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650 or 750 crank?


conqueror
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Hi guys. Tom from the forum brought up a new issue I hadn't thought of before because I just assumed it.

 

The cylinder heads off of my engine are the same ones as those of a 750, but does the crank itself look like it was manufactured for use in a 650 or 750 engine? What might be the repercussions of running a 650 crank in a 750 cc engine?

 

I went to Oldtimer and compared my pictures to the pictures he had of 650 and 750 cranks for sale (mine are the last 4 pictures)

 

I know the angles aren't too good, but these are the only pictures I have of the crank.

 

http://www.oldtimergarage.eu/store/product...roducts_id=2443

 

http://www.oldtimergarage.eu/store/product...roducts_id=1150

 

 

 

 

 

Vasile

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Vasile,

 

Putting a 650 crank in a side-valve 750 can be done, but it is difficult. If I remember correctly the bearing diameters are the same, but the oil seals are completely different. And the 650 connecting rods are wider than the 750 ones.

 

There was a guy in Moscow who was modifying 650 OHV blocks to take the 750 cranks, both for the extra displacement and because the 750 cranks don't break as often as the 650 ones do.

 

Putting a 650 crank in a 750 block would not make sense on several levels. Of course, if a guy out on the steppe somewhere had a broken 750 crank but could only find a 650 replacement crank, who knows what could happen.

 

If you did have a 650 crank with standard 650 pistons, they would be 10mm too short and the motor would have a very low compression ratio. It would run, but never produce much power. But if someone got inventive enough to swap in a 650 crank, they may have found some piston combination to restore the compression height.

 

I think you have a 750 crank. Lysek's photos are a bit misleading. The 750 crank should have the same round oil slinger the 650 does. It is missing in his photos; perhaps he expects you to re-use your old one.

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Vasile,

 

Putting a 650 crank in a side-valve 750 can be done, but it is difficult. If I remember correctly the bearing diameters are the same, but the oil seals are completely different. And the 650 connecting rods are wider than the 750 ones.

 

There was a guy in Moscow who was modifying 650 OHV blocks to take the 750 cranks, both for the extra displacement and because the 750 cranks don't break as often as the 650 ones do.

 

Putting a 650 crank in a 750 block would not make sense on several levels. Of course, if a guy out on the steppe somewhere had a broken 750 crank but could only find a 650 replacement crank, who knows what could happen.

 

If you did have a 650 crank with standard 650 pistons, they would be 10mm too short and the motor would have a very low compression ratio. It would run, but never produce much power. But if someone got inventive enough to swap in a 650 crank, they may have found some piston combination to restore the compression height.

 

I think you have a 750 crank. Lysek's photos are a bit misleading. The 750 crank should have the same round oil slinger the 650 does. It is missing in his photos; perhaps he expects you to re-use your old one.

 

Ok that's a relief! Thanks again kugelbake. :thumbsup!:

:cheers:

 

Vasile

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Putting a 650 crank in a 750 block would not make sense on several levels.

 

But putting a 750 OHV crank will make a lot of sense hence the market is flodded with beautiful chinese quality parts. I already have an obsession about how to do that...

 

I dream about a "bullet proof" M72 engine with NEW parts.... :thumbsup!:

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You have a regular 750 crank repeteadly hit with a hammer, which is not a good thing (picture 3).

 

AHH!!! Damn it! damn romanians always fixing things with hammers :thumbsup!: I know I'm guilty of that! :cheers: Thanks valicaddy, now I for sure have to replace the crank

 

Vasile

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You have a regular 750 crank repeteadly hit with a hammer, which is not a good thing (picture 3).

 

AHH!!! Damn it! damn romanians always fixing things with hammers :thumbsup!: I know I'm guilty of that! :laugh: Thanks valicaddy, now I for sure have to replace the crank

 

Vasile

 

No, you don't. Take it off and then you will see exactly how good it is. :cheers:

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You have a regular 750 crank repeteadly hit with a hammer, which is not a good thing (picture 3).

 

AHH!!! Damn it! damn romanians always fixing things with hammers :thumbsup!: I know I'm guilty of that! :laugh: Thanks valicaddy, now I for sure have to replace the crank

 

Vasile

 

No, you don't. Take it off and then you will see exactly how good it is. :cheers:

 

I just have to find a place that can test it's balance, do you know of anywhere like that in Sibiu or maybe Valcea or Cluj-Napoca?

 

Vasile

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I will ask a friend from Dej. Don't have any ideea for now. If he has time, he can help you, he has the know-how FOR SURE!

 

Ok thanks! But he's probably going to have to talk with my cousin (Daniel, the one you gave the flywheel to), and I'll let Daniel know what is going on. Since I'm not in Romania right now and I really don't know when I'll have the next chance to go back.

 

Vasile

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