Race modified 175CS Modello Sport? - MVAgusta.net
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post #1 of 15 (permalink) Old 01-26-2015, 07:28 AM Thread Starter
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Race modified 175CS Modello Sport?

Initially thought (by an MV novice!) to be a Raid 250cc because of its capacity but discounted due to the fact that it was SOHC, the bike I have been assembling from boxes of parts appears to be a 175CS that has been upgraded to 250cc (or thereabouts) and was possibly raced in Rhodesia (now Zimbabwe) in the late 1950s and early 1960s. A note in the bottom of one of the boxes states that the bike had been raced by Gerry Wolhuter, a local amateur racer, who achieved reasonable success in the Southern African region. (He features in at least one race programme for 1959 at the Grand Central Circuit, Johannesburg, riding an MV Agusta 250.) If this is the bike raced by Wolhuter, it was subsequently registered for the road again and bears an old registration number for Salisbury (now Harare). The bike is currently in Bulawayo, Zimbabwe, where Wolhuter lived before he was killed in a racing accident in circa 1960

The machine has a number of parts missing and others are not original, like the silencer and tail lamp.

The enlarged capacity has been achieved by widening the crankcase mouth from the standard 72 mm to 76,5 mm and increasing the bore to 71mm. The latter is a significant increase from the 175CS bore of 59,5mm and I am unsure whether a standard 175 barrel could accept a liner as wide as 71mm bore. A spacer 6mm thick has been fitted under the barrel to increase the stroke and presumably accommodate the non-standard piston. The bike also has a larger 175CSS SS25A carburetor instead of a 22mm item. Other indicators of race preparation are a rev counter drive in place (seems it is usually blanked off) and a blanking plate over the speedo drive instead. The tank has an indentation at the rear top where it is surmised a bump pad was fixed for prone position riding, and modified top fork covers (shortened), presumably to fit clip-on bars.

The Frame No is 417159 and the Engine No 472934 S. Stamped on the barrel and elsewhere is the date 6/56. Crankcases and covers have matching numbers 296.

Hauti has (privately) suggested that extent of the engine modifications may indicate that it was modified by the MV factory, but stamped on the barrel is the bore in Imperial measurements 2,796 (= 71mm) whereas this would be expected to be in metric units if applied by the factory?
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post #2 of 15 (permalink) Old 01-26-2015, 11:13 AM
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A standard 175 cylinder has an extension in the casting that mates into the crankcase, but the standard cylinder could be bored to accept a 76.5 mm diameter liner- the liner would replace the stock cast extension- most manufacturers use just the liner to extend into the crankcase.
Your 71mm piston with the standard 62mm stroke would give you around 245cc displacement. The spacer under the cylinder won't change the stroke- probably done to allow for wrist pin location on the new piston or to compensate for piston crown height, but seems like a big difference.
There are 71mm pistons to choose from, a stock Triumph 650 bore was 71mm, and would have been easily available.
Have you measured the stroke of the crankshaft?
This could have been done by anyone- no reason to assume it was a factory modification.
Rick

Last edited by chip; 01-26-2015 at 11:48 AM.
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post #3 of 15 (permalink) Old 01-27-2015, 04:47 AM Thread Starter
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Thank you, Rick, for your input on the engineering aspects. I will attempt to upload some photos which may assist in illustrating what the modifications look like. Of course the 6mm spacer would not increase the stroke - unless a different conrod was employed......! I had originally written that it was to accommodate a different height gudgeon pin and/or piston crown. I cannot measure the stroke because the crank is disassembled.

