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 Tom Snow's 175 engine

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mscutt

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Registration date : 2011-10-21

PostSubject: Tom Snow's 175 engine   Wed Dec 07, 2011 10:36 am

Dear All

Tom Snow prepares his 175 himself but recently brought his 175 engine back to Tom Miller for a winter inspection and crankshaft rebuild. While there, Tom M took some pictures and thought they may be of interest to others doing a 175 build. (to avoid confusion, from here on 'Tom' means Tom Miller)

The pictures show several features so, starting from the top

It has a Brian White head machined with a simple hemispherical chamber with 8.9:1 ratio as measured on the whole of the stroke (23.4cc head volume at tdc).

The cylinder is also a Brian White casting and you can see there are 4 transfer ports in the liner. The inlet and exhaust are simple non-bridged, well rounded ports and a "rub" of the barrel shows the main dimensions. Note that the widths quoted are the arc lengths not the chord lengths you might measure with calipers in the barrel.

The crank cases are standard sand cast 175 cases with no welding or external modification which limits the tranfer ports to a quite modest entry area.

Tom uses second hand Yamaha TZ 350 pistons sourced on the ebay. He turns them down from their normal 64mm to 63.7mm giving a capacity of 184.8cc.
Turning the piston down in this way not only gives a supply of cheap pistons but allows Tom to prolong the life of the cylinder by going up by a couple of 0.1mm steps when the cylinder wears and needs honing.

The con-rod is a Maico one with 125mm between centres. At one time the supply of these dried up so Tom had to find a replacement. A Yamaha YZ250 rod looked a good substitute but has a 24mm diammeter crankpin that will not fit in standard flywheels. Fortunately the Maico rod you see in the photos is now easily available and can be fitted in standard flywheels by boring the crank pin hole to 22mm. Tom chose to experiment with solid flywheels as they are heavier but there was no apparent advantage. The rod has a 22mm small end eye and is fitted with 19 crowded rollers of 3mm diameter and 19.8mm long. To keep them in place are two 1mm thick hardened steel shims.

The balance factor of the flywheels is 50%. Tom did lots of experimenting on this and found 50% the best compromise and smoothest setup in his frames. A smaller factor would lead to fractured front down tubes or clipons and too large a factor gave cracking problems around the rear engine plates.

The clutch body is a design that Tom has developed over many years and his clutches are now in use by quite a few Bantam riders. The body is turned from solid and has 12 slots. It uses hardened and soft steel plates fitted alternately making it almost indestructable. Having said that, the plates do wear so a gearbox dipstick made with strong magnets is fitted to collect the particles of iron shed by the plates and this is removed and cleaned after every race.

Ignition is provided by a PVL straight line unit set at 0.080" btdc. and the carburetor is a 36mm Lectron.

So, the $64000 question - do the extra transfer ports in the liner make any difference ?

The simple answer is we really don't know if four narrower ports are better than two wide ports but the hope was that scavenging would be improved. As mentioned above, the limiting factor for gas flow is the available area for the transfers in the crank cases so there may be nothing to gain by making the area in the liner larger. Despite many hours of work or calculation you never really know in advance if an idea will bear fruit but Tom is always keen to try out the challenge of a new idea - its like the lottery - if you never try it you won't get anywhere.

Mick



























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john bass

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PostSubject: Sick!   Wed Dec 07, 2011 10:32 pm

Sick!
I AM SICK WITH ENVY!
What wonderful work in a wondeful workshop.

Thank you for the pictures!
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mjpowell

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PostSubject: Re: Tom Snow's 175 engine   Thu Dec 08, 2011 7:22 am

Excellent! ... This is a great insight into what can be done ....
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nigel breeze

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PostSubject: Re: Tom Snow's 175 engine   Thu Dec 08, 2011 7:57 am

wow.. those port timings alone blow the original cast barrel out the water.. in fact your pretty much wasting your time without the alloy b tw casting by the looks of things...the larger of the two transfers although smaller in width by approx 5mm from the cast barrel is more than made up for by the second set.. two different roof angles can only improve the scavenging and then the increased blow down period.... im looking at a cast barrel sitting in front of me on the coffee table and all i can think is i hope the
table legs can stand the wieght......... affraid
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Nick B

