When reboring a villiers 98cc to +15 what is the required oversize?

Question

For restoration of a 1938 excelsior autobyk with villiers junior engine. It is a fixed head cast iron barrel.

Answer 1

Wow,,that's an OLDIE!
Sounds like a very interesting project.

I am not certain exactly what information you're seeking.

Or whether You are asking as a "Customer" who will be having the boring job done,,
Or as the actual person who will perform the Boring operation yourself.

Plus,,I just do not recall the Specifications of a 1938 Villiers off the top of my head,,,,LOL :)

I can offer you a little info about Boring,,in GENERAL TERMS

Ok,,,, Generally Speaking.

Replacement Pistons are offered in standard INCREMENTS of Size,,including Oversize.

The size designations are such as;
Standard Size,,,+.010" +.020" + .030" +.040"
Or,,marked in Metric Sizes

Each Mfgr has a Range of Sizes,,,,some Bikes only have a couple available,,some have several.

The Steps or Graduations might be .010" Increments or .020" steps.

Which doesn't really matter.
What's more important to understand is that Each Engine will have a Fixed Range of Pistons/Rings that are Available to chose from.

Even MORE Important to realize is that the SIZE MARKINGS are a NOMINAL DESIGNATION.
They are NOT an actual,Precise Measurement of A Specific,Individual Piston.

Most Pistons are finished VERY Precisely.
But they are Not ABSOLUTELY PRECISE in their diameter.

Even Direct Replacement STANDARD-Bore Size pistons can VARY from example to Example.
Most DO vary,,,even if it's an insignificant amount.

What all that means is,,,
Even when You have a Piston of Known & Marked/Labelled NOMINAL SIZE,,,,
You have NO IDEA what the True,Actual Size of THAT Specific Example until You Measure it.

This is where the Engine Machinist's SKILL and Experience come into the picture.

The 1st step is the Precisely Measure the Piston.
A NOMINAL 2" Piston might be 2.0005,,,or 2.0008 or 1.996

Those are all "2 Inch Pistons" in NAME,,for Labelling Purposes.

But by having Precisely Measured,,the Machinist NOW has an EXACT figure upon which to Calculate his desired Working Clearances.

It would be Very Hit-or-Miss to use a 2-Inch Piston with a .0015
clearance,,
by simply Boring the Cylinder Arbitrarily to 2 + .0015,,,and having a 2.0015" Finished Bore.

What IF... ACTUAL,Specific Piston Size was 2.0008?
Or,,1.996?
In a 2" + (.0015" Clearance ) Hole....
Neither piston would TRULY have the desired .0015 Wall Clearance.
One would be Tight,,one would be Loose,,,Right?

So Regardless what a Piston's Nominal Size is,,,,
(literally the NAME of it's size,but Not an Actual ,True DIMENSION)......
....A Boring Operator will measure the Exact Piston at hand and Bore>Hone to the Required Clearances.

Give the Machinist a CORRECT Piston and all he needs to know is "How Much Finished Clearance" is required.

Nothing else Matters to him.
To YOU,,it may be a 2" Piston.
But to HIM ,,it's Not...it is an UNKNOWN till he Measures it.
He does not care what you may CALL it,,,he's going to first determine Exactly what it REALLY IS ,as far as size.

Rarely will he find a genuine 2",,
it's almost always a Big 2" or Little 2",,,so to speak.
There's most often a variation from actual 2".

Clearances:
Each Engine has a "Factory Specification" regarding working clearances.
Most all Mfgrs also Specify the METHOD to measure the Piston.
They stipulate WHERE on the piston it should be measured.

And that Measurement Location is NOT ALWAYS at the absolute "largest Point"/greatest diameter,,,as would seem most logical.

Pistons' Dimensions are VERY DYNAMIC as they operate,,it Changes in a variety of both Areas and patterns/or "habits".

So,,an Obvious Measuring Point may not be the IDEAL point to measure.

Because the Piston might be Known according to Mfgr's Research to Expand during Operation in a specific area which is Most Crucial to having the Proper Clearance.

In cases where there Is NO specified Measurement Location or Procedure,
or it is simply UNKNOWN....
Any Competent engine machinist knows by Training & Experience a "Satisfactory,Workable Average" to apply.
A sorta of "Universal Location & Procedure" to measure the Piston.

