Engine rebuild after EFATO

After landing I noticed a lot of oil coming out of the exhaust pipe which had coated the lower fuselage and tailplane.

Off with the top cowling and a removal of a spark plug from each cylinder with my friend turning over the prop it was obvious that there was no compression on Cylinder 1. Which is the front most SB cylinder.

Next step was to remove the head from Cylinder 1 which immediately revealed the cause of the problem. The piston was cracked in half!

I next used a borescope and after removing the sump was able to look at the rear of that piston to see if any big chunks had come off - luckily none were visible - just a hairline crack across the back of the piston - same as the front. Also the piston webs that are behind the gudgeon pin were still intact which had kept everything together.

So thankfully no further damage was done to the engine. That broken piston was all that was damaged.

Piston cracked in half on Cylinder 1.

These pistons were originals from new which was 2013. With approx 650 hours on them. They are made in the USA for Chrysler car engines and are of a cast type. 

Since 2022 UL Power do not sell an engine with these pistons. Having upgraded to a forged type. 

I had thought I should upgrade my pistons some years ago and this failure now gave the perfect excuse to do so. 

Rather than just upgrade one cylinder I decided to treat this as a top end overhaul and bought new forged pistons and barrels for all 4 cylinders.

Upon further investigation I found a little corrosion in Cylinder 4 bore and some wear in the other cylinders so I'm glad I made this decision as it has future proofed the engine somewhat.

As well as the pistons and barrels there were quite a few small items that were needed as well as some specialist Loctite products and tools. All in it cost me £3k in parts and tools as I did the whole job myself labour was not a factor.

While I had the top end apart I took the opportunity to clean up the heads and also found some pitting on all of the exhaust valves and some of the inlet valves - no doubt caused by my forced usage of AvGas 100LL at various stages, although I do try to minimise the use of it as best I can, usually running on UL91.

So I ground all the valves too.

Pitting on the exhaust valves - removed by grinding them using paste.

Valve seating face looking much nicer after grinding.

Sadly UL Power would not offer any kind of discount on the parts - despite them using me as a sales tool in their marketing for the last 9 years.

Also disappointing was the arrival of the 'new' pistons which were all damaged in some way or another - most likely cause was a poor job of packing them in the box and maybe rough handling by couriers.

Another set was sent and some of these had damage too. After a 3rd delivery I was finally able to start putting my engine back together.

'New' forged piston - damaged in transit

Slowly but surely I reassembled my engine - using a step by step guide kindly supplied by UL Power agent Metal Seagulls. Although I did find an error in the document - ring placement was switched! I was able to confirm the error before proceeding thankfully.

Some specialist tools are required but it is a fairly straight forward job really.

Engine going back together with new barrels and upgraded forged pistons

If you are thinking of upgrading yourself and want to borrow the specialist tools then let me know - I'm happy to share them so long as they are returned.

Piston half (cut across the gudgeon webs with a hacksaw) showing the fracture across the entire top portion.

No clear cause for the failure was established. Obviously the cast type is inferior to a forged type and I since the incident I have had several other UL Power users tell me they have had exactly the same failure.

The fact that UL don't sell an engine with these old cast pistons anymore tells you all you need to know really.

In the higher compression 'i' versions of UL engines I would not want to fly behind one with cast pistons.

When everything was back together I did a couple of ground runs to prove everything was ok and nothing was leaking.

After my inspector signed off the work I set about flying off the 5 hours of run in time.

To date (Jan 2026 - 8 months later) and with approx 56 hours on it the engine seems to be behaving as it should.