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PostSubject: Re: Tom Snow's 175 engine   Fri Dec 09, 2011 9:24 am

Good evening Mick & the 2 Toms,
very nice to read about & see a top class motor.
I hope this gives a few of the builders some extra oomph.
It would be nice to know a bit of development history that has got you to where it is today.
To finish i would like to commend Tom for his honesty and openess on what has previously been a sensative subject, many others would have not bothered.
All the best Nick

p.s. how many wins with that motor ?
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Ned

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PostSubject: Re: Tom Snow's 175 engine   Fri Dec 09, 2011 12:12 pm

Wow how times have changed ! In the past current port timings and configuration would never have been reviled. The tape measure by the expansion pipe for anyone to make calculations from ! I recall one occasion when the author (Sticky) discretely dropped a fag packet under a bike in the paddock before photographing it so that he had a known measurement (the fag packet) to calculate the measurements of the expansion pipe.
While the rest of you were looking at the more technical aspects in the photos I was noting other changes in Toms workshop since my day.
Noted new welding gear, Pete has got himself a reactor light mask. Toms got a new engine wash and a convector heater !!! He always screamed "MIND THE HEATER" at you as you entered his work shop. Kicking fan heaters would invariably blow the element. I think I must have shortened the life of a couple. Embarassed
But some things never change - Swarf on the deck. I never went home without taking some with me in the soles of my shoes. Embarassed
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john bass

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PostSubject: Re secrets!   Fri Dec 09, 2011 10:49 pm

Wobbleyman -- the late Colin Aldridge, RIP -- who was known as the Vulture before his proper nickname was frequently seen in the pits with a vernier checking on exposed barrels when the fast men were making checks on, or fettling pistons etc... etc... He blatantly carried a steel tape measure and could be seen at circuits measuring exhaust pipes of Bantam racer campers at circuits like Llandow & Cadwell at 5am in the morning. With a grossly oversize motorcycle coat (name begins with B... but cannot remember...?) wrapped around him, and a flapping, hanging panel between its legs, the monk like mutzer pulled over its head he looked like some monster from out of space at llandow on a foggy morning at 5a.m.....

Such was his determination!
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mscutt

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PostSubject: Re: Tom Snow's 175 engine   Sat Dec 10, 2011 12:22 pm

Hi Nick and all

Mike Powell put some info on the history of this bike on his thread "Confessions of a 175 race bantam engine - tech info...."

Originally built for Jimmy with the intention of it having a wide powerband and being relatively easy to ride, it was later developed by Mike for a while and ended up as Tom Snow's. As Mike mentions, he reduced the flywheel diameter and found it was not so good so Tom Miller rebuilt the motor with another set of flywheels and reduced the port timings a bit. At one time it was 98 exh and 71 transfer but is now 96 exh and 65 transfer.

In the past we have had higher port timings resulting in motors with great top speed but a relatively narrow power band but since you spend more time accelerating than at top speed it made more sense to go for lower timings and try to get a wider spread of power but now I hear Tom Snow wants more speed!

Mick
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johnSbantam

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PostSubject: Inlet tract length ? TS pipe dimensions   Sat Dec 10, 2011 1:47 pm

Great post Mick and excellent pictures of the neat motor.

Common tuning philosophy suggests the shortest inlet tract length, but somewhere many years ago I read an article by Brian Woolley with a formula for inlet tract length. Do you consider the length for a particular rpm peak power or is that manifold simply to move the carb back from the barrel fins ?

I can see some approximate lengths for Tom's pipe back to the beginning of the middle straight section.
Can you hint at the length to the middle ?
I have had several formula and simulated pipes made with vastly different lengths to the middle that don't seem logical nor work very well, compared with a shortened ye olde "sprint" pipe.
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mscutt

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PostSubject: Re: Tom Snow's 175 engine   Sat Dec 10, 2011 11:06 pm

Hi John

a while back experimenting with the inlet tract length made very little differece to either power output or carburation so I gave up messing about with it but in the last few years something has changed and our engines are more sensitive to such changes. This one is 178mm to the piston, not including the bell mouth in the carb.

