Just another opinion, don't get offended...

I usually measure crank pins in inches but here is my take, I rebuild hundreds of engines a year.

Most crank pins are .196-.1965" new (4.98-4.99mm)
I consider only running as back up engine at .194-.193" (4.90+mm)

I also consider rod slop, if the rod is worn this often doubles the slop.

Rex

Thanks Rex,

I was under the impression from GO that a new 21-0600 con rod big end bushing was 5.000mm and a new crank pin was 4.997mm - giving a brand new factory tolerance of .003
How correct that is I don't know, but it came from the horses mouth at the time, so pretty reliable info.

So on a set of basic verniers you would likely see a reading of 5mm for a new crank pin - not 4.95mm

Like you I've measured a fair few new and used crank pins, and to the best of my knowledge all GO cranks from the GXR Series going forward have a 4.997mm pin to match the 5.0mm bushing on the 21-0600 Pro Series con rod used in all recent models (GXR Series, GXII-5RHO, UDP, Champion and GXII Plus).

I like to err on the side of caution with the forces involved inside these little motors spinning at up to 40,000 rpm.
If there is slop at those revs on a single cylinder motor, something is going to give sooner rather than later

The conrods measure 5.00 when new. The crank pins of various brands usually measure 4.95 boardering on 4.96..
As for "factory tolerance of .003", not even Novarossi or OS build their engines to that tolerance, the brands all build crankpin/rod to about 0.03~0.05mm smaller than the rod, every single one I've measured.
If the "factory tolerance" was .003mm they'd not be able to assemble the engines.

It's not realistic to have to discard a crankshaft after 0.025mm of wear as you suggested.

My grand prix went 280,000kms before i traded it and i never changed a water pump or the timing chain/dampers ect. Sometimes you get lucky. Other times you maintain your stuff and it lasts forever. But theres nothing to maintain on a nitro engine. Its not like you have to keep up with oil changes and topping up coolant, your basically gambling with your money every time you fire up a nitro engine. All you can do is keep your air filters changed and oiled properly and hope the guy who built your motor wasnt still hung over when he got to work on monday. Id like to think you get what you pay for but that doesnt seem to be the case all the time.
I realize the forces at work in these motors but theres gotta be something companies can do to boost reliability a bit.

Top fuel engines are a little different than rc engine too btw. A top fuel engine makes 8000+ hp running 98% or more nitro. We run 20-30 for the most part and the methanol stabilizes it. Im not sure you could even get 98-100% nitro to light in our engines. Top fuel engines run twin magneto ignitions with sparkplugs. Although by the 60' mark the plug is burnt out and the motor is dieseling the same way our engines do, but it takes spark to initally fire the motor up. Those motors also come close to 20,000rpm near the end of a run with a ton more stroke, even in scale, than an rc engine. I would say in comparison a top fuel motor has way higher forces to deal with. Also, the engine internals get tested and re-used. Not that weekend obviously but you dont throw away a $3-4000 set of conrods after one use. Bearings yes, but bearings arent that expensive. No reason to toss something that can be rebuilt good as new with a $10 bearing.

Our engines should have replacable bearings for the conrod. That would make things a bit cheaper. Youd have to press them in for sure but a tool could be made so that anyone could replace thier own. I dunno, id just like to run a motor for a few gallons and not have to rebuildit twice. Call me crazy lol

The Gm 3.8 v6 is awesome, we have 3 around 230-260k, lasts longer than the car usually.

Top Fuel turn less than 10,000 and turn less than 900 revolutions on 1000 ft pass.

Some .21 engines I have modified run on 50-60% nitro.

I have messed around making my own bushings for rods and even replaced a pin on a crank. I have also put 2 small ball bearings in the rod. (couldn't find needle bearings) None of these lasted very long, the forces on these little engines is much greater than you think.

As far as maintaining a nitro engine, I recommend changing the front bearing after 2 gallons or so with almost any brand. At 4-6 gallons it may need a pinch and a rod and maybe a rear bearing. This is not really expensive in most brands, especially Go/Argus/ Alpha where rods are around $20.00.

Rex

What is a good bearing for replacing the bearings in my GO Engine (GX5R) its only had about 10-12lts through it & the rod was changed after break in, well, after about 4-5 lts anyway.
It still goes great but would it be worth changing them to something better, like a rear ceramic & one of them double sealed Nova bearing if they fit.

Incorrect. If you can find an incar view of a top fuel rail dragster. Yoy can watch the tach go up over 13,500 at the end of the track. Your right about total rpms for a run tho. Its like 1400 total including burnout

Incorrect. If you can find an incar view of a top fuel rail dragster. Yoy can watch the tach go up over 13,500 at the end of the track. Your right about total rpms for a run tho. Its like 1400 total including burnout

Holy double posting batman!!

Whats the best rear bearing for nitro engines? Is there one company that got it right moreso than others? Is there anything that can be done to help prolong the bearing? I find most of them ive had leak but they still roll and function great. What to do?

The rear bearing is the larger bearing, the front bearing is the one with seals. There are a few good ones, I sell Avid and they last very well for the $ (both front and rear). The 17011 Novarossi is good and it has 2 seals on the outside. I also think Os front bearings are good, you will pay a lot for the Nova and Os.

If you run in dirty conditions the Buku cap is great and the bearings should last longer

Rex

Ohhhh ok. I always thought the rear was the outside bearing. What do the numbers of the bearings represent? Inside diameter, outside diameter and thickness?

yes, the Go uses a 14x25.4x6mm

Rex

Any novaS pipe combo works with go engine, any opinion?

So i picked up 2 GX7Rs today. One with a Jammin pipe, the other with a Caster pipe. Both have new front bearings and conrods done. TONS of pinch left. Got them with a Caster ZX1.5r pro kit roller and a rather large grocery bag full of spare parts including at least half a dozen sets of shocks. 4-5 sets of different springs 5-6 sets of a-arms, rear hubs, front steering blocks, servo savers etc etc etc. Probably almost everything to build another buggy minus the chassis. Got everything for $200. Im fairly happy. Havent made it home yet to look through everything but ive seen some of what i have at the lhs and if i paid full price for everything i got today id be into thousands. Smokin deal i couldnt say no to. Just the motors alone ive seen for $300+ and these have the bearings and conrods done already.
Im a happy guy today lol