Advice on an air cooled single top end failure.

[luke22]

Hi,

hope it's ok to ask this here, it is about a bike engine but really it's a technical issue and I could do with some expert imput.

Friend had a 400cc air cooled engine fail recently, it is reasonably new 2018 with 30K miles on it and EFI fueling. The exhaust valve head broke off and totalled the head and piston after a long run with the engine making a noise, he had to keep going as there was nowhere to stop in foreign savanna. What we are trying to figure is the failure reason and I have a couple of questions really but will first describe the engine and what was found.
So long stroke 400cc (78mm bore x 86mm stroke) 24Hp modern engine. Noise appears to have been from a head gasket leak that was big but he couldn't place the noise at the time, failed gasket was under the exhaust exit and deep in the fins. The 2 valve head is OHC with rockers with rollers following the cam, one of the rollers, exhaust, had failed and all its needle bearings were still in the head. The roller had stayed in place and had lots of flats worn on it and the cam lobe had considerable wear into the sides of the lobes. Everything else was fine up top.

So firstly we are not sure if the gasket failure led to overheating which seized the rocker roller and then the valve had loads of excess lash and broke.
Or if the roller had started to fail a long time ago and had nothing to do with the valve and gasket leak.

So first question is would you expect to see a slow revving 2 valve lose a valve head if there was excess (should be 0.25mm lossened to maybe 2mm over a few days) valve lash? Engine was revving at maybe 4 -5K rpm for over 500 miles.
Second is to do with the EFI. The bikes in question run quite lean to pass emmissions and are known to have high head temperatures, the previous carb versions ran between 30 and 40 degrees cooler. A lot of them have surge issues and have starting and tick over issues till they are richened up a bit. I understand that during steady throttle openings the EFI will go into closed loop and read the lambda exhaust sensor. Would a gasket leak give a rich reading and potentially cause the EFI to have run even leaner? Causing excess heat to duff the rocker and valve. Or would that cool the head?
Third is does a gasket leak without the EFI being involved cause higher cylinder head temperatures?

Don't think we will ever be 100% about what happened but want to look into what are the weaknesses of the engine to basically know how to make them more reliable and possibly tune-able, 24hp is pretty low!

I have never seen a valve head break on an air cooled low rev single, only ever from over revving high rpm engines bouncing valves. Seen bent valves but never a broken head. My guess is the valves are a little soft, the roller was just unlucky and we have heard of a few head gaskets failing so probably should include retorques as a good maintenance precaution.

Any thoughts greatfully received.

[lowscorts]

I think the first question is what bike is it?, many small capacity bikes are made in the far east and the qc is terrible

[luke22]

It's a royal enfield LS410 engine, very basic 80's design rather than their older 50's design era engines.
If you look online there is a lot of bad press from the indian sold machines but they are made in a different factory to the uk and us machines. Hitchcocks and owners describe them a reliable and with few problems over 30 to 50K miles. But there are some random failures and one of them is exhaust valves, some dish at the stem top, some have lost heads. Not many considering the numbers sold but enough to question if the QC/tempering on the valves is very good.
Would you expect a valve to lose its head like described?

[lowscorts]

yes, but its a bad copy made in india, i've recently done a new 650 for a customer, big bore, cam, mapping etc, nothings round or square in the engine, needing quite alot of rectification of the poor manufacturing.
The older singles were just as bad and nothings a surprise with random failures as they're just a poor copy of the originals

[luke22]

That's surprising about the 650's I thought they were a new designed engine. They definitely are budget machines when you look at the price of them.
The LS410 engine is not one of their copies, it was developed in 2016? with Ricardo (apparently) and the frame with Harris to make the Himalayan. Must be one the best value adventure bikes if you are going fully loaded where there are no roads, decent fuel, garages etc
They put up with a hell of an abuse carrying stupid touring weights and can be fixed with a pair of pliers and a hammer 100 miles from anywhere! There just isn't any jap or euro machines that do all that, unless you buy an old 80's thumper, they are either too tall or too expensive or too delicate. They are made better though.
So I've not heard of a single lower engine failure with the LS410, apart from burnt out clutches and the odd rattly oil pump chain, they seem to be bomb proof. Still not convinced there wasn't a QC problem with the valve that broke, I suspect that you are right that the odd random top end failure is down to QC/metallurgy issues.
So question is would you expect a well made valve to break because of too much valve lash?

[lowscorts]

Unlikely on a lazy low powered single, the 650 had 3 high spots on each valve seat from where they'd been pressed in unevenly from factory, regardless of who designed the engine if its machined and assembled badly you'll always have issues

[Graham]

huge valve lash wil massively strain a valve, cams have closing flanks that gently close teh valve against the seat, with a big lash the closing ramp is missed and teh valve crashes hard into teh seat, this is what actually causes what we call tappet noise. 500 miles with huge lash on an aircooled engine, id almost be suprised if the valve didnt fail!

[luke22]

So valve would have been getting hammered as the roller on the rocker failed, from the notes he took he checked valves at 500 miles pre failure and the next day and all was good. The sound he heard at 500 miles was probably the head gasket leaking, a blat-blat sound. He noted a tinkling sound from the top end at about 200 miles pre failure which was maybe the roller bearing needles going awol, so increasing valve lash, from 200 ish or more miles.
Another question would be does a head gasket leak on an air cooled make them overheat?
Trying to figure if the gasket and rocker roller failures would have been connected, or if there was just 2 different problems at the same time. If the engine was overheating then could imagine the top end being fried and bearings failing or valves starting to stick or something.

[lowscorts]

It's a badly assembled bike, made with poor quality parts, I wouldn't waste too much time analysing it personally