Leak Down Test (Don't Laugh!!)

OK, some more datum and some confusion for me!! (what's new!!)

OK, good news, RH gudgeon is a good fit in the conrod small end, good solid dry hand fit, no decreeable 'rock' from spindle.

Not so good and the confusing bit. I orientated the RH piston in the bore as it would be installed. I placed bigger & bigger feeler gauges along the side 90degrees from the gudgeon pin hole.

When I got to .010 it still slid down, just, but with no 'rocking' the next size up was .012 which would not fit.

My confusion is this gap is VERY big, because I'm dealing with a circle do I halve the gap? which would be .005 (I think!)

If my assumption is correct then from this check the bore clearence is not the issue

The other not so good news is the piston, have a look at the pictures below. There's been intrusion into the oil ring land and the front of the piston has a smear going all the way to the top of the piston, with a small smudge/nub sticking above the piston edge height!!

click said:

OK, some more datum and some confusion for me!! (what's new!!)

OK, good news, RH gudgeon is a good fit in the conrod small end, good solid dry hand fit, no decreeable 'rock' from spindle.

Not so good and the confusing bit. I orientated the RH piston in the bore as it would be installed. I placed bigger & bigger feeler gauges along the side 90degrees from the gudgeon pin hole.

When I got to .010 it still slid down, just, but with no 'rocking' the next size up was .012 which would not fit.

My confusion is this gap is VERY big, because I'm dealing with a circle do I halve the gap? which would be .005 (I think!)

If my assumption is correct then from this check the bore clearence is not the issue

The other not so good news is the piston, have a look at the pictures below. There's been intrusion into the oil ring land and the front of the piston has a smear going all the way to the top of the piston, with a small smudge/nub sticking above the piston edge height!!

View attachment 3917 View attachment 3919

View attachment 3920 View attachment 3921

Click to expand...

Are you inserting the feeler gauge from the top, or from the bottom? Pistons are tapered and bigger at the bottom, so you should insert from the bottom.

I’m not sure it’ll tell you much though. The piston is shagged! You’re not measuring a piston any more... you’re measuring the remains of a piston that might not quite be the same size and shape it used to be!

Fast Eddie said:

Are you inserting the feeler gauge from the top, or from the bottom? Pistons are tapered and bigger at the bottom, so you should insert from the bottom.

I’m not sure it’ll tell you much though. The piston is shagged! You’re not measuring a piston any more... you’re measuring the remains of a piston that might not quite be the same size and shape it used to be!

Click to expand...

Inserting from the top!!! Fair enough about the piston, I didn't have a really good look until today.

I'll try the LH piston which is in slightly better condition & this time I'll insert the feeler gauge from the bottom!

Are you sure the piston is rubbish, maybe a bit of spit & polish could sort it out . . . . . . . . ONLY JOKING

click said:

OK, some more datum and some confusion for me!! (what's new!!)

OK, good news, RH gudgeon is a good fit in the conrod small end, good solid dry hand fit, no decreeable 'rock' from spindle.

Not so good and the confusing bit. I orientated the RH piston in the bore as it would be installed. I placed bigger & bigger feeler gauges along the side 90degrees from the gudgeon pin hole.

When I got to .010 it still slid down, just, but with no 'rocking' the next size up was .012 which would not fit.

My confusion is this gap is VERY big, because I'm dealing with a circle do I halve the gap? which would be .005 (I think!)

If my assumption is correct then from this check the bore clearence is not the issue

The other not so good news is the piston, have a look at the pictures below. There's been intrusion into the oil ring land and the front of the piston has a smear going all the way to the top of the piston, with a small smudge/nub sticking above the piston edge height!!

View attachment 3917 View attachment 3919

View attachment 3920 View attachment 3921

Click to expand...

That looks like a piston that ran out of oil.

No you don't divide piston to bore clearance by two.

