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Balancing conrods-pistons / Tapping threads

Practical Engine Building

Discussion and questions related to the course Practical Engine Building

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Hey there, really love the course!

I'm actually trying to rebuild my SR20DET with stock pistons and rods since it's my first and want to try rebuilding my first engine with cheap parts. (Instead of breaking forged stuff on my first build). I've been trying to adjust the overall weight of my conrods, but it seems that one of them is 3g under the others. I've started removing material from the heaviest one, removed all I could but still weights 3g more than the lightess one. I guess the engine was running with these rods from the manufacturer so they might work as well. Matching my heaviest piston and pin to that light rod, still managed to get 1g difference between my heaviest and lightess match. Any advice? 3 rods would weight around 666-667g, #2 would be around 662-663g.

Also, been following your advices and decided to try and tap my headstud holes with my generic M11x1.5 tap with clean engine oil. I was quite surprised with how much material would come out of the holes, kind of worried I'm gonna strip the threads when putting the torque ARP recommends. Any advice here also? Some friends told me if I have no resistance in putting in the studs by hand I should be fine not tapping them. I've tapped 2 holes at the moment and waiting for help ahah! :)


Simon :)

Hey Simon,

It's not uncommon to have an imbalance in factory components however 3 grams is quite significant and a little surprising to be honest. Most late model Japanese engine components are actually pretty good from the factory these days. You can find also that with an imbalance this large that it can be tricky to remove enough material to get the rods balanced without weakening them to the point of being dangerous. Knowing the SR20 rods quite well however, there is quite a lot of material on them and I'd be surprised if you couldn't safely get 1 gram off the heavier rods. It's also important to know if the imbalance is at the big end (easier to remove material from here safely), or on the small end (much less scope to remove material here).

Just to be clear, you are balancing the pistons and rods independently?

While it is surprising how much material you end up removing from a head bolt hole using a tap, provided you're using a sharp/new tap and it's well lubricated, you should be in no danger of stripping the threads out. This is something I do on every engine I build.

I'm running into this with stock components of a 1987 VW engine I'm doing my first balanced build on. The conrod big ends were almost 3 grams apart, then the small ends were 2. I've finished balancing them and the difference between them is significant enough that I re-weighed them twice on separate days. Now I'm on to the pistons and they are 3 grams different, the pins 1 gram, and by some miracle all 4 sets of lock rings weigh 1.53g. I'm betting these pistons are going to look interesting, too. I'll post pictures when I get back to work where that stuff is.

Edit: If your tap is removing metal shavings from the hole, that is not good. Read the whole post below, but the short answer is that you should only be removing dirt/burrs/minor thread galling with the right size tap.

As with most things, there is a series of tolerance ranges with threads - a cheap tap may be a little on the oversize and, as you have found, remove excess material - there should be NONE removed!

There is another problem, the thread form - shape of the cross-section - there are some slight variations in this (been caught before) and a 'generic' tap may not follow the same profile and remove excess material, worse, it may be a different shape!

That last may be VERY IMPORTANT! Some fasteners intended for high loads may have a shallower, flatter side that better supports the load, and a more steeply inclines side that is unloaded (what caught me out), a little like the 'buttress' - I believe some ARP fasteners use this asymetric design and, if so, this may be a case where doing the 'right thing' isn't ;-). YES, I have found several references to this asymmetric ARP form in connecting rod listings!

Rather than using standard plug taps, you should be able to find 'chasing' taps for your thread size - bearing in mind the ARP proviso - that are specifically designed to clean out, even reform damaged threads, without removing material! ARP do sell them, and I presume they are already profiled for thair thread form, if different from the conventional.

You do have a couple of options, short of that, that will come close to doing the same job - cut some slots/reliefs along the threads of a spare bolt and use that, or lightly relieve (blunt) the cutting edge of an old plug tap.

Because it was an internal thread, mostly in compression, in an area where there is less of a failure rate, you 'should' be fine to continue using the 'rods but, because the friction values on the load bearing faces may be different, I would take the bolts up to around half torque a few times to help burnish the mating surfaces, and use the stretch method rather than torque when tightening.

You can see some illustrations of what I mean by thread forms, etc, here -