conrods.

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937carrera
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Re: Conrods weight difference

Post by 937carrera »

The Samba thread was in the performance engine section......... 0.1g for race spec was what was being talked about, the 5g etc was OE.

The approach I have taken so far on my WBX unknown thread was to simply measure each of the entire rotating assemblies. Can you get a good balance by mixing / matching components, pistons, pins each vary too.

Balancing is far more important for high revving engines, piston velocities are much faster and I suspect if you were to go looking that the energy involved would be squared or cubed with speed
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

thenomadcat
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Re: Conrods weight difference

Post by thenomadcat »

Fortunately my new pistons are all within 1g of each other. Unfortunately this means I can't use the piston weight to balance the conrod difference.

Will do more measurements and experiments this week-end. I'll let you know how difficult it is to get a conrod properly balanced.

Andrei

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937carrera
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Re: Conrods weight difference

Post by 937carrera »

DIY or engineering workshop ?
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
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bigherb
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Re: Conrods weight difference

Post by bigherb »

VW make no reference to weight only to be replaced in sets. As a diesel runs on full compression all the time variations in compressions would negate any differences in weight balance.

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thenomadcat
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Re: Conrods weight difference

Post by thenomadcat »

bigherb wrote:all the time variations in compressions would negate any differences in weight balance.

That's very good news but I'm afraid I don't fully understand how this works. Could you please explain why compression will negate the weight difference?

Thanks a lot.

Andrei

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Re: Conrods weight difference

Post by BiTurbo228 »

Ye olde thread, but it comes up on google searches for conrod weights so I thought some clarification might be helpful!

Manufacturers don't tend to spend time individually balancing conrods. What they'll do is weight-match them into sets (usually with a fairly generous tolerance, I know Maserati BiTurbos are +/4g but some others can be larger). That's why they tell you to replace all conrods with a matched set, as they should be grouped together by the manufacturer.

What it looks like has happened is someone's bunched a set of conrods from a couple of different batches, and ended up with some heavy ones in with some light ones. If anyone's in a similar situation you might be able to bring the weight down to within factory specs. If not then I'd expect the engine to feel a little rougher/be harder on bearings at higher rpm. Depends how exacting the factory specs were, and how robust your bottom end design is to imbalances.

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