This has been a very interesting read. Even if most of it is above my pay grade. First I think you have mentioned anti-drain sump valve. I have some negative experience with those and have a theory. The damage/ seizing started prior to owning it with lack of oil from the anti drain valve. Possibly from the initial start up. What was the reason for the initial head work? Oil burning? Just a layman’s theiry...

Triton Thrasher said:

That looks like a piston that ran out of oil.

No you don't divide piston to bore clearance by two.

Click to expand...

Thanks for clarity on bore clearance

MichaelB said:

This has been a very interesting read. Even if most of it is above my pay grade. First I think you have mentioned anti-drain sump valve. I have some negative experience with those and have a theory. The damage/ seizing started prior to owning it with lack of oil from the anti drain valve. Possibly from the initial start up. What was the reason for the initial head work? Oil burning? Just a layman’s theiry...

Click to expand...

Above my pay grade as well!!

Yes, the original head work was because of oil burning, I went through the whole gamit of things like position of rocker arms, valve seals, porosity, leaking head gaskets. When I originally fitted the head I thought I had it licked but the work I got done on that head was OK but a basic error of, I think, fitting the rocker spindles cold meant 2 of the spindles were quite loose. I then decided to bit the bullet & get a head worked on by Jim C.. There's more detail to this but I won't bore you all with more information!.

It's possible I'm dealing with an issue caused before I purchased the bike. All I can say is my time with the bike has been by the book. I've changed the oil probably every 1000miles sometimes less!! due to it sitting for long periods. Although I've used the bike I've never abused it. It has never seized on my watch.

As I said the chap who built this bike was very well respected amongst Norton club owners in the UK. The previous owner, who I purchased the bike from, very rarely used the bike but on one spin ended up seizing the engine due to lack of oil (I think this might have been from not priming the one way valve, vaguely remember this being said but it was a long time ago now!!). This is when the bike was given back to the original builder & the engine was stripped, cleaned out, crank re-balanced & the Norvil 920 conversion done. The owner then put 500 miles on the engine before I purchased it.

I think earlier I said I had put 14K on the bike, I was thinking of another bike I have, I've only put 5000miles on this bike!! This is over a long period of time. Some of the summers I spent just testing the bike & brought it for short runs for testing only but never went for any decent spins. Some summers it never got used due to biking on other bikes.

OK, as per Fast Eddie's instructions, I've measured the LH piston in the LH & RH bore.

LH bore, piston inserted from the bottom + feeler gauge, could fit a .006 which was tight & the piston could just about move.

RH bore, using the LH piston, .006 would not fit, too tight. A .004 did fit but tight, piston could just move.

So, the RH bore has less clearance than the LH bore, not sure if this is a significant difference?

So, not extremly tight clearances, maybe the RH is a bit too tight for a 920? Looks like it will need a re-bore if I go for the 920 refresh route.

Here's some close-up pics of the LH piston (a little bit better than the RH piston! no incursions into the oil ring land & no smudging above the piston skirt)

The more I look at these pistons the more I count myself lucky that I took the advice from lads on this forum to take the barrels off, although I did plan on taking the bike out for a bit of a blast before taking the barrels off but the bike would not start, maybe the Norton gods were looking after me!!! or did one of you guys from the forum fly to Ireland, break into my garage, rig the wiring so it wouldn't start, re-lock the garage & then fly home. Whoever it was, thank you!!

Cold seizures are caused when the piston expands faster than the cylinder when the engine is first started cold. Since the Norton engine uses a low expansion grade of cast iron it does not depend on the barrel being up to temperature to provide piston clearance. So cold seizures do not happen with an iron barrel Commando.
Rich mixtures will cause wall washing which cause severe ring wear and will scar the cylinder wall but I would doubt that is what caused this problem.
What you have is a classic case of "the piston grew too big for the hole"
This could be caused by:

1.insufficient piston to wall clearance.
2.too much heat in the piston from over advanced timing.
3.too much heat in the piston from an excessively lean mixture.
4.too much heat in the piston from a spark plug of the wrong heat range.
5.too much heat in the piston due to compression ratio too high for the fuel's octane rating.