Quite why recent engines have become sensitive to inlet length I'm not too sure - it may be just that we have a tighter fitting piston skirt these days or maybe its the exhaust pipe working better.

The pipe gives a good spread of power peaking at 9000 but power remains much the same up to 9750.
From piston to the end of the exhaust stub is 95mm at which point it is 39mm diam.
Cone 1 length 282mm expanding to 55mm diam
Cone 2 length 178mm to 87mm diam
Cone 3 length 95mm to 110mm diam.
Centre section 104mm long
Reverse cone 228mm long to a 23.5 diameter tailpipe

I dont think it has had any wins in its current setup but it was a very close thing at Cadwell Smile

Mick
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PostSubject: Re: Tom Snow's 175 engine   Sat Dec 10, 2011 11:58 pm

As the engine stands now since Tom snow had it I believe it's done 3 meetings and had 1 win at Mallory.
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mjpowell

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PostSubject: Re: Tom Snow's 175 engine   Sun Dec 25, 2011 9:50 pm

Just thinking there is nothing to stop anyone having the same spec motor as this,
as Brian could do you a BTW barrel to this spec (I bet he'd prefer to put more ports in
though) and Mick's exhaust spec has also been published. This is a sure way to get a
22bhp plus rocket ship!!
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Derek

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PostSubject: Re: Tom Snow's 175 engine   Thu Apr 12, 2012 5:14 pm

Hi Nigel thanks for those drawings, what a pitty they can not be posted on our Forum ! may be you could PM them to alan and he could do a global PM to all the people looking in, in one go "im not sure but its very very interesting Im sure Trevor and John would love to see them, I recon actually Trevor may already have seen them as his radii on the transfer to my last reply is so similar I recon he lifted it from them. "Touche"

anyway I thought I would post on this article before the start of the season, reference my objections or disagrement with a number of aspects, in this post.!

Anyway to anser your question which I see no one covered.
nobby1 wrote:
wow.. those port timings alone blow the original cast barrel out the water.. in fact your pretty much wasting your time without the alloy b tw casting by the looks of things...the larger of the two transfers although smaller in width by approx 5mm from the cast barrel is more than made up for by the second set.. two different roof angles can only improve the scavenging and then the increased blow down period.... im looking at a cast barrel sitting in front of me on the coffee table and all i can think is i hope the
table legs can stand the wieght......... affraid

Peter Tibbets 152cc Bantam uses a std cast Iron Barrel, and produces as much as anyone except pehaps Ian and Mark Bikes, well Im sure its within a 1 or 2 BHP, it would be advisable if either Peter or Alan are at Mallory and you having no issues to grab some time with them, certainly give you some usefull advice. !

i think its the the weight issue, people too sencitive to the advantage some of these lightweights have obver normal people, Imean have you seen how light some of these lightweights are like "Mike Mcdonald he should be carrying a 3 stone weight under his saddle, for the likes of others to compete against. !!!

but make sure you have fun on sunday, thats what its all about.

regards Derek
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john bass

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PostSubject: Not only the engine...   Tue May 01, 2012 9:34 pm

With Tom Snow it is not only his wonderful engine -- it is the way he rides.

Some would say I have no right to say that but I can by just seeing two pics of his on this website. Whilst cornering he is tucked into his fairing just as much as he is whilst in the vertical. And I mean tucked in. Much like another pic we had on here of Peter Tibbits leading Mike (The Guy, yewknow!) Powell who whilst cornering are wrapped into their fairings. That is one thing I didn´t attend to closely enough all those yonks ago: keeping the FAST SPEED ON is attributable to optimum, reduced air drag as it is to reductio of too much excess BODY WEIGHT.

Good luck Tom, keep it up -- or rather, down!

Yaaaawwwnnnnn!

CheerS!
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PostSubject: Re: Tom Snow's 175 engine   Thu May 03, 2012 5:57 am

Hi John been asked to Add a bref reply by Tom Snow just to say thank you for your comment on riding style and effort thanks for the encouragement . Tom
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john bass

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PostSubject: Thanks Ian!   Thu May 03, 2012 10:06 pm

Thanks Ian! That was a pleasant surprise! I am usually told, on here, I am Old Hat and that sort of thing but I am well sealed anyway.

