Piston fitting in 70cc engine

Hi everyone,
Reaching last steps of my engine rebuild for my honda chaly, I was given a spare standard 70cc piston for the engine with new rings.
I did a test fit into the barrel and it is a very tight fit, it requires two fingers pushing the piston up and down, however this is with a dry barrell without any oil at all

Obviously the rings will expand with heat, I am scared they will seize. I tried test fitting a ring into the barrell and it has a 0.3-0.4mm gap, it is hard to measure.
Is this fine?
Am I overthinking it?

The piston is marked STD. and the rings came in a seperate packet, same brand, old school Japanese replacements still in original 80's packaging.
Just fit it up and go? or maybe the rings are oversized? Is there such a thing?

You can get different sizes of oversized rings, yes.
But sine it's marked STD I guess these are not.
The gap should be indeed very small.
The piston shouldn't be moved inside the cylinder without any sort of lubrication. This could damage the cylinder wall.

Some brands aren't interchangable.
For instance, fitting a Kitao piston in a Takegawa cylinder is a bad idea because of slight differences in tolerance.

If it's an OEM Honda cylinder and the correct 6v piston, I wouldn't bother.
Check however if the rings are mounted correctly along with the shaft and new closing rings. (circlips)
Just break in the engine correctly.

I was a bit worried how tight the piston felt but after measuring the ring gap I just carried on with the rebuild , I have had the engine running and just left it ticking over for a while but you will be able to read in an other post the problem that i have had with someone fitting a 50cc head to a 70cc barrel .

Waynus wrote:Just break in the engine correctly.

Ahhhahaa! And how do I do that correctly?

I bought a 2013 Honda CB125E a while back and I decided after reading on the internet to break it in by riding it hard for first 30km.

35km in and the engine sized up and self destructed. Apparently from manufactuaring swarf caught in an oiling hole or something.

I recently fitted a big bore kit to a Yamaha BeeWee and I broke that in by thrashing the balls out of it for 50km , it had great compression and power for a long time.

Der..................I think you've answered your own question !!!

All the best Si

My sugestion:
correct oil, oil level, ignition timing and cooling
low to mid revs until engine on temperature, do not let the bike idle to raise temperature, but easy on the throttle here!
do NOT stay at the same revs in ANY range
Open up throttle and don''t stop untill you reach the high revs (do not push to max, and don't stay at this high level for more than a couple of seconds)
Back up completely, run in mid rev range and re-accelerate
Don't put high loads on the bike while breaking in (uphill, passenger,...)
Don't apply full throttle from very low revs! (this puts a lot of stress on the engine)

If engine is overheating, stop, let it idle a few seconds, kill it, wait a couple of minutes and re-start.
If it happens again, drain oil and check oil pump.
Get an oil temp meter. You shouldn't go far above 110°C with most engines.

While it's important to treat the engine right, it's even more important to break it in troughout the whole rev range! Don't be a chicken but don't overdo it either!
Change oil (and filter if required) far sooner as recommended by the manufacturer
Listen to the engine noise while breaking in and get to know eachother
Make sure oil is applied to cylinder + head when building the engine.
The first moments after the initial start up, no oil will have reached these parts, and the damaged could already be done.
New engines come with running-in oil. Make sure to replace it soon enough.