Leak Down Test (Don't Laugh!!)

Dances with Shrapnel said:

So you really need to answer the fundamental question of "why did the pistons scuff"?

If not mentioned already, try finding an undamaged and minimally worn portion of piston skirt and corresponding barrel and check the clearances with a feeler gauge to get some idea of where these piston/bore combinations started out at. My hunch is inadequate clearance but you don't know that until you or someone checks (if possible). I can speculate all sorts of other exotic reasons for the scuffing but no point in going there ...yet. Hopefully it was just inadequate piston skirt clearance. If you cannot do the checking then definitely have the machinist conduct the checking.

When you removed the pistons, were any of the rings hung up (stuck)? Though I saw the skirt damage I don't recall seeing any heavy smearing of the skirt into the ring lands.

Apologies if some or all of these questions have been asked already.

Click to expand...

That's a good point, whoever I pick to do whatever work I decide on getting done I'll send them the used pistons & get them to check the bore clearances.

I want, as much as I can, to get a reason for why this happened. I don't want to spend my money & other peoples time rebuilding an engine that does the same thing again!!

The rings were free & and had no 'gumming up'. Apart from the 'scuff' marks, they look in great shape.

My money if firmly on inadequate clearances. I think it’s been bad for a long time and only recently became bad enough for you to notice.

+20 920 still gets my vote.

Cam looks perfectly fine to me. No idea which cam it is though.

Yep i agree it looks like it was bored too tight I had this happen on a BSA b44 the skirt of the piston looked very much like yours from cold seizure

Click,
Option 3
1) +20 now and on the road, then
2) Shop for 850 conversion parts at a leisurely pace and if you don’t like the 920 rebuild in to a 850 at a leisurely pace and sell 920 parts to recoup some $

Or keep all the parts because I don’t think there is a Norton owner that doesn’t have boxes of extra parts laying around, because you never know..........

What ever the case good luck, but that 920 with a restrained right sounds pretty nice.

Pete

I tend to agree with keeping it a 920 for now since everything is there. I don't know anything about the 920 as far as actual operation - never seen one - but with proper assembly and clearances, can't see why it wouldn't be perfectly OK. And, as noted by Deets55, it could then be a leisurely effort to make it back into an 850 if, at some point you decide to do that.

Click, before making any decision, I would look at the condition and fit of the existing 13/16" Gudgeon pins in the conrods. If the pins are not worn but not a good fit in the rods you will need to replace the rods anyway I think. I'm only saying this based on the hatchet job somebody did on the bore honing. ( so unfortunately you need to check everything before proceeding).

I do feel for you though, quite a dilemma - but I agree, keeping it a 920 seems the simplest option at the moment.

If the rods seem ok, and you opt to send it to Pete Lovell, why not ask him if he can bore the oil drain for a hollow dowel between the head and barrel. If not done, I see that as an ongoing problem as you only have Maybe +/- 3mm? between the bore and the drilling, and that 460cc piston can do a hooooole lot of sucking on the overrun! To stop it encroaching on the liner it would need to be eccentric to the existing drilling, but any engineer worth his salt should be able to do that easily. Then all he or you'd need to open the head gasket hole a bit, and bob's your uncle. Ideally as it's a drain you'd want a press fit in the head, and a good fit in the barrel. Then if (God forbid) it starts smoking again you'll know it's not that.

Cheers,


cliffa.

cliffa said:

Click, before making any decision, I would look at the condition and fit of the existing 13/16" Gudgeon pins in the conrods. If the pins are not worn but not a good fit in the rods you will need to replace the rods anyway I think. I'm only saying this based on the hatchet job somebody did on the bore honing. ( so unfortunately you need to check everything before proceeding).

I do feel for you though, quite a dilemma - but I agree, keeping it a 920 seems the simplest option at the moment.

