Etz 250 head change to 300

[Maj333]

Hi all
I have a 1988 etz 250, I have a 300 barrel, head and piston and looking for any advice on fitting and is there anything I need to know adjust etc.
I am sure this question has been asked before but I can’t find anybody who has attempted this??
Thoughts and advice welcomed
Thanks guys!

[Blurredman]

As someone who has done this.. I can confirm that it 'can' be done.

However. I have an over-bored 250 barrel, rather than the dealer modified type that was sold, and also not the original 300 that MZ supplied in the last years..

Can you confirm?

[Puffs]

Hi Maj333,

I have no experience with this, but if I remember correctly, there are 2 types of 300 barrels: the ones with a sleeve (as sticking out from the base of the barrel) that fits into the standard 250 crankcase, and those that are slightly bigger. In the latter case work is required. Take the barrel off & have a look. I do not know the ID of the standard 250 crankcase, maybe someone else does.

The 250 & 300 have an identical stroke & crankshaft, but the 75.5mm OD piston of the 300 will also exceed the ridge at the top of the standard 250 barrel, which fits snugly inside the head. A 250 head also requires work if the barrel is bored to that OD.

[Maj333]

As someone who has done this.. I can confirm that it 'can' be done.

However. I have an over-bored 250 barrel, rather than the dealer modified type that was sold, and also not the original 300 that MZ supplied in the last years..

Can you confirm?

The guy I brought it off says it’s a original 300 barrel and head and should fit straight onto the 250 (with new gaskets) the piston and rings came out of that engine, there is a slight scorch mark beyond the rings on a small section of the piston but generally in very good condition with no polished areas (Will rehone with new piston at some point)
The barrel visually is spot on with no marks so the plan is to fit it and see what happens? Possibly the carb might need adjustment but is a upgraded one.

[Blurredman]

All I can say then is this:

Compare the barrel and head of the 250 with the 300. Does the machining of the sleeve measure the same and fit in the same way?

If it is an official 300, I am unaware whether the piston ingresses into the head or not.. Never seen one. As Puffs said, if it does ingress into the head then spacers/thicker head gaskets need to be used.

See if the piston lines up with the ports when at BDC.

If the piston clearance is fine and condition is good. Why bother changing it?

[Maj333]

Thank you for all your comments.
I have now done the change over and it has gone really well! A lot more torque with less gear changes, going to look at the gearing now and perhaps try a 18T front sprocket to see if I can bring the revs down a little bit on long run outs.

[Blurredman]

Ah good. Glad it went well for you.

Coincidentally I have an 18 sprocket and find it a little short actually. I might be putting the standard 19 back on..

[Puffs]

While I believe the rear sprockets & gearbox ratios are all the same, the size of the rear tire has quite an impact too: bigger tire --> bigger circumference = more distance per rotation (& v/v).

You guys with those 18" rear wheels have smaller front sprockets than the 251 with it's 16" rear. For info: for my 257cc 251 with a 110/90-16" rear I landed on a 20T front sprocket, and I believe original was 21T (@ 3.25"x16").

Raising the stroke volume to 291cc should indeed give more torque & allow a taller gearing = lower revs = bigger front sprocket.