23 replies to this topic

[cal844]

Repeat the test with wide open throttle and with the spark plugs out.

You'll also want to do it wet and dry.

Youre Only testing the strenght of the battery and starter that way. Absolutely pointless.

Actually no, I have always done it this way, readings are the same as with the plugs in.

Theres many ways to do it, that's the way I was taught and how I've done it on every engine I've worked on, not just minis....

Some reading....

https://www.cartechb...mpressionteting

Edited by cal844, 23 May 2020 - 01:46 PM.

[Dusky]

Repeat the test with wide open throttle and with the spark plugs out.

You'll also want to do it wet and dry.

Youre Only testing the strenght of the battery and starter that way. Absolutely pointless.

Actually no, I have always done it this way, readings are the same as with the plugs in.

Theres many ways to do it, that's the way I was taught and how I've done it on every engine I've worked on, not just minis....

Some reading....

https://www.cartechb...mpressionteting

Good for you. The OP already established even compression and no big rise on the wet test.To test his rings he now needs a leak down test.

While at it, what would be considered too high compression on a mini, what's the factory quoted spec for a compression test? ?

Edited by Dusky, 23 May 2020 - 01:51 PM.

[Cooperman]

A few points from the above if I may.

 

1.  The compression test MUST be done with all the plugs out, the engine fully warm and the throttle fully open.

2.  The valve stem seals may be the issue, but it is unlikely as new inlet seals were fitted. The lack of exhaust valve stem seals will not cause ingestion of oil on low throttle settings into the chambers.

3.  How many miles has the engine done since re-build and was it rebored/new pistons? A newly built engine often smokes a bit until fully bedded-in.

4.  The last time I had experience with a smoking, oil-butning engine which had allegedly been rebuilt by a professional engine company, it turned out the bores were, on average, 0.003" oversize. As it was bored to 1380, it cost the owner a new block and another lot of re-boring. The so-called 'expert builder' didn't want to know and he had been paid in cash with no receipt (!), so no come-back.

[cal844]

Repeat the test with wide open throttle and with the spark plugs out.

You'll also want to do it wet and dry.

Youre Only testing the strenght of the battery and starter that way. Absolutely pointless.

Actually no, I have always done it this way, readings are the same as with the plugs in.

Theres many ways to do it, that's the way I was taught and how I've done it on every engine I've worked on, not just minis....

Some reading....

https://www.cartechb...mpressionteting

Good for you. The OP already established even compression and no big rise on the wet test.To test his rings he now needs a leak down test.

While at it, what would be considered too high compression on a mini, what's the factory quoted spec for a compression test? ?

Whilst no big rise in reading the test was performed incorrectly. Simple as that. Imo the OP should repeat the compression test then as you say leak down test.

Id expect around 200psi on a new built engine

[Dusky]

Repeat the test with wide open throttle and with the spark plugs out.

You'll also want to do it wet and dry.

Youre Only testing the strenght of the battery and starter that way. Absolutely pointless.

Actually no, I have always done it this way, readings are the same as with the plugs in.

Theres many ways to do it, that's the way I was taught and how I've done it on every engine I've worked on, not just minis....

Some reading....

https://www.cartechb...mpressionteting

Good for you. The OP already established even compression and no big rise on the wet test.To test his rings he now needs a leak down test.

While at it, what would be considered too high compression on a mini, what's the factory quoted spec for a compression test? ?

Whilst no big rise in reading the test was performed incorrectly. Simple as that. Imo the OP should repeat the compression test then as you say leak down test.

Id expect around 200psi on a new built engine

To show what? A tad higher with the same result? Waste of time.

I'm not asking what YOU would expect. I'm asking what the BMC engineers would expect. This will be entertaining.

[MacFreuden]

Thanks for the replies guys and to update, compression test done with accelerator fully down, all plugs removed, ignition coil all unplugged so I dont get any sparks and a warm up engine...

Dry
1. 127.5
2. 128.5
3. 130.0
4. 131.5

Wet
1. 144
2. 141
3. 142
4. 147.5

Am I right in thinking this shows that all four piston rings are worn and need replacing?

In reply to some questions...

I have re-used the old pistons and piston rings as I was told they hadnt had much mileage (pretty much just run them in) and someone also told me you couldnt replace the rings without replacing the pistons too which at the time money didnt allow. Is this true or could I buy a new set of rings and fit them with no issues?

I have driven it around 130miles staying below 3000 rpm since I put it all back together as I still need to run in parts of the gearbox which are new... but I guess I don’t know the previous owners mileage

Regarding the low readings in general, any ideas as to the cause of this? I have read and heard that they should be circa 180 as you guys have debated so was wondering about other components and if they might cause this, like valve lapping etc..

Edited by MacFreuden, 25 May 2020 - 12:49 PM.

[Cooperman]

If the bores are ok when rebuilding with no 'barrelling' and no lip, you can normally just hone the bores and fit new rings, subject to everything being within tolerance when accurately measured.
Piston wear is not such an issue as bore wear.

[sonscar]

In my opinion only based on my experience running in is best achieved by not sticking rigidly to 3000 or under but by lightly using the motor through most of the rpm and by where safe snapping the throttle shut at medium rpm reasonably often in the first 50 or so miles as this action beds the rings in.
You may have glazed the bores.If the smoking g is bad you may consider honing and new rings.
Running in is a very emotive subject.In my working life I must have had 10 or more new vans and never ran them in and they were OK.Steve..

[Dusky]

As said before. Leakdown test.