If the rods seem ok, and you opt to send it to Pete Lovell, why not ask him if he can bore the oil drain for a hollow dowel between the head and barrel. If not done, I see that as an ongoing problem as you only have Maybe +/- 3mm? between the bore and the drilling, and that 460cc piston can do a hooooole lot of sucking on the overrun! To stop it encroaching on the liner it would need to be eccentric to the existing drilling, but any engineer worth his salt should be able to do that easily. Then all he or you'd need to open the head gasket hole a bit, and bob's your uncle. Ideally as it's a drain you'd want a press fit in the head, and a good fit in the barrel. Then if (God forbid) it starts smoking again you'll know it's not that.

Cheers,


cliffa.

Click to expand...

Thanks for the heads up on the gudgeon pin fit. I had to remove both of them with a pin remover. The RH one (the side that was smoking and the worse looking piston) was tighter than the LH one, which came out easier.

I presume both should be a press fit with no discernible movement?

Interesting about using a hollow dowel between the head & the barrels to 'bridge' the drain hole. I don't think I've ever heard of this being done. Is this something you think is a good idea or something you have seen working? Not being critical just curious if this has been done before

Hi click, no you should be able to slide to the pins in and out of the conrods by hand, but should be a nice fit with no discernible play in any direction. Maybe try them dry, as you'll be able to feel any excess play easier.

Honda have used hollow dowels for many years on oilways (even when they have much more than 3-4mm of gasket surface), not sure if Norton or any other Brit bikes have though.

Look at the gutter downpipes outside your house, that's exactly how they work - i.e. the upper pipe always fits inside the lower pipe. The same with those builders chutes that they drop rubble into from a height.

How's that whiskey coming along

cliffa said:

Hi click, no you should be able to slide to the pins in and out of the conrods by hand, but should be a nice fit with no discernible play in any direction. Maybe try them dry, as you'll be able to feel any excess play easier.

Honda have used hollow dowels for many years on oilways (even when they have much more than 3-4mm of gasket surface), not sure if Norton or any other Brit bikes have though.

Look at the gutter downpipes outside your house, that's exactly how they work - i.e. the upper pipe always fits inside the lower pipe. The same with those builders chutes that they drop rubble into from a height.

How's that whiskey coming along

Click to expand...

Thanks for the extra info.

Had to get another bottle of whiskey, using more whiskey than oil burning in my Norton

Ha Ha, which Whiskey do you prefer?

cliffa said:

Ha Ha, which Whiskey do you prefer?

Click to expand...

Jameson on this side of the pond but when I'm stateside Seagram's, neat NO ice.

click said:

Although I had a good chat with PL, I didn't make any definite arrangements with him. I explained I was not sure what I was going to do next. I've asked him a few more questions about the 850 option. Apologies if I gave the impression I had engaged with PL's services.

I've got two options as I see it & I'm looking for as much info. as I can before making a decision:

1. Get the existing 920 barrel bored out to +20, fit new pistons, bike is back on the road. Least expensive option.

2. Change back to a std 850, replace conrods, barrels, pistons. Bottom end needs to be split. More expensive & will take more time.

All I'm doing is asking the guys who are regarded as some of the top Norton machinists over this side of the pond what they think. As has been said I'll probably get 5 opinions from 4 machinists! but that's OK. Ultimately it's up to me to make my own mind up!!

Click to expand...

1. Did you check if the new +20 pistons you are considering are the same weight and gudgeon pin size as your existing ones as the ones RGM sell now are the same weight and gudgeon pin size as standard 850 pistons I believe, so won`t fit your rods without bushing and won`t suit your crank balance.

Option 3. Buy a hone (less than £20) hone your bores to clean up and have my old 920 pistons to check clearance. If clearance is ok (4-5 thou) fit new rings and you`re back in business.

http://s628.photobucket.com/user/skip-rat/media/Norton/IMG_3233_zpsktvmd2ix.jpg.html?sort=3&o=1

Regards, Al.

