1957 VH - Piston Slap, new piston & rebore?

[carl.horrocks]

Hello All,
Well, after doing a top end overhaul (new valve guide, ex valve & springs) and fitting a hi-d-hi in-line oil filter, I finally got her back on the road and was looking forward to returning to a club night after a long absence.
I took her out for a 'shakedown'. After about 30 miles I felt very comfortable and decided to open her up a bit.
I reached just over 60 then a very nasty clattering sound from the engine started. After getting home I stripped the engine to check the various culprits and all was ok except for some new scoring on the piston. I did some investigation on the web and found that Piston Slap was the likely culprit. First time I have encountered it.
So, I have a JP 0797 (+ 40 thou) piston that is scored, but a barrel that appears to be in fine shape. I initially wondered if I had a worn piston and could just replace that with another 40 thou one but I do not think that is a good idea.
I guess that I am in the realms of getting a +60 piston and reboring the barrel accordingly. Can an iron barrel for a VH be taken out to 60 thou?
If not, will I need to get the barrel relined and hence fit a standard size 7.5:1 compression piston?
I have never had to rebore/new piston before so I am not sure of exactly how the relining of a barrel goes - any advice gratefully received.
Regards,
Carl

[tony.barnett]

I'd think the first thing you should do it measure the bore and the clearance between the piston skirt and the bore (in an unworn part of the barrel - maybe halfway down the barrel). Piston slap is a piston running in a worn barrel (or a too small piston I guess). Sounds to me like you had a seizure so you really want to find out what caused it. Might be too small a clearance - might be that your engine was running a bit lean (check the carb settings too while you're at it). Are the piston rings OK? Post some pictures of the piston scoring on here.

[brenton.roy]

Hi Carl, You would have felt a seize that did enough damage to cause serious piston slap. Like someone threw an anchor out the back?
JP pistons do have a tendency to seize and need more room than other brands. Bore it to worn out please...
Is the motor still noisy or did it just make the slapping noise as it nipped up?
If the bore measures Ok and it hasn't been scored, you might get away with a new piston or be able to clean up the one you have.
If you don't have a micrometer to do the measuring Tony suggests, maybe take it to a machine shop and have the bore measured top, middle and bottom. This will give you an idea of it's actual condition.
Measure across the thrust faces of the piston.
I hope this helps. Brenton

[carl.horrocks]

Tony & Brenton,
Thanks for quick response.
There was no sudden slowing down of the bike although the engine cut out briefly.
Rings are fine. The clattering noise remained after the onset. Took bike home, started her up again and although she ran smoothly, the clattering remained. Checked little & big ends, Valve cam & cam followers and they are ok.
Fuel Mixture: a nice straw to dark brown residue on the plug. Do you think it might be worth raising the needle one notch?
Attached are some photos of the piston. I will get measuring the barrel and piston clearance as advised.
Thanks for help guys.
Regards,
Carl

[nevhunter]

That is a good seizure.( I don't mean it is good for the engine however) When This happens the piston may/will increase it's clearance because the very hot part shrinks. Some pistons will acquire a hollow where the heavy seizing has taken place.
As pistons that are made of aluminium expand more than cast iron barrels, you will experience more piston noise when the engine is cold, than when it gets hotter. IF the noise doesn't diminish when the engine is hot, you might have another cause.
A seizing piston does put an extra load on the rest of the motor. This can cause the flywheels to go out of true and put a large load on the big-end. If it was on the way out the extra load could finish it off. Sometimes some ot the aluminium from the piston skirt will weld on to the cylinder surface and has to be removed. Check that this hasn't happened.
I've had pistons seize like this and re=used them. You have to check what actual clearance the piston has. They are oval as well as tapered and depending on the material used in the alloy will require an appropriate clearance. It is most likely that the clearance was not enough in this case. If you are working a bike, it is better to err by more clearance than the minimum, than not qute enough.
The design of the piston can have a bearing on the clearance as well. The T slot piston can have less clearance. because it has "give" in it's design. The solid skirt type, ( preferred for hard work) has nowhere to go when it gets hot, so a bit of piston "rattle", when the engine is cold, may be prudent.
Regarding the rebore to .060" this is only .010 bigger than at present and your rebore man may find that the previous rebore wasn't quite true so more might have to come out, than .010". The barrel of the 500's will bore to .060, but you are always better offf with as much of the original barrel there, as possible.oversize pistons are always heavier than they should be because they become thicker than the originals. JP pistons are usually a bit heavier to start with, compared with the OEM pistons. Nev

