Bsa Bantam Engine Mods

[higgo]

Hi all, i have a friend who is without internet access, and is currently building a Bantam, he now has a rolling chassis, and the next stage of the build will the motor, without having to take a second mortgauge out what would be the best mods he could do to the motor. ( 187 cc big bore ) And are there any web sites where i can print info off for him. Many thanks HIGGO.

[smelling123]

Hi Higgo,

Ive taken a lot from Trials Central forums so I think its time I give some back - hope this helps...

You can have a competitive motor without spending much. The 185 big bore piston helps but remember to make sure the head is machined to match the new bore (squish gap should be set with gaskets at about 0.75mm/30 thou"). Compression ratio is fine between 10:1 and 12:1 - 12 will make more power but 10 will be a bit 'softer'. You can get a cheap suzuki TS175 piston off ebay for about £30 but you need to make a phosphor bronze busg for the little end as the gudeon pin is a different size to the Bantam. You also need to shorten the Suzuki piston to stop it hitting the crank wheels at BDC (shorten carefully with a hacksaw and tidy with a file to the same skirt length as the Bantam). An easier option is to buy a big bore piston from Rex Caunt racing (they are on ebay) - think they're about £45 but are built to suit and have a needle roller small end bearing.

The iron barrels are fine for power (but heavy and run hot) either D14 or B175 but the ports are a bit smaller on the D7. Ive raised the transfer ports on mine but it isn't noticably better. Buy a smaller primary drive sprocket (again Rex Caunt) to gear the engine down - when this is done the clutches can slip so also need stiffer springs.

There are several ways to improve the clutch action - the main problem is that the cable gets full of mud and the clutch action goes heavy. I will post a picture of what I think is the most simple method.

Change the gears over (2nd and 3rd) - this is FREE and gives a useable 2nd - see previous post from a few days ago.

I find the standard amal carb is good, some use Mikuni. I also fitted my own boost bottle which helped tickover and really made the low down response crisp.

A good ignition helps - the ratio of engine revolutions per kick of the kick start is less than other bikes so most Bantams seem to need a good kick to start (and a good flood of the carb).

I haven't seen anything radical in any of the alloy barrels ive seen - so I haven't bought one yet but they are a LOT lighter and will no doubt run cooler.

If I think of anything else I will re-post.

cheers

Stu

[martin 305]

Hi Higgo all Stu said plus I fitted an 8 spring clutch to mine as action was heavy with up rated springs , this made clutch slip a thing of the past and you can use it all day without tiring ! Also fitted a primary chain tensioner as the 12tooth primary sprocket is a pain to get the tension right even with a half link both these are available from RCR For not a lot of money Might pay to have the compression plates welded to the crank webs as a friend had a rivet come away and wrecked the bore and piston ! Just done both of mine Cheers Martin

[smelling123]

Where'd u get the 8 spring clutch plate Martin - Rex? Think I need one.

Agree on the tensioner, I made mine but not worth it given the price to buy - rattle like hell without it (and snatchy).

Also should have added, always fit new main bearings on a rebuild - because the crank seals are inboard of the main bearings, the lubrication comes from the gearbox oil and im not a fan to be honest - there's no flow/return it just relies on some being flung down the feed holes.

[andy m]

I have a Rex Caunt big bore piston in mine and had the barrel Fahron tuned for trials use which gives it a bit more grunt when needed also used Electrex world ignition. You could try to find a centre plug head which helps.

Alternatively there is a rare B175 engine for sale on the classifieds page.......

[martin 305]

Where'd u get the 8 spring clutch plate Martin - Rex? Think I need one.

Agree on the tensioner, I made mine but not worth it given the price to buy - rattle like hell without it (and snatchy).

Also should have added, always fit new main bearings on a rebuild - because the crank seals are inboard of the main bearings, the lubrication comes from the gearbox oil and im not a fan to be honest - there's no flow/return it just relies on some being flung down the feed holes.

Hi Stu got the clutch from peter savage just google RCR villiers about 45 quid very well made and as you say primary tentioner is not worth making as it cost 15 quid and smooths the bike out quite a bit

[higgo]

Thanks very much chaps for your input, i will relay all this to my mate.