Grumpy knob said:

1. Did you check if the new +20 pistons you are considering are the same weight and gudgeon pin size as your existing ones as the ones RGM sell now are the same weight and gudgeon pin size as standard 850 pistons I believe, so won`t fit your rods without bushing and won`t suit your crank balance.

Option 3. Buy a hone (less than £20) hone your bores to clean up and have my old 920 pistons to check clearance. If clearance is ok (4-5 thou) fit new rings and you`re back in business.

http://s628.photobucket.com/user/skip-rat/media/Norton/IMG_3233_zpsktvmd2ix.jpg.html?sort=3&o=1

Regards, Al.

Click to expand...

1. If I go with the 920 refresh option the pistons are supplied by Norvil, who supplied the original 920 kit for the bike I have. I talked with Les & he says he has a set of 920 +20 pistons in stock that use the 13/16 pins. Same weight piston as the ones I have. I think Pete L. can also supply a piston set as he machines the pistons for Norvil.

3. That's a very kind offer. I'm aware of my strengths & weaknesses and on this occasion I'll get a more experienced person to do the work. Although I understand the theory of what's required my lack of real world experience is a concern for me. My goal is to get the bike back in running order so I can use it for this summer. I'm not looking for another project .

Once again thanks for the offer of the pistons. I'm going to see if I can measure an undamaged part of one of the pistons I have & then measure the bottom of the bore & compare. I don't have any bore measuring tools, I only have a digital calliper.

I'll also check the ring gap in a few places on the bore & report my findings.

I also want to check again the oil drain hole in the barrels & the rest of the journey into the timing chest. I already checked all this before I put the head on for the first time but I might have missed something!!

OK, as I said above I only have a digital calliper & do not have bore measuring equipment.

I measured the piston across the bottom in line with the gudgeon pin hole and got 80.46mm

I measured the bottom of the barrel letting the calliper rest on the bottom of the barrel so as the measuring tangs were as deep as possible inside the bore, probable no more than 30mm. I kept this as square as I could across the bore. I got a reading from both bores of 80.64.

5 thou is about 0.127mm

So based on very inaccurate readings!! I have a clearance of: 0.18 at the very bottom of the bore.

I'll do the rings tomorrow.

As nothing busted up and seems simple lack of clearance why not just hone the bores to match up with cleaned up current pistons? Just quessing till a real bore measurement done. I on purpose ordered Mz Peel's bores .005" bigger at bottom as always cooler less expanded than top area. Point being ya may of lucked out just needing upper bores re-honed to proper .005" clearance with some taken off piston sides. If paying attention to hobot and JMS on piston thrust surface relieving and texturing for less friction better oiling, now is opportunity to do it for a win win. Otherwise follow seasoned advice to fully rebuild from bottom up expensively. Oil drain mod seems good way to go regardless.

Am inclined to agree with Hobot. Nothing seized, so if you clean the piston faces with a smooth file ( do NOT use emery paper ) and get the bores rehoned to get the worst of the score marks out then it should be 0k. Precaution would be to find out from Norvil which piston suppliers they use and what the recommended bore clearence is. And one final point , do ensure that the rings were fitted the right way up by the original builder.

hobot is never capitalized by hobot for reasons you'd not believe and I'd still have kill ya if told why. Pre-no name-Ms Peel had serious loose over clearance issues as demo Combat for sales with a couple hot shot afterwards leaving decades long - counties wide reputation before I acidently acquired. Cheap effective solution was knurling the pistons to take up some slack as well as oil retention distinctly surprisingly lowering hi rpm friction. Point being clever bootlegger hot rod machine shop with SUN brand knurler can easy make up for fudging bore and pistons to get back in saddle wearing out rear tires and grin muscles. Peel could out leap wheelie limited moderns to 90 mph/2nd with them climbing air out of peripheral vision shooting into turn only room enough for one, smokeless quiet and survived tach needle bouncing off both sides of peg so fast was invisable for a few seconds then slowly bouncing off both sides a few seconds before stopping level testing. On opening saw most the kurls wiped off but still nice fit and no bore scores. Your call on how long to delay big block Commando grins before life time catches up. I'm kind of scared of hi speed but am addictive to pulling hard up to saying Uncle! But mostly slow down so much for all blinds/crests get annoyed/depressed if not able to just roll on throttle for scary enough return to illegal cruise speeds.

