Forum » Practical Engine Building » Balancing conrods-pistons / Tapping threads

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! :)

Thanks,

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.