Crossposted on Bobs, but I know there’;s a few big block experts here too…
I have a bunch of VC460s on our rental karts and we’ve popped four rods in the last two weeks. Not exactly sure if they just had too many miles on them or not. We keep RPMS under 5000, use about 1.5qts of 5w40 Rotella T6 oil, (Previously 1QT of 5w30, which we ran for a weekend or two this year and this was probably not enough oil). We let them warm up before they take a beating etc.
Most have the have stock GX390 cams in them as the decomp went out on most of the VC cams. Carb is the typcial 21mm Gx390 type “clone” carb with a 15mm intake restrictor plate too, so I wouldn’t regard their use as being very high stress… Yet they’re breaking a lot recently with the rods exiting the block on the top. We’re running the Arc rods too.
Now we’re at a point where we’ll have to start going through our boneyard of parts (About 16 engines) and start measuring to build engines from what we have. The first one I took apart was marked as “good” it had signs of piston head contact when I pulled it apart. Which leads me to my first question.
What should the squish gap (head\piston clearance) be in these engines?
Can I stack MLS gaskets (I have 0.020 ones on hand) to drop the compression and/or open that gap up a little?
I’ll circle back with photos of the rod failures see what people’s thoughts might be. My application is different than most, but still it seems odd they are coming apart like this. We bought the fleet used, so it’s hard to know exactly what the issues are… All I know is that I need to have four or more of them built by the weekend 