[brenton.roy]

Hi Carl, I think the back of the piston is the main thrust face, so this makes sense. I have a piston that looked just like yours, which is still in the bike. I did try the things you and others have talked about, including stressing over oil feed and jetting, running a 220 main, etc, etc. Just as Nev says, it rattles a bit when cold and gets better as it warms up. After the seize, I ran a hone to clean up the marks and filed down the rough edges around the damaged area of piston.
JP's are very well known for this. As Nev says, they are also heavy and they can affect engine balance.
Brenton
I would be very interested in an alternative source of pistons other than JP if anyone knows of one?

[nevhunter]

Just following up on that. The JP people do use a high silicon alloy that should have a coefficient of thermal expansion less than some other alloys. I think the problem is the selection of a clearance figure that is not enough. I would be pretty sure that with a bit more running clearance they would be fine. They certainly won't break. A solid skirt piston can be reversed in the bore unless the gudgeon is offset. The thrust side (rear) takes the most load
The extra weight would require a rebalance to get the smoothest possible result or the alternative is to remove metal from the inside of the piston to get it's weight close to the original. (Tedious and time consuming). The smoothest engines reduce the reciprocating weight to the minimum. A good Ariel piston has a tapered bore gudgeon pin also , to reduce weight. A weight going up and down cannot be balanced by a weight going round and round. It's only partially effective. I think the factor for the singles is 65%. OTA.. Nev

[carl.horrocks]

Nev & Brenton, thanks for the info.
Done some measuring and the barrel is fine with a 10 thou piston ring gap all the way down. The piston body to barrel clearance (on the non-scored bit) is between 8 & 9 thou at top of the skirt and just uder 5 at the bottom of the skirt. These are on the thrust edges. Do these seem OK? Cannot find this info in my ariel w/shop manual.
So, it looks like I can carefully clean things up, turn the piston round and expect a rattle on initial statup and quitening as it gets warmed up. Brenton, Draganfly M/Cs here in UK have several variants of Ariel pistons but I am not sure if they are JP.
regards,
Carl

[john.nash]

JP and Gandini seem to be the only pistons I could find, new, for a "straight" application.
I know Robin Parker (on this forum) has been talking to Peter Kemp (AOMCC single spares) about having some "made" but that involves the right specs, materials and probably testing........

Len Ore tells me he has a vincent piston in there, which requires modification.

Apart from that it's NOS stuff.
There were some at the AOMCC AGM, back in April, and I unashamedly bought the next four oversizes (with rings), which should see me ok for a few years ( I run two singles).

Maybe we should open a new thread with the title "pistons" to see what everyone is using...

[keith.mettam]

Carl, from the Ariel information I have, the new ring clearance for all 'Ariel engines' as measured in an unworn part of the bore (i.e. top or bottom) is 0.006" to 0.008" (except Colts which are 0.008 - 0.012"). Rings don't usually need replacing until there is a 0.025-0.030" end gap. Note that you will need to check each ring for the minimal clearance.
The piston clearance for all 500cc models with cast iron barrel and solid skirt piston is specified as 0.005" - 0.007". This clearance is quoted for both 'Below Rings' and at something called 'Extreme Skirt' and is measured at the front and rear piston thrust bearing face. As this is Ariel data, I am guessing that the pistons are likely to be Hepolite ones or similar so I would be tempted to heed other people's advice with your JP piston i.e a bit more clearance than usual so maybe get the barrel lightly honed to achieve a higher clearance figure. Looking at the images, as already stated, it has clearly suffered a piston seizure but the piston looks quite re-usable. Clean it up with some fine wet and dry or similar and also check the ring grooves have not been distorted above the points of seizure. New ring clearance in the grooves should be 0.003".
With the clearances you quote I would not expect your engine to rattle with 'piston slap'. Something else may be wrong. Have you double checked the ignition timing? Could it have slipped slightly? Is the standard carburation running with the Burgess air filter or without any sort of air filter (in which case perhaps the main jet should be a size or two larger) ? You say the big and small ends seem fine, are the main bearings also ok ? Good luck with it Carl.