Credit where credit is due. You look at my avatar on here and you´ll see my sit-up-&-beg frontal area is large when compared with Dave Hunter (also naked at taht time) so I put some numbers together -- not necessary for on here -- the calculations of which showed the air-drag to be worse for acceleration than the extra weight I was carrying (dieted down to 12stone 3 lbs body weight and wearing thin leathers...!!).

So looking back to the time your Dad fell asleep in my Barking garage I was lacking in the knowledge which I have today.

All the best when you meet up again -- on the battle-track -- with Messrs Snow, Bramley, Powell?, Carcreek and even your Papa!!

Cheers!
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Derek

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PostSubject: Re: Tom Snow's 175 engine   Sun May 06, 2012 4:54 am

my goodness the site's gone to sleep !

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john bass

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PostSubject: No no!   Sun May 06, 2012 5:32 am

No no! The site and Sticky Mick are truly awake.

Cheers!
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PostSubject: Re: Tom Snow's 175 engine   Sun May 06, 2012 7:23 am

actually John spot on I just realised I just missed all the latest and very technically interesting stuff, "helpfull to outsiders looking in too!!!.


I did look at the video off the line ? from Mallory interestingly only one bike making any real progress off the line, well from the angle that is
The rest all seem really fair'ly matched 'off the line', or did anyone else see this.

you know I miss so much these days, so john can you just tell me what I am clearly not seeing !!.



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PostSubject: Tom Snow´s 175 engine...   Mon May 07, 2012 4:07 am

Must be something to do with the gearing. Maybe a very low bottom and using the other two gears with lots of clutch slip and keeping the speed on -- ie hardly using the brakes at all. Saw similar at the last Sunday (2011) of VMCC, Cadwell where everyone else seemed to be standing still with one sprinting away like a robber´s-dog***. Yes I was there! That was after having paid homage to my old army school reunion at Chepstow and a week in Essex. Andy Boyle, my ex-250 ABS sponor, insisited we go and we were just in time for the 1st Bantam race on the Sunday. Smell of methanol in the air had me wishing I had my leg over again....

*** Sickly nostalgia again: reminded of grass track days where I used 1st to get away and then only top and 3rd during the racing -- scrub off the speed by sliding the bends with no braking, of course, very much like being on downhill skis. High Wycombe was fast -- several riders were speed trapped at just on the ton and two at just over -- on my JAP it was 99.8mph. Can be done on a Bantam, in the rain of course, but not quite so quickly ....



How boring -- yaaawwwnnn -- snore!!!


Wake me up in a hundred years....!!!
Cheers!
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mscutt

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PostSubject: Re: Tom Snow's 175 engine   Sun Feb 10, 2013 10:55 am

to make this more complete -

the Inlet timing is 97* btdc

carb is a 36mm Lectron and inlet tract 178mm from piston to where the inner end of the bellmouth begins

the bellouth is about 19m long blending out to 58mm diam.

several larger diameter carbs were tried but they all suffered from flooding. It looked like a resonance problem but as that went away with a 36mm carb we stuck with that.

Mick
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john bass

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PostSubject: Hi Sticky Mick!   Mon Feb 11, 2013 4:58 am

Not the hundred years yet!

Never mind.
I was highly impressed by what you said at Cadwell, Mick! You said something like, bloody heck John this bike is really fast.

Gentleman! When Sticky Mick says that you just pack your bags and go home. Just a silly comment -- but first season back and 175 Champ --- WOW!

All the best in 2013!

All of you.

Cheers!

Yaaaawwwnn! Snore..................................
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PostSubject: Re: Tom Snow's 175 engine   Tue Apr 08, 2014 6:30 am

Just dragging this article back to the top of the tree so to speak. With renewed interest in 175s. Snowys bike had 3 wins out of 6 races in 2014 and set the new Cadwell Park lap record... nuff said..
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PostSubject: Re: Tom Snow's 175 engine   Tue Apr 08, 2014 7:02 am

My barrel has an uncanny resemblance to the ports on this one ,
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ROBBIE

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PostSubject: Re: Tom Snow's 175 engine   Sun Apr 20, 2014 7:17 am

sure it was my barrel lol
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