fitting new piston

Jason Luther /

so i got a new piston for my ward riverside, the old was was pretty much destroyed. the cylinder walls were fine, no scorches our gouges. the new piston is the stock diameter bore (40.2mm) and it wont fit. if you hold the new and old piston together there isnt any noticable difference in diameter, so it has to just be barely too big. i bought a cylinder honer and have been using that, but as you can imagine it isnt a speedy process. its gotten a little better i can slide the piston in a little but still gets jammed in there. is this a common problem and the right approach? or are there any tricks to fitting a new piston? thanks for any help-jason

Re: fitting new piston

pretty odd.. i doubt you bought the correct piston.

A 40.2mm piston will _not_ fit into a 40.2mm hole..

Re: fitting new piston

Gregory Mcintire /

I am not familiar with the Wards Riverside, but I can tell you a few things that may relate to this.

I have 5 Puch motors all with a 38 mm bore. But, the bore size varies on 3 of them. Some are marked with a 3 on the top of the cylinder and one marked with a 33 and one marked with a 1. The pistons have similar markings. They vary in diameter a small amount, on the order of less than a thousandth of an inch.

The piston to cylinder clearance for a chrome bore Puch 50 cc motor is supposed to be from a minimum of .0008 inch to a max of .0016 inch. The cast iron bores are supposed to fit looser (from .0014 to .002). But, there are I believe 5 different sizes of 38 mm chrome bore cylinders, each with a number on top that specifies which of the five it is. These cylinders vary enough that my piston marked 3 will go into the cylinder marked 1 but with NO clearance. The number 5 piston (and cylinder) is .0016 inch larger than the number 1.

If I were to buy a piston for my 38 mm bore Puch, it could come in 5 different sizes. Unfortunately I cannot bore out a chrome cylinder at all. You can bore a cast iron cylinder, but a hone is not a good way unless it is a "rigid hone". A non-rigid hone can very easily make your cylinder less than straight. It may get larger on the top or bottom, but even more likely, larger in the middle. A rigid hone used properly will make it straight. The piston is supposed to be larger on the bottom (skirt)

Your best bet would be to find out what the piston clearance is supposed to be and then take the piston and cylinder to a qualified shop and have it bored properly. Alternatively you may find that there are various "sizes" such as mentioned above and then order the correct one, if the Riverside uses a sizing scheme similar to the Puch stuff mentioned above. Keep in mind that you positively do NOT want to bore an aluminum cylinder with a chrome bore.

Re: fitting new piston

Gregory Mcintire /

It would fit perfectly, but with zero clearance and therefore unacceptable.

Re: fitting new piston

nobody is talking about clearance (which is the original problem, imho) but my point is not arguable.. its Machineshop 101 .. a 40.2 mm piston cannot be inserted into a 40.2mm bore.

Re: fitting new piston

Jason Luther /

i went through several rounds of questions to make sure that i got the correct piston. getting the old piston out was no easy task, it wasnt seized, but i still had to beat it out. its an iron cylinder, and it looked as though it was sitting for a long while. could the cylinder have 'tarnished' or someting making it difficult to get the pistons in/out? though looking at the inside walls, they are smooth. after honing for a while the old piston slides in and out easily now. should i keep going until the new piston fits?-thanks for your responses-jason

Re: fitting new piston

Gregory Mcintire /

If you consider how a bore is measured (inside micrometer) and how the outside of a piston is measured (regular micrometer) you will see that if you were to measure a 40.2mm bore with the inside mic then measure the piston with the other mic, you would then discover that you could measure the inside mic with the outside mic. Perfect fit. Think about it... the tools used to do the measuring.

But more to the point of the original poster's question... when a cylinder is called a 40.2 mm bore in its specifications, the piston will also be called 40.2 mm even though it is likely to be from .001" to .002" smaller. It is simply the way it is done. The piston is not listed as a 40.1746 mm piston in a parts book.

Re: fitting new piston

i won't argue semantics.. nor will i suggest that if your inside mic fits into your outside mic while they both read the same number, that your tools might not be as accurate as they could be.

As far as jason's piston fit problem, it would seem that, since the original piston had to be hammered out of the cylinder, there may be more to this than replacing the piston.

When jason mentioned "... it wasnt seized, but i still had to beat it out..." (??) my first thought was that a shop should examine that cylinder.

Re: fitting new piston

Jason Luther /

okay then, what kind of shops are equiped to accurately bore the cylinder to fit the piston?

Re: fitting new piston

Just about any shop that does bike or auto engine rebuilding can bore it oversize if it's necessary. You may get away with using that same new piston but with oversize rings.

The cylinder wears out faster at the top than the bottom because the rings rub only in the top portion. Without knowing the true bore size of the cylinder at all points it's kinda wasted effort to try to home it just so the piston slides through. The cold piston isn't truely round nor is it a true cylinder lengthwise.. its a tapered oval. Exhaust heat makes the piston expand unevenly.

Specs from an owner's manual doesn't say much about that particular cylinder. Even new engines are sometimes way off of spec sizes.

What i'm wondering is if that cylinder is warped or out of round or something.. Maybe it's cracked.. have it magnafluxed or whatever .. maybe it's toast.. just have someone look at it.

Re: fitting new piston

John Joedicke /

Any machine shop can do the bore for you.

Re: fitting new piston

Jason Luther /

thanks for all of your input-jason

Re: fitting new piston

Jason Luther /

one more thing. without knowing the correct bore, what should i tell the machine shop?-jason

Re: fitting new piston

find the clearance in the manual, and tel them the ring end gap clearance and the skirt clearance, if you don't have a manual my owners manual may have the numbers

Re: fitting new piston

All that really matters is that the piston matches the bore. Bring both the piston and the cylinder down there. You don't have to tell them anything. They do this stuff all day long.

Re: fitting new piston

Jason Luther /

alright then, how much can i expect to pay? i realize all shops are different, just ball-park. moneys a little tight.-thanks-jason

Re: fitting new piston

how much will _what_ cost? You don't know what is needed.. They might tell you "yer doin a fine job.. take it home and hone another couple thou' off it and it will be OK"

Re: fitting new piston

Jason Luther /

or they will probably say, "what in the hell did you do!? this thing needs to be rebored. next time leave it to the professionals!" what do you think is the going rate for a rebore?-jason

Re: fitting new piston

no idea..

anyway, you aren't taking it down there to be rebored. All you want is for a pro to look at it and evaluate the situation.

If it makes you more comfortable, take it to someone who doesn't have the equipment to fix it but does have the knowledge to advise you.. like a chainsaw/garden equipment repair shop.

Re: fitting new piston

Jason Luther /

okay, i like your optimistic approach. it just seems that my mopeds are governed by murphys law. i think its just the area of the cylinder where the piston doesnt travel that is blocking the piston from getting into the cylinder. it seems as though it was sitting for about 20 years before i bought it.-jason

Re: fitting new piston

Jason Luther /

alright, so im a big do-it-yourselfer. what do they actually use to bore out a cylinder? i cant imagine there is a bridgeport sitting there that they use. and if that is the case, then it would cost a fortune for them to set up the machine, calibrate for each and every cylinder thats brought in, accuratly attach the engine block to the machine, and then finally do it.-jason

Re: fitting new piston

around here it is about $40-60 for a single cylinder

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