comnoz showed why big blocks were rpm limited d/t long stroke friction but rest of the engine could take beyond redline just fine so missing out on full potential. A real Nortoneer all would nod in respect would split cases and clean the slug trap and replace the rod cap fasteners after sending crank out for dynamic balancing with upgrade=safety of steel flywheel. If not keeping spare rear tires to renew each season or month, then why bother with quirks of big blocks? There are stumps to pull at each and every run up to cruise or line to traffic to pass or elite modern to embarrass, sheeze, think ya live forever?

Oh yeah Ms Peel ran another 2000 miles immediately after smoke and ear ringing cleared but wiped off cam tensioner so lost sport bike power only pulling hard to 110 with full fairing so big could light cig's at 90 mph no problemo but not up to dicing with Ducati Monsters so shot her like ole Yellar ending.

You posted a question about balance some where's back on this yellow brick road relative to getting new (larger) pistons. Consider this: each gram of imbalance can generate a kilogram of apparent weight at 1000 Gs of acceleration, which is conservative for a Norton. Selecting a balance factor has shown to be a highly personal choice, based on other threads dedicated to that topic, but whatever factor you choose at least the parts will be matched to a uniform scheme.

I recall you writing that you didn't want to split the bottom end, but when you ask questions about balance splitting comes with that territory. Another truism: Open the bottom end, open your wallet.

I wish your Norton a speedy recovery and understand that life gets in the way often...

forgot to leave the best piston surface url. Pleased on doing it to mower with chain saw file.
http://www.federalmogul.com/en-US/Media/Documents/HighqualitySAEpaper200909PFL1163manuscript2.pdf

Btw its a freaking Isolastic Commando for gosh sakes so a few points either side of ideal is a non issue, especially if already run with current pistons w/o hands numb or various brackets light filaments or connections fracturing. If not isolating by about 2000 then just bevel the front iso's 45' with slight rim left to get back disappearing act.

click said:

I measured the piston across the bottom in line with the gudgeon pin hole and got 80.46mm.......................

Click to expand...

If I read this correctly, you really need to be taking the measurement on the piston skirt at right angles to the gudgeon pin, not "in line with".

Fast Eddie said:

My money if firmly on inadequate clearances.

Click to expand...

I tend to agree but it's click's money so he really needs to confirm the root of this problem.

A quick assessment is sliding the piston in to the bore starting with say a 0.003" feeler gauge between the skirt and bore and increment up until it feels like it there's really no slack. The measurement should be at 90 degrees to the gudgeon pin.

I like hobot's assessment on the pistons. They really don't look too bad other than the scuffing but I would give them a good cleaning and inspect the ring lands with a jewelers loop to see that they are not damaged or scratched if you should choose to take that path. From your assessment of the bores (lack of feeling a ridge where the vertical scuff marks are) you may be able to clean them up with a light proper hone.

Back to the root cause. This motor has a habit of seizures; see a pattern here? Did you ever have an oil pressure pressure gauge to rule out oil pressure problems. If this is an oil starvation problem, things to look at are the feed side starting from the tank screen and lines. The lip seal on the timing side end of the crankshaft has been known to blow out or maybe the incorrect shim was used behind the doughnut seal on the oil pump. The oil pump could also be a problem though unlikely. You would be amazed at how little indicated oil pressure is needed to keep the rod journals from seizing. So these are all straws to grab at if that time comes. It is rather odd that you have been going along with this conditions without any indication of "an event" like a rear wheel locking up on the motorway at speed.

Again, I would slip the feeler gauges in, even if it's where there's scuffing as that will give an indication of where the clearance ended up. As straight forward as this may seem, it sure is an odd one you are dealing with here.