TL125/150 swap

[solvent3]

Few weeks ago i managed to get hold of a TL125 engine with a 150 kit already fitted. Dropped the engine straight in and away we went. 1st gear is fine, but whenever i shift to 2nd or above the bike is jumping out of gear.

Rather than stripping down the box on this engine i thought i would swap the top end off the 150 onto the bottom of my 125. Is this as straight forward as i am thinking or is there anything i should look out for.

the 125 engine originally had the 122cc cylinder and the 150 one was the 124cc.

thanks.

[craig10]

Yes it's do-able, but watch out for the following:

The 122 had a one piece head with a 100 link cam chain and 'longer' tensioner blades as the centre line of the camshaft is higher than on the 2-piece head which was fitted to the 124 engine.

The 2pc head 124 engine had a 98 link cam chain and 'shorter' tensioner blades.

I'd be tempted to just swap the head, barrel and piston complete with the cam chain and tensioner blades from the 150 onto the 122. If you just swap the barrel and piston you'd have to check the piston to valve clearance to be sure that there's no danger of valve to piston contact with the different head. Also the combustion chamber on the 122 head is quite small (never seen a TL 2pc head so don't know if it is also small) so it may push the compression ratio too high with the 150 piston.

Might be worth checking that there's not a wear ridge at the top of the 150 barrel as the top of the stroke on the 122 engine may be in a very slightly different place due to production tolerances on the crank and casings. Easy enough to check the piston 'deck' height before you remove it from the 124 against the height when fitted to the 122.

Your piston rings wouldn't thank you for clattering into a ridge several thousand times a minute.

Edited November 26, 2010 by craig10

[solvent3]

Thanks for the reply.

Yes the idea is just to use the box off the 122cc and transfer everything above from the 150.

thans for you response i will look out for what you said.

Yes it's do-able, but watch out for the following:

The 122 had a one piece head with a 100 link cam chain and 'longer' tensioner blades as the centre line of the camshaft is higher than on the 2-piece head which was fitted to the 124 engine.

The 2pc head 124 engine had a 98 link cam chain and 'shorter' tensioner blades.

I'd be tempted to just swap the head, barrel and piston complete with the cam chain and tensioner blades from the 150 onto the 122. If you just swap the barrel and piston you'd have to check the piston to valve clearance to be sure that there's no danger of valve to piston contact with the different head. Also the combustion chamber on the 122 head is quite small (never seen a TL 2pc head so don't know if it is also small) so it may push the compression ratio too high with the 150 piston.

Might be worth checking that there's not a wear ridge at the top of the 150 barrel as the top of the stroke on the 122 engine may be in a very slightly different place due to production tolerances on the crank and casings. Easy enough to check the piston 'deck' height before you remove it from the 124 against the height when fitted to the 122.

Your piston rings wouldn't thank you for clattering into a ridge several thousand times a minute.

[tltel]

Hi

Before doing all that work, take the clutch cover off the 150 engine and check the spring arm and roller are engaging on the end of the selecter shaft. ther is a star on the end of the shaft, you can see it move when you select a gear,the small roller engages in the star and holds it in gear.

sometimes the arm comes loose and stops it working. if my memory is correct you can see it with the clucth drum in place but you cant get to the 10mm bolt without removing the drum. still easier than stripping two engines and you will still have a spare.

Hope that helps.

TLTEL

[bunyan]

i have an 1974 engine and i tried fitting a 125cc barrel to it and it didnt fit. the hole in the cases is smaller than the later engine. i now have 1976 cases.

[craig10]

i have an 1974 engine and i tried fitting a 125cc barrel to it and it didnt fit. the hole in the cases is smaller than the later engine. i now have 1976 cases.

Thanks for that Bunyan, I'll check that out when I next open the CB 122 motor in my XL. Just checked here http://www.powersportspro.com/pages/parts/viewbybrand/7/Honda.aspx, the part nos. are different for the cases on the TL from the CB / XL 122cc models.

Some good stuff here http://www.users.globalnet.co.uk/~worsfold/TLpage1.htm

Edited December 3, 2010 by craig10

[tlrider]

Hey guys,

Regarding the 122cc and 124cc top end swap here is what I have found.

I have a 166cc engine where a 200cc (192cc) cylinder is shortened and put on a TL125 'K' bottom end (122cc). I'm building another and want to put the two piece head from the 76 'S' model on it (124cc). It's going in a Fraser frame so I want the two piece head.

My modified 192cc cylinder and the 124cc cylinder are the same length at 2.677" (68mm). I don't have a 122cc cylinder, can someone measure for me?

The 124cc cylinder OD is 2.455" (62.35mm)

'S' case ID for cylinder is 2.5" (63.5mm)

unmodified 'K' case ID is 2.515" (63.88mm) Bunyan, I can't explain why yours didn't fit, manufacturing variances?

Combustion chambers visually are the same size between K and S heads.

Cam chain and tensioner: craig10 is correct in the 124 two piece head has a 98 link cam chain and a shorter tensioner blade due to the cam shaft lower in the head. However, the adjuster in the bottom end is higher than on the K engine.

Using the the 124cc S head with a 122cc length cylinder (needs verification as stated above)and the 98 link S chain, the S tensioner is too short. With the adjuster all the way out I can not get enough tension on the chain.

I replaced the original longer K tensioner and with the adjuster all the way in, bottomed out the tension is just about right without gaskets. I would prefer it to be a bit looser at full adjustment.

Once the engine is together with base and head gaskets, if the chain is still too tight I plan to remove the adjuster screw and remove some material between the threads for the case and the shoulder for the o-ring. This will allow the adjuster screw to go deeper in the case elongating the tensioner.

I found the 100 link chain to be one link too long for this combination.

I hope this is not too confusing.

Revision: Upon further inspection the lower adjuster pivot points are in the same place. The S tensioner is shorter than the K but they do not have the same radius leaving the 98 link chain too slack. The slack is taken up on the S engine with the changes to the guide blade opposite. It pushes the chain closer to the centerline of the cylinder taking up the slack.

Edited December 6, 2010 by TLrider