What does piston slap sound like?

treebor99 · Post by **treebor99** » Sat Mar 17, 2007 10:11 pm

Showing my lack of experience with 50's singles here.

My father in laws 1952 G80s makes a knocking noise when under power.

Brief history.
Whilst we had the head off last year to sort out a sticking exhaust valve we noticed slight score marks on the barrel. Removal of the barrel discovered a broken oil scraper ring.
Rebored and new piston (purchased from the club)
Big end does have a very small amount of up-down play, we did clock it at the time but I can't remeber exactly, I think it was just a few thou. Big end is original.

Whilst running in it is just about possible to cruise at 30mph without it knocking, so we have done about a hundred miles, as yet no improvement.

Ever since reassembling there has been this knocking noise.
A colleague who used to do hill climbs and sprints on a Matchless 500 had a look, he reckoned it was piston slap - but I can't help wondering if the play on the big end is actually what we are hearing.

Listening using the age old piece of wood against the back of the ear, the noise is louder and clearer at the head than the crankcase.
Is it the big end amplified through the conrod into the piston?
Is the piston 'slapping'?
Or another theory that was suggested to me today - if the head gasket bore is undersize the piston could be contacting the gasket (head gasket was purchased from the club).
Or is it somethng else?

Your thoughts, comments and experience most gratefully received.
Many many thanks, Trevor.

Biscuit · Post by **Biscuit** » Sat Mar 17, 2007 10:15 pm

By your description of the play in the bigend, I think that is your problem.

john · Post by **john** » Sat Mar 17, 2007 10:57 pm

From bitter recent experience. (crank pin photo)

If the sound reduces or goes away when cruising on a neutral throttle (on my recent failure I could make rattle disappear with careful use of the throttle) but comes back clearly when the engine is pulling, then it is a big end failure.

In “Osborne” it suggests this noise is caused when there is between 0.0015” and 0.002” clearance in the big end assembly. If you are measuring a number of thou, I would think that you have a shot big end assembly, and you have my sympathy.

John T

treebor99 · Post by **treebor99** » Sat Mar 17, 2007 11:13 pm

Biscuit and John,
Thanks for such prompt replies - my gut feeling was the big end, but that's obviously a complete strip down and we didn't want to do all that if it's just piston slap.

I have read some related articles searching the forum and read with iterest comments about sludge blocking the oil ways in the cank.
The Bike did stand for a number of years - the only bit we didn't get checked out when the bike was returned to the road was the big end.

Hind sight is such a wonderful thing!
Although my father in law has had 5 summers of Saturday afternoon pootles around the local lanes so not all bad.

We won't be stripping her down just yet, may be start in a couple of weeks, so anyone else with pearls of wisdom or advice most welcome.

Thanks again, Trevor.

Merlin · Post by **Merlin** » Sat Mar 17, 2007 11:33 pm

I always understood piston slap reduced as the engine warmed up.

Eric · Post by **Eric** » Sat Mar 17, 2007 11:42 pm

In your description of the problem you said that the big end does have a small amount of up and down play that you had clocked but could not remember what it was. I would suggest that this is almost certainly the cause of your problem as there should not really be any discernable up and down play in the bearing.

itma · Post by **itma** » Sun Mar 18, 2007 10:38 am

you don`t say if there was any scoring or scuffs on the piston, if a piston has lost any metal from the contact surfaces, it can be noisy.
Another source of bigendy noises is the splines on the drive shaft;if these are worn and the shock absorber cam wears to match this gives a sound very like a worn big end.
According to the wisdom of the day you could get away with up to 15 thou play inthe big end.
I bet its on multigrade oil?
fill it up with 50 grade and even a dollop of STP, that `'ll quieten it down for the summer

treebor99 · Post by **treebor99** » Sun Mar 18, 2007 11:34 am

itma
The piston is new, rebored last summer so I really hope ther aren't any scuffs or scrapes. We will obviously check for this when we strip her down, I will also check the piston to bore clearance.

Oil is Rock oil 50, and it's fresh too.

One point I hadn't mentioned yet, there was a noticable increase in compression after the rebore, more effort required to kick her over.
That will obviously be putting the big end under more stress and thus the knock is now so much more obvious.

The drive shaft splines are definately worth a look before we strip the engine - thanks for the tip.

15 thou play, in the owners manual it does say you can carry on for another 5000 miles or until you can't stand the noise anymore!
It's my father in laws pride and joy, he's had her since 1953 - he wants her to sound as she should.

I have acess at work to a hydraulic press, and all the kit to clock the crank afterwards to ensure it's true.
But are there any pitfalls to rebuilding the crank yourself, is it worth sending it away to have it done?

Again, many thanks for your wisdom!
Trevor

crawsue · Post by **crawsue** » Sun Mar 18, 2007 11:58 am

I can't enlighten you any more on your "noise" ,Trevor,but if you decide to replace your big-end then my experience with Alpha Bearings was very positive.

itma · Post by **itma** » Sun Mar 18, 2007 1:09 pm

Big end noise after a rebore used to be favourite complaint for exactly those reasons.
piston slap is very obvious when running at light load in a high gear, you should hear mine!! The pre war engines were very prone to this.
re big end renewal if you have the kit thats a plus, pay attetion to the lining up of the oilways and keep it all clean, is the main thing.
the worst that can happen is that you find the holes in the flywheels are stretched; the best cure for this is a Saggers BSA conversion.
If you have an original two piece pin, and the facilities then its worth giving thought to just replacing the journal itself.
I really twitchy about one piece pins, having had a disaster once, thats why I went the Saggers route.
you pays your money etc....
54 yrs, thats some time to have a bike isn`t it?