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Discussion Starter #1
Hi Guys
I am restoring a 53 speedtwin and have gotten into opening up the motor and everything is worn out. The main issue is the crank and babbit rods which both have excessive wear. Being that the set up does not use shell bearing rods on the big end, I have 2 choices.

1. Have the crank turned down and have the rods sent out to have oversize new babbit installed and machined to fit the turned down crank. I have talked to some old ford guys and this is the process they use.

2. Buy new standard size rods and have the crank journals welded up then turned to stock spec.

I was told if it was a 650 that you could modify a latter crank to fit but since the stroke on the 500 & 650 are different this is out.


Has anyone else been through this ?

thanks
 

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Hi Guys
I am restoring a 53 speedtwin and have gotten into opening up the motor and everything is worn out. The main issue is the crank and babbit rods which both have excessive wear. Being that the set up does not use shell bearing rods on the big end, I have 2 choices.

1. Have the crank turned down and have the rods sent out to have oversize new babbit installed and machined to fit the turned down crank. I have talked to some old ford guys and this is the process they use.

2. Buy new standard size rods and have the crank journals welded up then turned to stock spec.

I was told if it was a 650 that you could modify a latter crank to fit but since the stroke on the 500 & 650 are different this is out.


Has anyone else been through this ?

thanks
First, have the crank journals ground, not turned.. However, I thought that the stroke in the 650's and 500's were the same ? and that the 650 Thunderbirds's were obtained by boring out a 500
Cetainly from about 1956 the parts were interchangeable, and a few 500's ended up as 650 sheep.
 

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53 speed twin

You may also want to post this question to www.britbike.com as there are more old pre unit guys that have discussed this topic before. You can probably do a search on that topic. Has always been very informative to me in the past.
Good luck,
Cliff
 

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Discussion Starter #5
The rods are the same between the 500 & 650 the throw on the crank journal and the bore is diffrent. From what I have been told you can not bolt a 650 preunit top end to a 500 lower end, or I would just put in a newer 650 crank and shell rods.

I did score a set of NOS 500 babbet rods, still in the wax, so that solves that problem. The crank plan is to have the bad journals hard chromed up to 3,000 over then take to a crank shop and have them put it back to stock spec. You can not get oversize rods anymore and the rod journals are scored bad so they need built up.

I talked to a few guys and this seems the way to go. Now I need some cams,tapps & blocks. Always something
 

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The rods are the same between the 500 & 650 the throw on the crank journal and the bore is diffrent. From what I have been told you can not bolt a 650 preunit top end to a 500 lower end, or I would just put in a newer 650 crank and shell rods.

I did score a set of NOS 500 babbet rods, still in the wax, so that solves that problem. The crank plan is to have the bad journals hard chromed up to 3,000 over then take to a crank shop and have them put it back to stock spec. You can not get oversize rods anymore and the rod journals are scored bad so they need built up.

I talked to a few guys and this seems the way to go. Now I need some cams,tapps & blocks. Always something
Yes the hard chroming is the way to go to restoring the crank....
I know the bores are different between the pre unit 500 and 650's but believed the crank throw to be the same... It was on the later post '56 bikes.
 

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Additional information

HI
Today whilst searching for information of a 350cc engine I came across this site. http://www.alpha-bearings.com/index.htm
from it I learned that the big ends, main bearings, small ends for the 5t's and 6t's are the same. I am sure they used the same crankcases and crankshafts... I do know of one 5T that had a 650 barrel fitted to it back in the early sixties. Dagad would be the one to ask. The original 1946/7 650cc Thunderbird was a bored out 5T.
 
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