Holy low compression batman

[cleverusername]

In an attempt to find the root of the Sprints over flowing expansion bottle problem, I did compression test.

1:120 2:120 3:115 4:120

Then I retorqued the head, the straight head nuts were loose in the centre. Retested giving:

1:120 2:120 3:120 4:120

So low but consistently low. So have I got a blown head gasket? I suspect the low compression could be down to bore wear, this is my first engine rebuild, so I fitted new rings and standard pistons without measuring the ring gap. Trusting my luck, the gamble doesn't seem to have paid off.

[marshman]

Nothing wrong with fitting new rings but did you hone the bores? If fitting new rings - either in old pistons or new rings and pistons - you should always give the bores a light hone. Crudely speaking the hone breaks up the smooth possibly glazed cylinder surface and allows the new rings to bed in. Did you check the bores for wear?

Also how long has the engine run since rebuild. If only for a few minutes the rings will not have bedded in, you really need to run it for a good few miles with the engine under load to bed the rings in and get a decent seal.

You can rule out (or in) the cause of low compression by putting a bit of oil down the bores and the testing, the oil will temporarily help form a seal and if it is the bores the compression numbers will come up.

I would be inclined to drive it for a few miles and see if it beds in and improves - you have nothing to lose, if it improves all is good, if not the remedy is the same, strip it down and find the cause. Just keep and eye on engine temp and coolant levels in case it is the head gasket.

Roger

[cleverusername]

I honed all the bores before fitting. As for running it, it is sorn, so it has barely been run.

[jikovron]

One note regarding rings is that the gap is ultra crucial, I've found aftermarket rings have all needed gaps opened up to spec even on old but good condition cylinder bores but especially on rebored blocks where they couldn't be fitted till the end gap was opened up enough to go in for checking.

I unwittingly didnt check on a 1500 engine 12 years ago assuming the gap would be to spec on a +20 block that had seen miles but it broke up its top ring fairly swiftly.
You might be ok with yours but it's worth a thought.

[Carledo]

I think the compressions you have are OK for the state the engine is currently in. It's just been rebuilt, has done minimal hours run time on freshly honed bores. So it's not yet had a chance to replace the coke lips above the top ring in the bores that the hone will have wiped away. Put a few hundred miles on it and check again! I wouldn't be concerned unless the compressions were wildly different, or one or more was less than 100psi.

Steve