Cylinder head small leak fix and big bore. - 912ULS

[cirrusjetpilot]

So, I notice during my last oil change that cylinders 2 and 4 have a very small oil leak. I looked everywhere to confirm it was actually coming from the head and it seems true.

Speaking with my mechanic, he told me about the lapping procedure which entitles of shaving a very small wall and reseal back.

I have been considering the big bore conversion and wondering if this would be the right time to do it? I believe the big bore entitles of completely replacing the cylinders and head correct?

Thoughts on this lapping procedure invasiveness vs biting the bullet for the big bore?

Thanks
Alex Di Sessa
N171KK
#REDFOX

[mr bill]

If you have a 912ULS, you use the heads as is. If you have a 912UL, the heads need to be bored out to accommodate the larger cylinders. Either way, it's a chance to clean the heads and valves, especially if you have been using 100LL.

[Av8r3400]

The big bore kit replaces the cylinders and pistons. The heads are reused as is. I'd recommend lapping the head and cylinder mating surfaces anyway big bore or not. It isn't a difficult task.

From the factory this mating joint has no gasket, o-ring or sealant at all. The big bore kits use a small amount of sealant there.

As a suggestion, re-torqueing the heads may solve the leak issue.

[cirrusjetpilot]

The big bore kit replaces the cylinders and pistons. The heads are reused as is. I'd recommend lapping the head and cylinder mating surfaces anyway big bore or not. It isn't a difficult task.

From the factory this mating joint has no gasket, o-ring or sealant at all. The big bore kits use a small amount of sealant there.

As a suggestion, re-torqueing the heads may solve the leak issue.

Yes, thanks for that re-torqueing suggestion. We will try that first and fly few hours and see the results.

Alex

[mr bill]

Rotax SI-912-025 gives the correct torque sequence for installing the heads.

[HighWing]

If you have a 912ULS, you use the heads as is. If you have a 912UL, the heads need to be bored out to accommodate the larger cylinders. Either way, it's a chance to clean the heads and valves, especially if you have been using 100LL.

Not sure this is accurate. I watched Hal install the 105HP zipper on a local guys 912UL and it was simply remove and replace. In his hands, it took a couple of hours. And as mentioned, the only parts in the kit are the cylinders and pistons.

A couple of pictures showing the differences between factory and Zipper components.

[mr bill]

Just talked to Hal Stockman. The bigbore kit will fit the cylinder heads of a 912 UL, but it is a rather snug fit. To provide more clearance, he recommends turning the outside diameter of the cylinder, rather than re-bore the cylinder head.

[Av8r3400]

I never heard from Hal about any fitment issue. My big bore kit went together on my 912UL without any issue of fit between the cylinder and the head.

I did purchase the kit in 2015, so there may have been changes made to the cylinder design in the mean time. I guess it is possible that Hal knew I was putting this on a 912UL and machined the cylinders differently?

[mr bill]

Sorry for the confusion, installing ULS cylinders on a UL requires boring the cylinder heads.

[WWhunter]

I believe Ronnie Smith of Mississippi Sport Aircraft, also makes a big bore. Maybe this is the one that the poster is referring to. Just an idea.