Is the piston you are using forged or cast?

If it's cast then #1 is the likely problem as a cast piston is likely to just burn a hole in the top if combustion temps get too high.

If it's a forged piston then any of the above problems will likely cause a seizure like you have. Jim

PS, both pistons have had seizures. Just one has advanced a lot farther than the other.

Ok Mr. smarty pants comnoz, what's clicker's most efficient way back in the saddle? Stumped all us all's so far on myriad of causes. Jim ya give life back to myriads of Commandoneers too but who'd want your work load dealing with picky Commando owners sheeze.

Here's a stock MK3 piston and bore after 21,000 miles.
The bike suffered a trans. layshaft bearing seizure and was never put back on the road. It ended up with a breaker who sold the parts on EBay . I bought the main lumps
I've only cleaned the piston top to find the size stamp. It shows STD, so I guess it's the factory piston.
There wasn't much carbon on it.

This gives some idea of what things should look like at that mileage. There is no cylinder ridge, in fact the honing marks still clearly visible.
So when all is right they can go a long way!

comnoz said:

Cold seizures are caused when the piston expands faster than the cylinder when the engine is first started cold. Since the Norton engine uses a low expansion grade of cast iron it does not depend on the barrel being up to temperature to provide piston clearance. So cold seizures do not happen with an iron barrel Commando.
Rich mixtures will cause wall washing which cause severe ring wear and will scar the cylinder wall but I would doubt that is what caused this problem.
What you have is a classic case of "the piston grew too big for the hole"
This could be caused by:

1.insufficient piston to wall clearance.
2.too much heat in the piston from over advanced timing.
3.too much heat in the piston from an excessively lean mixture.
4.too much heat in the piston from a spark plug of the wrong heat range.
5.too much heat in the piston due to compression ratio too high for the fuel's octane rating.

Is the piston you are using forged or cast?

If it's cast then #1 is the likely problem as a cast piston is likely to just burn a hole in the top if combustion temps get too high.

If it's a forged piston then any of the above problems will likely cause a seizure like you have. Jim

PS, both pistons have had seizures. Just one has advanced a lot farther than the other.

Click to expand...

Thanks for the reply.

They are cast pistons, so No. 1. looking more likely, my basic measurements of bore clearance seems to indicate RH is too tight .004? and LH should have been OK .006?
I know I'm trying to measure worn/damaged parts but its the only thing I can do.

Most of this is just academic but I would like to know why this happened.

Once I chat to a few of the machinists tomorrow & get lots of conflicting opinions I'll make a decision. At the moment I'm leaning towards getting Pete L. to rebore the barrels to match the 920 +20 oversize pistons Norvil have in stock and put the bike back on the road.

I've checked both gudgeon pins in the small ends on the conrods & they are a good tight hand fit, no movement/rock etc.

worntorn said:

Here's a stock MK3 piston and bore after 21,000 miles.
The bike suffered a trans. layshaft bearing seizure and was never put back on the road. It ended up with a breaker who sold the parts on EBay . I bought the main lumps
I've only cleaned the piston top to find the size stamp. It shows STD, so I guess it's the factory piston.
There wasn't much carbon on it.

This gives some idea of what things should look like at that mileage. There is no cylinder ridge, in fact the honing marks still clearly visible.
So when all is right they can go a long way!

Click to expand...

Hopefully I can get my Norton sorted & end up with pistons & bores looking like the ones in your pics!!!

Those latest close up pictures of your pistons are fugly. As fast eddie said - "shagged".

Dances with Shrapnel said:

Those latest close up pictures of your pistons are fugly. As fast eddie said - "shagged".

Click to expand...

I'm sure a few minutes with a Dremel & a course flap wheel would sort them out

click said:

Thanks for the reply.

They are cast pistons, so No. 1. looking more likely, my basic measurements of bore clearance seems to indicate RH is too tight .004? and LH should have been OK .006?
I know I'm trying to measure worn/damaged parts but its the only thing I can do.

