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Discussion Starter · #1 ·
I was going to post this question in JohnG's current thread with his T140 rebuild, but didn't want to hijack anything, so here is my question, kind of in reverse from his. I have heard mentioned several times that the T140 inlet cam is recommended in the T120 650 motor.

What would be the advantages/disadvantages of installing a T140 inlet cam over the stock cam for my 70 Bonnie, keeping the T120 exhaust? The bike will be a daily runner, so I am after smoothness, torque and reliability over top end or peakiness. From the response to JohnG's questions, I assume it would be a direct drop in.

Meanwhile, the restoration is gaining momentum. Although I am not ready to split the cases yet, the top end of the motor looked remarkably good once I cleaned the carbon and accumulated dirt off from years of storage. I am still waiting for my new bore gauges and larger micrometer to arrive, but the bore and pistons are in good shape, even though they will be replaced with a 0.010 rebore. Compression ring gap was 0.080" when inserted into the bottom of the bore, so there is no question they were well worn. However, there is almost no ridge at the top of the cylinder, and the bores look good. The tappets also look very good, with only a few light scratches, and the cams are smooth as far as I can see until I actually split the cases. The motor has only somewhere between 15-20K miles since new (the speedometer frequently didn't work), with regular oil changes. There is no detectable play in the pistons/wrist pins when I wiggle or try to move the pistons. Meanwhile, I have every bit of the frame apart, and am starting to clean those pieces up, to get ready for stripping and painting.

My general strategy for now is to focus on the framework for a while. Once I have the frame painted and all bolted together with new or replated nuts and bolts (still a major problem), then I will come back to the motor. No sense having the motor together, only to sit around waiting for the frame. Meanwhile, I will order all the motor items I know I need, such as seals/gaskets, bearings and new pistons.

Anyway, is there much to be gained by changing the inlet cam, or is the stock item pretty good for all around use? I will of course keep the exhaust cam stock. As a related question, if I have the tappets cleaned up, do I have to worry about the shop grinding down past the heat treated layer? I have heard MAP mentioned to do this service - would they be the best place for this (in the US at least)?

I will follow up with good photos once everything is moving along. Right now, there is lots of planning and studying, but not much "doing" to photograph.
 

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Hello upstatetrophy,

It seems like your post has falled between the cracks. I do
not know the answer to your question. But I do know
that someone on this page has the answer to the 140 intake
cam into a 120 engine on the intake side. If no one answers,
you can also try Britbike forum, a pretty good page, you can
do a search there, not the best search engine though. I know
this topic was covered over there in the past.

Pookybear
 

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Discussion Starter · #3 ·
pooky,

Thanks. I'll follow up your suggestions. It is just that I have been told twice now that the T140 inlet cam works well in the T120 (as does the T120 exhaust in the T140), but I'm curious what the tradeoffs are.
 

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There is a noticeable power increase using the T140 intake cams but as with any performance modification there is a trade off you lose a little on the bottom end and pick up a little on the upper end, it may or may not be enough of a loss for you to notice. As for the lifters if there is a noticeable groove worn in the face replace or reface them, map is the only place i have heard of that offers this service, that hard face is actually stellite and if too thin on the edges can chip and that will make a mess of your cam. i usually replace them but for the price these things go for now i would look at refacing them if needed. Hope this helps, Gearhead
 
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