Czech Motorcycles

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CZ 125 Type 476 & Type 488

150cc Conversion

An article by Andy Reid

July 8th 2015
panzergranate at hotmail dot com


I was working on my son's CZ 125(150)-488 toady with him and it is still in everyday use since your last E-mail in 2007.

Updated changes to the engine:

The guy who gave my son the bike on his 18th birthday milled 3.9mm. off of the top of a 175 cylinder and hand cut a new head gasket groove / indent into the new liner top.

The piston has had the crown indented so that the exhaust and inlet ports (which close too early by 3mm. at BDC) open correctly. Indenting the piston was the old traditional way of porting 2 strokes in the 1950's and 1960's for racing by Jawa and CZ. It is simply cutting in a ramp rather like a dropped pavement curb. It allows for porting changes to be made by simply swapping pistons, as opposed to whole cylinder blocks.

The cylinder head was a 175 item but has been replaced with a 125 head. This does mean that the bike will diesel when hot with the key pulled sometimes. Compression is 13:1.

The engine has been rebuilt into a 1968 CZ 175-477 set of cases and gearbox, fitted with a new 125-488 crank after the original engine suffered a blown left hand oil seal and I spotted stud thread damage cause by some ham fisted previous owner at sometime in its history.

This engine rebuild has been running for 4 years now.

I'm riding a 1977 CZ 350-472-1 fitted with an 80,000 mile old 1974 Aprilia built CZ 350 engine. This engine was popped open for inspection in May 2015 and had no signs of wear yet. All the original Japanese Koyo and Stanley bearings were good, so it was put back together again and refitted.

However the original Asso Werke pistons and cylinders were worn past tollerance and I'm planning to have a rebore and fit the new set of 2nd oversize Asso Werke pistons, rings and pins kit I have for it.

Compared to the lower mileage (15,000 mile) CZ 350-472 engine, the Aprilia version goes like the clappers as it picks up revs so quickly, despite running on a set of Jawa 350-632 pistons and cylinders temporarily.

The Aprilia built 350 motor comprise of a 250-623 gearbox and cases fitted with a 350-633 Bizon crank, Asso Werke high compression forged pistons and Koyo and / or Stanley low friction race grade bearings throughout. Because of the 350-633 Bizon crank the engine has an oil pump drive slot on the left so I do have the option of converting it to Posilube, something that CZ never did for production (The CZ 350-472-2 Posilube version was only a single prototype). However the oil tank would replace the tool kit panel, something that I'm not happy with.



May 20 2007
panzergranate at hotmail dot com
I recently tried out an interesting experiment to prove a theory that I could convert a CZ 125 into a CZ 150 and it works OK.

You will need a CZ 125 Type 476 or Type 488 engine obviously.

You will also need the following parts.:-

A CZ 175 piston and cylinder block.

A CZ 175 gudgeon pin (wrist pin to anyone in the US!!)

Four CZ 175 stud bolts.

(1) Remove the 125cc piston, cylinder block and stud bolts. These can be discarded.

(2) Fit the 175 piston and gudeon pin.

(3) Fit the 175 cylinder block with either a new gasket paper or silicon rubber (more 
effective). You will observe that the piston travels up the cylinder block until 4mm. 
before TDC and travels down the cylinder until 4mm. short of being level with the bottom 
of the exhaust and inlet ports. This is not that serious as it will run with this.

(4) Fit the 125 cylinder head, not a 175 cylinder head.

(5) Tickle the carb and kick start the engine into life.

The bike is now 153cc.

The bike we converted ran up to 59 MPH with a JIKOV 2924 carb.

The engine hits a power restriction at 15 MPH in 1st gear and will not increase speed. This is due to the exhaust and inlet ports closing 4 mm. too early, limiting intake duration.

However the bike does cruise more easily.

So far I've lifted the inlet ports by 4mm. This took 3 hours with various grinding bits and a power drill. This bossted acceleration but top speed in 1st gear was still limited to 17 MPH. I plan to lift the exhaust port by 4mm. to fix this problem.

I suspect that the early 1970's Aprillia assembled CZ 150 used Asso Werk piston which may have had cutout ramps in the piston crown as in the Type 559 JAWA singles of the 1960's. Only Aprillia made a 150 version of the CZ single under licence, CZ didn't bother.

This is a simple big bore conversion for any CZ 125 single.


I tried another cylinder 175 on the CZ 125 and this time only lifted the exhaust port by 4 mm. using a reverse cone/frustrum stone in a power drill working from 
the top of the cylinder to take off a 45 degree chamfer to 4 mm. of cylinder height.

Also I discovered that the Type 488 engine had a CZ 175 head fitted by the previous owner after comparing it to a nice Type 476 CZ 125 head I have in the personal stockpile here.

The bike now pulls 20 MPH exactly in 1st gear before hitting a wall and needing a change up into 2nd gear.

The 153cc (58 x 58 mm.) engine has noticefully more bottom end and midrange stomph thanthe 125cc engine did though so far top speed hasn't been tested yet. It should perform somewhere in between the CZ 125 and CZ 175 it is expected.

Carburation runs a 98 main jet but with a 50 pilot jet through a foam replacement airfilter. Jetting to the 98 mainjet from a CZ 175 is a common mod when fitting a K & N or foam airfilter to a CZ 125 anyhow.

The exhaust system is still stock apart from the standard modification of removing the large end cap stanction collar to eliminate the flat spot and seal leak in the rev range.

The bike sounds and runs like a CZ single should.

Anyone else attempting this big bore conversion will need a CZ 175 piston, rings, cylinder and studs.

Also a JIKOV CZ 175 main jet/carb and the 50 pilot jet from a CZ 125 carb. The god awful AMAL carb is usually ditched over here and replaced by a JIKOV anyhow as the bike is 20 MPH slower on the AMAL carb.

Note that post 1985 bikes in the UK have the AMAL fitted usually on Type 488 and Type 487 CZ "CEZETA" singles.

OK that's all for now on the conversion so far.

By the way, it is the annual JAWA/CZ owners club of Great Britain's national rally on the& 15th, 16th and 17th of June 2007 iat the Cooper's Arms pub campsite, Pewsey, Wiltshire.(Near Salisbury). It usually attract hundreds of JAWA, CZ and the occasional sad MZ rider, Cossack owner or Voshkod pusher.

For details check out the club's website.


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