Hi Folks,
I know this subject has been brought up in the past. Thought I'd share some findings with the compatibility of the 2 items.
The top end was apart on my 351 equipped KLX, so I checked out the valve clearance, with the cam timing modded.
Did not have a spare head gasket, so I used some aluminum shims that matched the approximate thickness of a head gasket(.037). With a spot of clay in each valve cutout, and running the engine through a few revolutions, I measured about .075 clearance.
JE pistons recommends .100 vertical clearance, so this mod may not work/or be safe.
Personally, I find my 351 equipped KLX to chug along nicely with stock cam timing, so it may be a moot discussion. I've been caught in the wrong gear, climbing a hill, and it surprised me how it pulled without protest.

 

Did you, by chance, also measure with stock timings?

Two members did run the MCM with BB351 kits - both successfully - with one admitting to lots of redline operation with the combo..

 

Maybe the shape of the replacement piston takes up some of the clearances? I contacted Bill about doing the MCM with his kit and said simply don't do it, engine damage will occur. I know someone on the forum did the MCM on his BB but reported very little gain.

 

I did not measure in stock timing form, Klxster.

 

I think that the piston is the difference here. Specifically, the valve reliefs must be machined with different dimensions because the deck height of the pistons should be practically identical. So, changing bore size is not responsible for any piston/valve interference it would be because the reliefs cut into the new piston are not positioned correctly.

 

I personally have no preference regarding MCM on a 351. But this subject has continually reappeared over the years in here. Last time it did, I researched the subject - even heard from one of the old members who ran the combo - David R ran the combo hard, lots of redline operation with no issues. jhoffy22 ran the combo, (in his words "+5" hours) with no issues. I am sure someone will point out that jhoff22's engine did fail - the reason was not attributed to MCM..

We know MCM radically enhances TRQ below 6500 rpm and pretty much equals TRQ above 6500 rpm when compared to stock timings. In layman's terms, the engine breathes better below 6500 and almost as well above - bigger pistons will only increase the these positive effects.

The proper choice of mods for a given owner is actually complex. Exhaust headers/slipons selections will tailor power curves, MCM tailors power curves, Airbox/Carb setups tailor power curves, etc etc..

I want the best of both high RPM and low RPM power that I can make - so I run "high RPM power" on my exhaust selection and airbox/carb setup and "low RPM power" by employing the MCM.. This balance has shown to make amazingly broad enhanced power curves..

 

When I did the MCM on my stock motor I was amazed at the performance gains. I ran the MCM for a summer and was very happy with the results. After I did the 351 I went back to stock simply because I couldn't find anyone who ran the MCM long term. I didn't want to chance an engine failure after spending the cash for the BB kit.

 

I would venture to say the clearance will depend on where the piston is as the valve opens to the max value, which is based on the cam peak. With the MCM the exhaust and intake are on the peak around 10 degrees nearer TDC. So that would mean the piston will be a bit closer, but how much I cannot say. Just pointing out there is a bit of difference.

The discussion in the MCM thread had some comment about it, but the rider who commented was mistaken, he was thinking of the non-interference designs he had been dealing with - Chrysler turbos. I had one - broke a belt, no valve contact. Not so on the KLX. Any cam timing change will change the clearance.

 

As I've said before, you can't blame Bill Blue - Imagine what you'd say to a customer who is asking about "monkeying around" with cam timings.. I certainly would just say "no"... What is important is that he did say - in the MCM thread - that valve-to-piston clearance is same as stock with the 351 kit.. Anyone can call him to verify stock clearances.

Stock clearances means MCM safe - Bill says they are stock, David R and Jhoffy22 ran the combo without issues..nuf said?

 

I guess if I was running the 351 kit, a full exhaust system is a given - no way would I want my 351 trying to pump exhaust outta that tiny stock header. Likewise, I would not want my 351 trying to suck air through a snorkel or even a hole in the lid.

I'd assume the stock ports and the stock cams are going to have a hard time feeding the bigger engine, and will increasingly fail to do so as RPM rise. So, with stock cams, I'd figure the MCM could maximize power levels best - the stock ports/valves stand a better chance of doing a good job under 6.5k and the MCM further optimizes that range.

So I'd end up with monster power levels below 6.5k and might not loose anything above compared to stock timings - due to the restrictive ports and stock lift/duration likely being the bottleneck up high that the stock cam timings just can't compensate for..