Lapping four stroke valve seats

Hi, I'm trying to sort out a SC70 four stroke. It was bought second hand in a model, and didn't look like it has done much running.

After treating the engine to new bearings, I bench ran the engine inverted, as it will be installed inverted in a model.

The engine would not peak to high revs, and this was traced to a bad / weak exhaust valve spring, as you could hear a weird 'pop' sound as the engine turned over. I ordered two new genuine OS70FS Valve springs, which are much stronger.

Before reassembling the engine, I lapped both valves. Mounted the valve in the lathe and cynoed the valve to a wooden dowl held in the tail stock, so the dowl is central to the valve. I used a bit of dimond lapping past to the valve seat and gently spun the valve via the dowl at low revs with a drill. After ultrasonic cleaning, reassembled, and I'm finding both valves hiss / leak quite a bit.

Is it possible to get a near airtight valve seal, or am I wasting my time?

Thanks in advance, Mike

You probably over did it, On model valve sizes only the gentlest hand lapping is needed if any.

Good chance your head seats and valves are fubar.

Engine Doctor may be along in a bit with more info.

Have found with a number of ASP/SC (Sanye) four strokes, having spent some time trying to seal the valves, that the hiss is caused by the bronze seat / valve guide not sealing the head. That is to say the valves are a good seal on the bronze seat but the gases leak between the seat and the head.

Have seen it on quite a few brand new engines too.

Edited By Martin Arnold 1 on 26/11/2020 16:51:42

Thanks for your replys. I understand these are more of a budget engine, and quality may vary.

The lapping wasn't done for long. With a jewlers high magnifying glass, there is no 'step' worn into the side of the valve stem trumpet edge, where it meets the bronze valve seat.

I've read reports of ASP heads being porous.

I expect it is possible to get an airtight seal, but you might need to ask Laser for a replacement engine...

I think you will find Martin was spot on with his reply. I bought an SC 70 fs on ebay. To all appearances it was a nice engine. I know that ASP and SC four strokes do not seem to have high compression, but mine had very little. I took it down and checked every thing. Fitted a new piston ring , re seated the valves which made no difference. I bit the bullet and bought a new cylinder head with valves fitted from just engines. It transformed the engine and really good compression returned. It now runs and idles as it should.

ASP/SC are known for pretty rubbish valve sealing. I repaired an ASP30 for someone and in the end had to replace the head as i was unable to get the valves to seal. The new head worked fine so that was that.

ED reckons its distortion in the head when its all clamped down, and he may well be right as it would explain why they seal ok on the bench under vacuum but are useless when the head is bolted down.

On my SC400 radial the valve seal was hopeless so i attacked it with the little diamond tool i have here at work. The seat had a very sharp corner on it from the factory so i gave it a bit more of a 45 degree and it worked a treat with compression on all 5 cylinders now and not 3.5 cylinders like before. I also gave it twin ring pistons too so that might have helped!

I think this may well be why I'm having seating troubles, as the head could be warped. Looking at the bronze seat in the head, with a magnifying glass, you can see the tool marks from manufacturer with a sharp edge. So it's not offering much seating face.

Thanks for everyone's response.

Hi Glow engine . The valve seating doesnt need to be very wide . A gentle grind with the fine grinding past thinned with a tiny drop of paraffin cuts quite well. I use a piece of neoprene tuning pushed over the valve stem to grind the valves as follows , Apply a little paste on valve , Insert valve into head then push a piece of neoprene or silicone tubing ' a tight fit onto the valve stem . Now twist the tubing back and forth between thumb and finger while pulling the valve gently into the head. Occasionally lift the valve from the seat and reposition it and you will find the grit starts to cut again . Don't spin the valve with drill or similar as this wont make a good seal. All you need is a matt ring around the valve head and the seating. Thoroughly clean and reassemble and test seating with some paraffin or thinners poured into the port .Any leaks will soon become apparent If there are any leaks then repeat the grinding process . If you get a good seal and it leaks when fitted to the engine then it will be either the head distorting or the inserts letting by the casting. No cure for these problems except to fit another head. If compression can be felt when turning engine over it will probably run OK but if compression is very low or almost non existent then engine will probably run but wont idle well or give a good top end.

Fitting a new head can fix it but there is no guarantee that the new head will be any better. SC, and ASP are prone to this as they are budget engines and have varying QC . Other makes have better quality control and materials so don't tend to have the problem .

Hi Engine Doctor. Thanks for your detailed reply. I will have another look at lapping the valves / seat with your method.

I'm residing to the fact I may never achieve a perfect seal, in which case I will call it a day on this engine.

Thanks for everyone's reply.

Hi Glow Engine, I don't expect you will get a perfect seal but it is still worth test running as even with some leakage they can still run well enough to use. They only really become difficult when the compression is very low . Provided you can feel resistance/compression when turning over by hand they will run well enough to use but idle and top end will be affected . Good Luck

If Comp is really bad after re-build then it can make a good cut-a-way or desk toy or look out for a crashed engine with a broken crankcase that still has compression .

E.D..