piston problems

pictures (or stories) of hideous injuries sustained by your ariel
david.anderson
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piston problems

Postby david.anderson » Tue Mar 26, 2019 8:21 am

The 2019 AAR annual rally was held from the 22 to 24 March. I loaned bikes to 2 people and rode my trusty 57VH.
Problem 1. I get a phone call that my 49VH has lost compression. I arranged a back up trailer and told the rider to collect my other 57VH.
Problem 2. I am told that the speedo on my 2nd 57VH has stopped working part way through the Saturday run. A quick check showed it is not the speedo cable. The speedo has only done 2500 miles since a costly rebuild.
Problem 3. I am told the stainless carrier on the loaned 57VH has broken. Two of the uprights have broken straight through.
Problem 4. A friend who was marshalling for us had his battery die. It was only a week old. I pointed to my square 4 which was on loan to another rider and said, the battery in that bike is undersize and it is 6 years old. That must have been like waving a red flag before a bull. The battery died about 20km from the finish of the Sunday ride. The rider managed to get a start and ride it the remainder of the journey. I hope the Bennet alternator is ok.
Monday night I stripped the 49VH. The top had come off the piston with a clean break around the oil ring area. The only sign of the oil ring was a small slither that was held under the exhaust valve. Only very minor damage to the bore, but any damage is of course the end of the bore. So a new piston and re bore and new valves with a complete engine strip will have to follow.
I thought rallies were supposed to be fun. Certainly an event to remember.
David
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roger.fellows
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Re: piston problems

Postby roger.fellows » Tue Mar 26, 2019 9:39 am

Sorry to hear that catalogue of events David.

I've never had a broken piston to look at closely before. Was there not much material connecting the top and body of the piston at the upper edge of the oil control groove? Have you discovered any bits in the crankcase?

john.whiting
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Re: piston problems

Postby john.whiting » Wed Mar 27, 2019 12:50 am

That is a very flimsily constructed piston...........the entire cross section of the gudgeon boss is usually connected to the crown ,and the crown is normally reinforced by strut across the axis................people who go on about cuts behind the oil ring weakening pistons dont understand that an important design factor is to stop heat from the crown area expanding the skirt....................and thin hard stainless tube is notorious for cracking under stress.

david.anderson
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Re: piston problems

Postby david.anderson » Wed Mar 27, 2019 4:43 am

The piston was actually a +.030” second hand Polson piston that had negligible wear. I checked the ring grooves for wear which was negligible then I took the piston to a local machine shop that has a piston grinding machine to finish the piston to +.020”. The cylinder was then bored to suit. There appears to be ample wall thickness behind the oil ring but the decision to use an old piston was clearly a mistake.
The piston was not a thermal slot piston so that is not a cause of this failure. I will be doing a complete engine strip to look for ring parts.
I am one who does not like thermal slot pistons. Thermal slot pistons were a disaster in the Norton Commando with most suffering the same failure as the piston above. In fact all the prior piston failures that I have seen were thermal slot pistons. There is a thread on this site regarding a similar failure to a VB thermal slot piston. Most of the conduction of heat away from the piston is by the rings. This article provides good ring information.
https://www.lsxmag.com/tech-stories/pow ... trous-kit/
David

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Re: piston problems

Postby roger.fellows » Wed Mar 27, 2019 6:04 am

Thanks both John and David for the info

nevhunter
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Re: piston problems

Postby nevhunter » Fri Mar 29, 2019 10:53 pm

While you can in theory use less skirt clearance with a "w" slot piston they are not good for harder work in iron cylinder (particularly) air cooled motors. The design works on a temperature difference between the top and the skirt being considerable and maintained and the heat has to go out via the rings as David says. They are mainly designed for moderate output cars that are water cooled and mechanical silence desired.. That early type Polson piston is OK generally though a simple design. Maybe the later deeper oil ring was fitted at some stage. Possibly the piston had insufficient clearance in the top ring land section and momentarily nipped up or may have been a bit brittle. I use them when I have them and don't use the W slot pistons at all as they used to break " lift the top off" in some OEM fitted car engines in "ordinary" (non racing) service. Nev

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brenton.roy
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Re: piston problems

Postby brenton.roy » Tue Apr 23, 2019 2:06 pm

Hi David,
That's an impressive number of things to go wrong.
Note to self about re using Polson pistons..
I'm trying to find the right wording to suggest that any of your VH's might have had some "spirited" riding along the way. :)
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

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Re: piston problems

Postby Bruce Saunders » Fri Apr 26, 2019 9:37 am

My 54 NH with VH conversion, came with a low comp
Polson piston, which has substantial webbing under the
crown. Thought it was a modified car piston, maybe it is an Ariel one. It certainly looks far stronger then your failed one. Can post a photo if you like to see it.

john.whiting
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Re: piston problems

Postby john.whiting » Fri Apr 26, 2019 11:23 am

After Nev ID it as a Polson......I had a look through my collection.....every Polson was the same design .......extremely feeble.So it must be the standard Polson design for a tourer ,at least..............

nevhunter
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Re: piston problems

Postby nevhunter » Fri Apr 26, 2019 1:58 pm

I think they are the earlier pattern pistons (pre ww2) and are not "W' slot so that problem is not present. Polson is a reputable brand, generally now out of production. Heavier webbing above the gudgeon bosses is required for Higher Comp pistons like speedway etc. I have 2 or 3 semi finished ones of the particular early type that I will certainly be using but they are all low compression design with a slightly concave top surface. the only thing that one might suspect is the depth of the ring grooves may have been just a little light on and with a bit of extra heat made the rings nip up. It only has to be momentary and could have happened sometime in the past. Nev


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