Most of this is just academic but I would like to know why this happened.

Once I chat to a few of the machinists tomorrow & get lots of conflicting opinions I'll make a decision. At the moment I'm leaning towards getting Pete L. to rebore the barrels to match the 920 +20 oversize pistons Norvil have in stock and put the bike back on the road.

I've checked both gudgeon pins in the small ends on the conrods & they are a good tight hand fit, no movement/rock etc.

Click to expand...

Yes it sounds like it was too tight. A 920 with cast pistons should be around .005" when it's assembled. Around .0055 after some mileage.
I would want to re-sleeve it. 920 sleeves are pretty borderline thin without going .020 over. Jim

comnoz said:

Yes it sounds like it was too tight. A 920 with cast pistons should be around .005" when it's assembled. Around .0055 after some mileage.
I would want to re-sleeve it. 920 sleeves are pretty borderline thin without going .020 over. Jim

Click to expand...

Since preliminary measurements indicate it appears to have been too tight, what say you about honing out to get the required clearance (assuming new pistons). If I recall correctly, "click" indicated the vertical scuffs in the liners were not (or were barely) perceptible so maybe the bores can clean up. This way click would not have to bore 0.020" oversize. Is a clean up honing and maybe 0.001" to 0.0015" increased bore diameter honing too much to hone out?

If it were me, I would still like to have a machinist have a careful measure of the bores and their respective piston skirt diameters to make sure I was getting the full story. click's bore comparison was a smart move from a smart man. Sure you have not done this before click, perhaps in another life?

Dances with Shrapnel said:

Since preliminary measurements indicate it appears to have been too tight, what say you about honing out to get the required clearance (assuming new pistons). If I recall correctly, "click" indicated the vertical scuffs in the liners were not (or were barely) perceptible so maybe the bores can clean up. This way click would not have to bore 0.020" oversize. Is a clean up honing and maybe 0.001" to 0.0015" increased bore diameter honing too much to hone out?

If it were me, I would still like to have a machinist have a careful measure of the bores and their respective piston skirt diameters to make sure I was getting the full story. click's bore comparison was a smart move from a smart man. Sure you have not done this before click, perhaps in another life?

Click to expand...

I doubt they would clean up with a hone to correct clearance. [at least they never did for me] Jim

comnoz said:

I doubt they would clean up with a hone to correct clearance. [at least they never did for me] Jim

Click to expand...

Hi Lads,

Thanks for the replies.

Don't worry whoever or whatever I get done it will not be me doing the work. I'm leaning more towards getting the 920 sorted and not, at this point in time, converting it back to an 850.

If I go this route I'll more than likely use PL due to his history with this particular conversion. I'll send everything over, barrels, old pistons, rings & head so he can get as good an overview of what I have. I'll ask about the liner replacement as opposed to getting them rebored. I'll ask him to spec. & check the pistons & bores & come up with his best educated guess at what happened. TBH I wasen't sure if you could get a liner replaced, it now on the 'list'!!

I've a few Norton engineering guys to ring today, might come back with more options so we can discuss them at more length I'll have a decision made today.

A waste of time trying to measure those, the pistons will have collapsed, wouldn't be surprised if the liners have distorted off the barrel in the middle.
Don't reckon they would have done 12000miles like that ,( just read only 5000miles)
Very clean ring grooves with next to no carbon in them, like they have been washed out, horrible marks in oil groove, ring appears to be bottomed out in groove
I'm guessing you've used something to clean the piston tops and bores and washed all the oil out of every where, mexicomikes famous dry start,
Personally I wouldn't trust the liners now, they will most likely have voids ,if money and time are a bit tight take up GRUMPY KNOBS offer but get the barrels resleeved, that also may not be that easy if the available sleeves are smaller than what's in there now,dont get to hung up on gudgeon pin fit alloy rods going to expand anyway