Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Ring gap or bore size overdone?

Practical Engine Building

Forum Posts



Tech Articles

Discussion and questions related to the course Practical Engine Building

= Resolved threads



I have been chasing a Crankcase pressure issue for a while now, and a leak down test showed me that 1 of my cylinders is leaking 40-50%. While testing this cylinder, I could feel and hear the majority of air coming through the oil fill hole. When pulling the dipstick during the test, 3/4 of the dipstick was covered in oil. I don't know if thats normal, but I think that's excessive blow-by through the rings pushing the oil up the dipstick. *Test was done on a cold engine, but cylinder 1 was the outlier in results.

The engine was built by me 40,000km ago.

Machinist had to go 0.5mm over on all the cylinders to accommodate for 1 of the cylinders being out of round. He also ground the rings to the recommended clearance. Machinist was provided with 0.5mm oversized pistons and new rings. I have caught the machinist on mistakes with main bearing sizes before, so I have trust issues with this machinist.

What I am wondering:

1. Is it more likely to be cause by a bore that is too large or because of the rings having a very large end gap? Is there any way I can test this without rebuilding?

2. I plan to band-aid this issue with a check valve (in parallel to my pcv valve) that only opens under crankcase pressure and releases excess pressure through a filter. My system can easily support that. Any thoughts on this?

Hindsight, I should have done a leak down test before installing the engine in the car.

Thanks in advance!


With 40k on it, it's unlikely to be a ring or bore machining error - unless it was like that from the start. A scored bore will have a similar affect.

You haven't provided any information on the actual engine, or the way it is being used - without pulling the engine down, I'd be thinking broken ring(s) or land(s), possibly from a poor tune. Back in the day, an initial check with the head off, would be to slip a thin feeler gauge down the side of the piston, as a broken ring would allow the gauge to go further down in some places than others. You can buy cheap borescopes that attach to 'phones, one of those down the spark plug hole might give more information if you don't want the head pulled yet.

There is a bit of a silver lining, though, at least you'll now have more knowledge for checking his/her work - although I would be looking for someone more competent, if (s)he has been making mistakes.

Hey Gord,

I was trying to reduce the amount of details to avoid a wall of text, sorry. It is a Mazda MZR DISI engine (4 cylinder turbo 2.3). It is purely daily driven, with occasional spirited driving (hard load, WOT). These engines are known to produce a ton of low end torque, its what makes them exciting.

I believe this problem has been there from the beginning, I just couldn't figure it out.

It is interesting that you mention the broken ring though, I was wondering what part of a bad tune might cause this problem? Detonation in the ring lands due to excessive fueling?

I did some research on this after you posted, and it looks like broken ring is the likely cause to my problem! Looks like I will be pulling my engine, just have to get my replacement engine ready first.

History with machinists:

I have a hard time finding a machinist I can trust, I am not looking for a deal, I just want to avoid headaches. I even paid a 3rd machinist just to take measurements of the main bearings to make sure there was no mistake.

I had to switch machinists from the head to the block because I found that a lot of these guys have their own opinion on what's "good enough" vs. what I requested. In this case, he machined the valve lash 3x over spec, and then replaced the valves with aftermarket without any consultation with me - good thing I was able to check his work and ask the right questions... But i guess he isn't familiar outside of the Subaru world.

I think we have all had problems with bad machinists - back in the last ice age the small town I did my 'prenticeship in had two "engine machinists", with a third garage that only did their own work. Both the locals were ****ing terrible and I once drove 300 miles (1200 with the pick-up) to a 'race machinist' to get a block bored with a deck plate (they were a new thing back then) - they were even more incompetent and got the centres so far out the block was boat anchor material... To add insult to injury, it was a 4 bolt, which was rather rare and expensive down this part of the world.

Canada is a big place, and if you're having problems finding a decent machinist you may need to look at the 'States to get it done - it'll be more expensive in the short term, but still cheaper than having it done incorrectly. Oh, I've been watching this Canadian chap's channel for some years, he seems a friendly enough chap https://www.youtube.com/channel/UCKxu_85y8YGFQcY11cJGNtA with an active interaction with fans, so you could try writing and see if he knows anyone reliable that is local to you?

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?