Feel of compression after break in
 

Hi folks,

Got a question
After breaking in an engine am I suppose to eliminate the feel of piston rubbing the sleeve when I turn the flywheel?

I used an entire gallon breaking in with engine peaking ( for the last 4 tanks) with running temperatures around 200 but I'm still getting the metal to metal feel. Did I just screw my engine up?

Nope, that is natural. Some brand of engines will take 1 gallon of fuel after break-in before they are fully broken in, others break in much faster. You will know when an engine is fully broken in when it suddenly goes rich on you in the middle of a race:D

Haha that sounds frustrating.

Thanks for the reply

It is wrong to think you must not feel any pinch in cold condition. The right fit of the piston and sleeve is needed at operating temperature and due expansion of both parts that will be different compared to room temperature.

Well I thought I should feel a slight pinch. but one guy on the track showed me his engine (newly broken in) when turning the flywheel (cold) i could feel a somewhat strong compression, without any
metal parts touching.

so i was a lil confused. Im always confused :D

One other thing, Is it normal to see fine scratches on the piston?

Even the feel between 2 same engines can be different due slight differences in the alloy of the piston and other tolerances.

And scratches are not good but hardly avoidable. A tight pinch during the break in wit normal racing fuel can cause this but also small dust particles comming through the air filter.

It's weird. I am to ALWAYS find lil specs of dirt/dust on all my engines previously. And im sure I clean my air filters thoroughly enough. And I oil them. Does anyone face something similar?

Did u check it when the engine was hot ?

Tip I was given was immediately after running on the bench remove plug and feel for pinch. If the piston pinches on the way up and down (ie double pinch) it needs more on the bench, if it's a single pinch at TDC get it on track. As someone else mentioned, you feel it want to go and richens up when its ready. Heat up to 80-100c prior to starting and don't run WOT with a rich top, you'll hurt your crank, rod and rear bearing.

These is what I do since I got this answer from master tuner Dennis Richey several years ago. Hope it helps as it has been to me.

AFM

Don’t confuse compression with pinch; Sleeve Pinch is not that important contrary to popular belief. The question is how well it seals as soon as the piston starts to close the exhaust port and if it holds compression after you roll it to top dead center.
Place a few drops of after run oil trough the top (removing plug), then place plug in place, then roll piston to top dead center and listen and look for air bubbles through exhaust port. If there’s none, then your engine still has compression, even if it has no pinch.
The fastest team motors we use, have zero pinch. I even had them run to over 140ºC and they have survived well. Though I certainly don't recommend it.
Dennis Richey
Richey Racing Engines

Again that is a cold test which can be total different compared when the engine is at operating temp.

Yes it is a cold test, but it's worked for me on every engine i've owned.

I understand his reasoning(and Yes, I know who dennis richey is, wonder what happened to him), but the problem with that test is that when the engine heats up all that changes. Whats important is how it seals at running temperature, as both the sleeve an piston expand.

This is good to clear up some confusion (for me at least)
What I'm feeling when the engine is cold is the pinch? But the real compression test is actually when the engine's heated up till operating temp am I correct?

Yes. But do not heat up an engine with a heater to get that feel because when running the piston does become much hotter than the 100 degree celcius.

The best moment is after running on the track to stop the engine by the flywheel and directly turn it arround

Cool. Thanks for the reply. Will try that

Do we still need compression on racing? Id prefer using engine which had 1 gallon running, than new breaked-in engine for racing..