Another thing,,,to the Machinist,,,it makes NO DIFFERENCE Whatsoever that his Equipment is Metric or English,or even a "MisMatch",,,Metric Measuring Tools & English Calibrated Machinery,,,
Or whether the Piston/Cylinder is Metric or English.

Any "MisMatch,,any Combination",,No Matter.

Honda for Example,,Obviously Metric.
For Years,,maybe even still??--their Oversize Pistons were Named and Marked in English Dimmensions.
Such as ,,A 50mm Piston +.020" Oversize.

If all that Metric vs English "conflict" seems ODD,,
Realize that The Machinist ONLY considers "A HOLE" needing to be a Specific Final Size.
He uses HIS tools & methodology to perform the measurements and calculations----and all that is relative to ITSELF.
Not to any external nomenclature.

A BUNCH of Machinists Routinely do Beautiful,Precision work on Machines that they " Set for 2,,but Know it cuts 3 "...LOL.
In otherwords,,They are Familiar with their Own Machines & measuring Tools..Deviations.or idiosyncracies...Quirks.
They know how to compensate,,and achieve reliable,precison final results.
(**Nothing is THAT sloppy in reality as my extreme example,,,but SOME degree of discrepancy always exists)
Which is why Skill & Experience are so necessary

ANBODY can Read Instructions>clamp a cylinder into a boring machine> set the bit> push the start button,,,and "bore a Cylinder".
The FUNCTION may be that simple,,,
But Reliable & Consistent Success is all about The Skill & Experience


Bottom Line:
The Engine Machinest KNOWS.

#1_Give him a Cylinder--even something he's never seen or heard of before
#2_Provide the Specific Piston to be used in That Cylinder
#3_ Tell him what Wall Clearance you want & any possible Specific Details on Measurment Procedures

If You do not KNOW the Details of Clearances & Measurement Procedures,,,
The Machinist will almost Certainly know a satisfactory "average" to apply.

.....And You'll get back a Properly Finished and sized Piston/Cylinder set.

.........................
From that Basic situation,,it can go to a few different stages/levels

For example;
A Motorcycle Shop experienced in Cyl Boring will SOMETIMES have a "Better,More Experienced" Machinist in the Realm of MOTORCYCLE engines-in-general.
Not Always,,,but sometimes.

A Specific MACHINIST may have Specific Experience with Specific Engines.
These type folks often have developed procedures/methodology which SUPERCEDES Mfgr's/factory specs.
For Example,,they may have discovered through experience that a Different Finish or Clearance than "normal" yields better results.

Then Beyond That,,,
What might be called A Custom Machinist ,,would Know a RANGE of Fit & Finish to better suit specific USES.
Race engine vs Street engine Set-Up for example.
The charateristics & demands of Custom,Aftermarket Pistons.
He might be capable of Development-Level work---beginning with an Unproven SetUp on a particular engine,,then Monitoring the Result and Adjusting his Set-Up Specs to Optimize the Results

Cylinder Boring Skill & Knowledge can reach a fairly high level of proficiency.

But even at it's most basic level,,any competent Boring operator/machinist will produce satisfactory results.

To Them,,it all boils down to
A HOLE,a Piston,A Surface Finish,and a Working Clearance.

All the fine details & considerations are simply part of their skill set that they use everyday in doing their job.

I might add,,,it's Generally HARD & Rare to find a "BAD" engine machinist.
Bore jobs are Retail-Priced CHEAP.
The Machinery & Tools required are EXPENSIVE and delicate in some ways.
And REPLACEMENT Cylinders & Pistons to "fix mistakes" are quite expensive.

So a "Bad Borer" will likely be GONE from a business establishment,,even before word of his bad-reputation gets around on the street.
Employers simply cannot afford it.


For Your old Villiers,,ask around "who does good work" in your area.
Look for Clearance specs,,if at all possible.
Bring YOUR Cylinder,,and THE Specific Piston you'll install,,,and tell them the clearance You want.

On a Cast Iron engine,,a 98cc which I assume would be around a 40~50mm Bore???....
The clearance will most likely be .0015~.002".

Be VERY Thorough cleaning the cylinder bore before assembly.
Even though it may LOOK clean & ready to install when you receive it back from Boring,,,,"Microscopically" it probably WONT be.

