Hitchcocks Forum

Today when riding home from work the power seemed down (hasn't felt right for a few days) and the vibration was higher than normal. So i dug out my compression tester. I was only getting 45psi. I repeated this a few times. No more than 45psi. After squirting some oil down the spark plug hole and re-doing the compression test i was getting 105psi. I guess i need a new piston and a re-bore? Advice please. I have previously been posting reference spark plug failure, maybe this was related?

Ok now you have compression back Mike give it a run Enfields can loose compression with sticky rings, got nothing to loose and see how it goes

Check the decompressor. Simple things first.

If a squirt of oil down the plug-hole around the piston gives good compression, then the top end (valves and decompressor) must be good. I have given the bike a run today but it doesn't feel good. I'll check the compression again and see what it says.

Well i've just checked the compression again and it's still around 40 - 45 psi. The engine has done 36000km, what sort of mileage is usual for a piston and re-bore?

I would bite the bullet (no pun intended)and whip the top end off, it's not a massive job. Clearly something is a miss with the rings or bore as compression is poor and it is restored with oil. It could be sticking rings, but it is more likely to be damaged or worn rings,has it got hot and nipped up?
Even if it needs a re-bore you may be able to get away with a new set of rings.

It's a pull it apart job I'd say. Low compression generally doesn't turn up without cause. It's only a thirty minute job to get the head off and maybe an hour to get it all back happily together again.

Well i've just got the head and barrel off. The piston is heavily scored in about 6 places, like someone's taken a file to it. The barrel looks smooth enough but must be worn. My concern now is how much sideways play/rock is acceptable for the con-rod? I'm no expert but i don't think it should be much.

Mike, I asked exactly the same question several months ago, the first time I saw the inside of an Enfield engine. A couple of millimetres of side to side play is probably OK, but there shouldn't be any vertical play.

I guess the real question is why the piston is scored. With mine, one of the gudgeon pin circlips had come adrift and was loose inside the cylinder, but that was pretty obvious. If it does come to needing a new piston/rings/barrel, I found by taking it steady it wasn't a difficult job.

I found your thread. I'm getting about 3mm side to side rock which seems far too much, but as there is no up and down play, and based on the comments in your thread that must be OK. So i guess i do need a piston and re-bore. Any recommendations on which piston?