MkIII Top End Rebuild Pics and Sanity Check

ag12680 · Aug 29, 2015

Finally got the top end apart to check for stuck rings on the right cylinder and for potential soft cam wear. Bike has 7,800 original miles and sat for a few years.

I don't have a bore gauge and will have someone else do the work but looking for:

1. Does the cam look OK? The lifters show no signs of gauling from cam contact.

2. The top compression ring gap is .016. What is the wear limit? Replacing anyway, but curious about the rate of wear.

3. Assuming the piston to wall clearance is good - would you reuse these pistons? Scuffing is a bit worse on right side (shown).

Right now, I'm just aiming to get it running in stock form with no racing/performance intentions.

Thanks much...

Andy --

MkIII Top End Rebuild Pics and Sanity Check

Fast Eddie · Aug 29, 2015

I'd re-use those pistons without hesitation.

Same with the cam.

Heck I'd even keep the old rings, the thing is only just bedded in !!

Madnorton · Aug 29, 2015

Likewise, I'd reuse the the lot nothing there to suggest that any of it needs replacing.

pete.v · Aug 29, 2015

Agreed. The hone marks are still there and ring gap is right on. Cam wear marks like that are quite normal. They must have either used good oil or changed it often.

I think changing pistons and rings would do more harm than good in this instance. Leave the ring in place, maybe rotate them 120degrees. and slap that bitch back together.

I see this as a post break in inspection with successful results.

ag12680 · Aug 29, 2015

Thanks guys... Read elsewhere on the forum that the end gaps should be .010-0.012 when new. So the 0.016 is OK and the cylinders do not need to be touched even with new rings?

gtsun · Aug 29, 2015

Silly question but why did you feel it needed to be taken apart in the first place? Was it just to look at it because it sat for a long time??

Torontonian · Aug 29, 2015

MK 111 's had a batch of soft cams so good insurance to take it apart for inspections. All looks good. :wink:

Fast Eddie · Aug 30, 2015

Of course, it would be quite a convenient time to skim 0.040" off the head and raise the CR a tad ...

ag12680 · Aug 30, 2015

Thought the rings were stuck on the right side and (based on input here) wanted to make sure the cams were OK. Now debating whether to put a base gasket back on. Would I need to smooth out the top of the bores if not?

dynodave · Aug 30, 2015

Torontonian said:
MK 111 's had a batch of soft cams so good insurance to take it apart for inspections. All looks good. :wink:

Out of well over a dozen MKIII bad cam failures I have diagnosed, not one was hardened over about 22RC. Plus of all the MKIII's i have been involved with through out the whole series early to very late not one yet has tested as hard. Many died in under 10,000 miles and the remainder before 20,000. Yes a few lucky ones survived to middle age but it seems to be a minority.

My latest MKIII with original 5 miles on the ODO, never been registered, will get a new NOS hard cam before I ready it for the road.
Already had one AN chilled iron cam disaster so none for me thanks...

swooshdave · Aug 30, 2015

dynodave said:
Torontonian said:

MK 111 's had a batch of soft cams so good insurance to take it apart for inspections. All looks good. :wink:

Click to expand...

Out of well over a dozen MKIII bad cam failures I have diagnosed, not one was hardened over about 22RC. Plus of all the MKIII's i have been involved with through out the whole series early to very late not one yet has tested as hard. Many died in under 10,000 miles and the remainder before 20,000. Yes a few lucky ones survived to middle age but it seems to be a minority.

My latest MKIII with original 5 miles on the ODO, never been registered, will get a new NOS hard cam before I ready it for the road.
Already had one AN chilled iron cam disaster so none for me thanks...

When you say AN you mean a replacement cam?

ag12680 · Aug 30, 2015

dynodave, I take it you mean the Andover Norton cam? Where do you source harder versions? I just want the stock grind...

Thanks,

Andy --

dynodave · Aug 30, 2015

ag12680 said:
dynodave, I take it you mean the Andover Norton cam? Where do you source harder versions? I just want the stock grind...