Vast majority of "Bad Bore Jobs" which shops get faulted for are actually a Sole & Direct Result of improper Pre-Assembly Preparation.

All just general info that may be of some use.

Good Luck with your project,,that will be a real thrill to get it going again.
I hope it turns out very nicely!

Answer 2

I'm guessing that your +15 is .015", or fifteen thousands of an inch oversize. That is over existing or standard size piston that is currently in the engine.

Don't know where you are located but 1938 Villiers spares are not available in the corner store around here.

I would advise stripping the engine, measure the bore for out of round and size, also measure the piston for same. If you dont already know this then you probably don't have the correct gear to measure it properly. Any engine rebuild shop can do this for you, or engineering firm for that matter.

I have not played with villiers motors for over 35 years but I can tell you that if genuine or replica spares are not available then you may have a lot of trouble finding another piston to suit.

The Villiers wer typical 2 strokes of their time, piston ported and long stroke. I n locating a non standard piston the critical dimensions, other than diameter will be height of piston crown above the gudgeon centre line and length of piston skirt below, the next most important will be the location and size of piston skirt cut outs where they open and close the inlet, transfer and exhaust ports. The gudgeon diameter is probably the easiest modification to make unless it is vastly different.
If you cant find a villiers piston then my first point of matching would be early Japanese small bore pistons. By early I mean 1960's. They used longer pistons similar to the British models.

The only other option after that would be to have a piston made ( hope you've got a fat wallet.)

Piston rings are generally not hard nor expensive to have manufactured, the Villiers used a very standard type of cast iron piston ring.

As an afterthought, all my villers engines were robbed from lawn mowers to make minibikes. Try a long established lawn mower shop for spares

Bottom line is.........do nothing until you have sourced the replacement piston and rings.

Answer 3

You will need to take the barrel to a good engine shop for careful measurement, as it will not have worn consistently in all areas. For example, usually the exhaust port area will wear far more than say the bore on the piston pin axis.

Careful measurement by a skilled technician using a Mercer gauge will determine how much metal will need to be removed, and therefore the piston oversize required.

The new piston is then measured and the bore cut to the same nominal diameter. Running clearance is then cut in with a fine hone, which allows greater accuracy and leaves a fine surface finish for oil retention and ring bedding in.

With a fixed head design, it may be better to have the cylinder sleeved with a new liner which is then shrunk into the cylinder and finish honed lightly to size. This will help preclude any "accidents" with the boring bar, should it bottom out, also fixed head designs are notoriously difficult to align correctly, resulting in taper or even doglegs in the surface. A pre finished liner will prevent this happening.

My personal recommendation for this work is:

Martin Adams
SERCO Engineering.
Brighouse
West Yorkshire.

Sorry his phone number and full address is at work and my memory has failed me, he specialises in Brit bikes and will make ANY engine part you require, but as with any talented individual, he is not the cheapest.

Answer 4

Are you asking what size to bore the cylinder to increase the engine size an additional 15cc?
If you are, ask in the "Science & Mathmatics" section. It's complex math.
CC's are determined by the bore x stroke of the piston.
PIE (3.14) x R (radius or 1/2 the diameter of the bore) squared (radius x radius) = area of the circle of the bore.
Divide that # into 98 = stroke (distance the piston travels from the bottom to the top of it's stroke.

Now you want to figure out how to get 15cc using that # you got for the stroke.
PIE x R squared x STROKE
PIE x ??? x STROKE = 15cc
I can figure out what ??? equals with trial and error. Ask in the Mathmatics section for the formula.

Answer 5

Not sure if there is a website to refer to. but I can share my idea with you. Fold the sleeves to the elbow. Fold it so that the wrist part is not concealed. Sew a button similar to the colour of the shirt on both side of the sleeves. Button it to the sleeves that you have folded up. Wear a belt around your waist. This works best with a black, cotton mens shirt. You can go with a thick white belt. Change the button into white or red ones depending on the colour of the belt you choose to wear.

Answer 6

I believe long skirts are great, you're covered up minus the restrictions of trousers.

Answer 7

I think short dresses, coz you show off more hip and legs, but really it doesn't matter both should go great

Answer 8

i would not recommend any boring ,especialy on that bike., unless it is absolutely necisary