The original modifications seem to have been done with considerable skill - but probably still locally in Rhodesia. However there have subsequently been some rough hands on the engine....
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post #4 of 15 (permalink) Old 01-27-2015, 05:10 AM Thread Starter
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and a pic of the bike itself
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post #5 of 15 (permalink) Old 01-27-2015, 07:53 AM Thread Starter
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trying again
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post #6 of 15 (permalink) Old 01-27-2015, 12:55 PM
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That's a very nice motorcycle- hard to find in any condition.
A reproduction silencer is available:
http://www.brezzisamuele.com/en/sile...gp.141.uw.aspx
And it looks like it has this tail light lens:
http://www.ebay.com/itm/Rumi-Formich...c9ce25&vxp=mtr

Just to be sure we're talking about the same thing, the connecting rod length doesn't affect the stroke, it's just twice the distance between the crankshaft centerline and the center of the connecting rod pin- probably 31x2 for a standard 62mm stroke.
The 71mm piston will probably be heavier than the stock piston, so you'll want to be sure the crankshaft was rebalanced to compensate.
And, a 25mm carburetor is on the small side for a racing 250cc engine, 27,29 and 30mm units were used on street 250s- I've seen 35mm carburetors on race bikes- SSI carburetors bring a premium price on Ebay.
Hope to see more pictures of your bike.
Rick
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post #7 of 15 (permalink) Old 01-28-2015, 05:08 AM Thread Starter
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Thank you again, Rick. The repro silencer looks superb but I am not sure that my bike should not have a roundish tail lamp? I have not researched it as yet.

I have certainly been getting my wires crossed on 'strokes'.........! Not thinking clearly..... Incidentally although the 250 piston is undoubtedly heavier, the conrod has been quite substantially lightened but has, I guess, been up to the job until now. Luckily I have an auto engineer mate who is something of an expert on crankshaft balancing, so will ask him to check when the time comes.

A friend recently sold several of the bigger Dellortos, including the 'baby' SS25, on eBay UK and the price they fetched was fairly staggering.

Meanwhile I have several collectors in South Africa keen to purchase the machine - sight unseen - but really don't want to sell and unsure as to its worth anyway.

Peter
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post #8 of 15 (permalink) Old 02-06-2015, 06:46 AM
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hi all..
this is wonderful to read, i have ideas of trying to do this . 250 cc would be nice .
Did anybody know what piston the use? i have an idea of taking a piston without dome. and weld the combustionchamber to increase the lost in compression , i have read about someone that the conrod broke
and a new piston will be heavier , so i thohgt about a newer light one,, i saw a picture om meccanicha vergera group a modified crank with a carilloconrod ,, so i think with modern part it will work..i think it is phil hitchcocks crank
35 mm intake valve and 32 out, i was measuring and it looks like 73 mm in piston diam was possible to use, a lot of modern 4 stroke Hondas use 73 mm ..so maby its possible to use these,,
wonderful bike you have Peter ..
kind regards Åke
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post #9 of 15 (permalink) Old 03-22-2015, 11:02 AM
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What a great find. I would be wary of building a 250cc motor using the stock connecting rod but 203cc (bored out) MV 175 CSS motors were very successful in racing in the UK... Just ask Mike Hailwood.

That is a 175 Sport as you know, confirmed by the long saddle and the front fender having the extra braces. The tank is 1956 or later and the motor date stamping indicates 1956 also.

The added cylinder height possibly conforms to exactly the dimension of adding 2 links to the cam chain. This can work as noted to accommodate a longer rod, longer stroke, or taller piston. But MV 175 motors already have good rod length to stroke ratio. The Italians knew what they were doing, and while fragile, these motors were fast from the factory. I would guess the owner wanted a 250 "advantage" for his racing and matched up all the parts needed to make it work.

It is lovely and will make a nice machine for you with a little work.

Best,
Ivan
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post #10 of 15 (permalink) Old 03-25-2015, 06:25 AM
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Hi Ivan
on your moto leggera side there is a picture of a crank with a carillo conrod..
i have thought about that,,
i was sending a mail to you about Squalopistons. Former Precident of mv club uk Alan elderton
did a batch of squalopistons 63 mm and 65 mm 65 mm do 203 cc
is anyone know if its possible to make a batch of these again?
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