Thanks,

Andy --

My 2 cams are "new" unused NOS, rockwell C tested, norton 06-1084 from 20M3S vintage. I used a $25 20M3S flea market "take out" for my first MKIII. You might get a new megacycle cam... The split genuine AN (IIRC Southampton) was replaced with a NR megacycle...a lot less hot rod than a combat grind.
IMO even a combat seems to run OK in a stock 850 (intake insulators removed). If spending money, the milder NR would be my choice after a hard stock 06-1084.
Jim C might even a suitable mild regrind for normal touring mode.

ag12680 · Aug 31, 2015

Thanks DD...

Torontonian · Aug 31, 2015

D.D. you have me even more concerned now as I head up to Ottawa to pick up my 76 MK111 with 27,500 on the clock.

fiatfan · Jan 1, 2016

Fast Eddie said:
Of course, it would be quite a convenient time to skim 0.040" off the head and raise the CR a tad ...

I´d like to increase the CR on my standard pre-Mk III 850 "a tad" as you put it, are you serious with the 0.040" ? That´s like 1 mm :shock: Not being an expert in any way, that seems a lot, or not? Is there a way to at least roughly calculate the increase of the CR in perspective to how much you shave off the head? Keeping in mind you deal with a standard motor?
Tommy

Fast Eddie · Jan 1, 2016

fiatfan said:
Fast Eddie said:

Of course, it would be quite a convenient time to skim 0.040" off the head and raise the CR a tad ...

Click to expand...

I´d like to increase the CR on my standard pre-Mk III 850 "a tad" as you put it, are you serious with the 0.040" ? That´s like 1 mm :shock: Not being an expert in any way, that seems a lot, or not? Is there a way to at least roughly calculate the increase of the CR in perspective to how much you shave off the head? Keeping in mind you deal with a standard motor?
Tommy

850s were quite low CR it seems. I know they were quoted at 8.5:1 but many seem to be closer to 8:1.

The 750 combat had 0.040" shaved off the head as standard.

According to my guesstimated calculation, if you 850 is on a stock bore and is currently 8:1 then shaving 0.040" off the head will give you approx 8.7:1. if its currently 8.5:1 it will take it to 9.2:1... or thereabouts.

Shaving 1mm reduces the combustion chamber volume by approx 4.65cc. I think...

You don't need to shave the head though, you could fit the super thin copper rings, sold here: http://www.jsmotorsport.com/technical_compression.asp

fiatfan · Jan 1, 2016

Fast Eddie said:
fiatfan said:

Fast Eddie said:

Of course, it would be quite a convenient time to skim 0.040" off the head and raise the CR a tad ...

Click to expand...

I´d like to increase the CR on my standard pre-Mk III 850 "a tad" as you put it, are you serious with the 0.040" ? That´s like 1 mm :shock: Not being an expert in any way, that seems a lot, or not? Is there a way to at least roughly calculate the increase of the CR in perspective to how much you shave off the head? Keeping in mind you deal with a standard motor?
Tommy

Click to expand...

850s were quite low CR it seems. I know they were quoted at 8.5:1 but many seem to be closer to 8:1.

The 750 combat had 0.040" shaved off the head as standard.

According to my guesstimated calculation, if you 850 is on a stock bore and is currently 8:1 then shaving 0.040" off the head will give you approx 8.7:1. if its currently 8.5:1 it will take it to 9.2:1... or thereabouts.

Shaving 1mm reduces the combustion chamber volume by approx 4.65cc. I think...

You don't need to shave the head though, you could fit the super thin copper rings, sold here: http://www.jsmotorsport.com/technical_compression.asp

Thanks Eddie, my 850 is +0.020", but otherwise stock (I think, don´t know what cam is in there yet). I´m going to have the top of the cyl trimmed, but just that, no shaving. I also need to do something about the head since it looked like I had a small leak between the bores and the pushrod channels. Hence the shaving idea.... Jims thin head gaskets seems like a great idea, not the .003 ones but the .020. Then maybe 0.2mm off the head, I think that'll be just fine. I don´t quite understand what he means when he say´s "don´t use paper base gaskets", does he mean that you should only use one of the two Permatex solutions and NO GASKET? The paper base gasket in my engine was glued with something, and sat really hard in place. But it was thick, and when adding all these gaskets up, I see that you have to do the maths properly if you change something.
Tommy

MkIII Top End Rebuild Pics and Sanity Check

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