E: Engine Decompressors

[Chris Launders]

In another thread (Shop built roller starter) chainsaw decompressors were mentioned and as my engine has spare plug holes I thought I would try them, the cheapest I found were £4.09 each so I ordered a pair, the thread is M10 x 1mm and the old plugs I have are just under 9mm bore when the insulator was removed.
So I cut a pair down, drilled 9mm and tapped I soon had a them ready. Dragged the bike out and started it just to make sure it was running (not used her for a month) popped the decompressors in and gave her a swift kick, started no problem with much less effort (I normally don't use the valve lifters being overweight and it starts first kick), repeated several times and started every time.
So I shall be keeping them in, eventually, but I need to make new adaptors as cutting plugs down removes the hexagon to tighten them in properly.
I shall make some from hex brass bar and not take the 10mm hole all the way through but the last bit at the cylinder end I will drill smaller than the decompressor valve head, this is in case it ever fails nothing can drop into the cylinder.
A very useful little mod and one would be a benefit on a couple of different engines I have.

Chris

 

[greg brillus]

You can buy them with 14 mm threads although they are harder to find. The ones I used in the racer on the left picture worked very well when starting on rollers.

 

[bmetcalf]

Here's an adapter, although the page doesn't say what the female thread is.

https://shop.circlecycle.net/Alloy-Art-Compression-Release-Adapter-detail.htm?productId=23101069

 

[Bill Thomas]

I paid £ 120 ish each for one for me and one for Ron, From USA ,
The seller said the cheaper ones can fall apart !,
But I guess he would say that !,
But Chris has made his so it can't ?,
But mine go straight on, 14mm and I think they call it 3/4 length.
I was going to do like Chris, Then thought it was worth having a spend up !.

 

[Chris Launders]

Bill, I was going to get branded ones (like genuine Husqvarna) but thought I would get the cheapest first just literally to try them, then thought why not just build in a safety precaution. But I will probably buy a pair of branded ones anyway as they're not that expensive.
This is what I have now, as you can see there is only the small hex on the decompressor itself to tighten it in/get it out, which is why I will make proper adaptors.

 

[Peter Holmes]

If the decompressor separates into its two component parts whilst unscrewing it out of the cylinder head how would you remove the main threaded body from the cylinder head?

 

[Bill Thomas]

I was worried we couldn't kick it over quick
Enough, But it was fine.

 

[Chris Launders]

Peter, That's why I'm going to make proper adaptors out of hex bar, these were just to try the decompressors, they only needed to be just over finger tight for the trial.

 

[Sakura]

I tried several, including Venhill, but couldn't keep them oil tight

 

[Marcus Bowden]

An "easy out" would do it so long as you have a 10 mm tap to retap the damaged thread.