Milled cylinder barrel deck

NickZ

NickZ

VIP MEMBER
Joined
Oct 31, 2018
Messages
457
Country flag
I've gone thru several threads discussing milled heads, but this 1973 750 engine I'm rebuilding came with barrels that appear to have been milled down about 0.050", to where it is practically flat with the fin at rear.

Milled cylinder barrel deck

The RH-6 head that came with it appears to be normal.

Can I assume that the issues this presents are to be dealt with in the same ways as discussed for milled down heads?

Has anyone else out there seen this done?
 
750 cylinders are around 4.560" - 4.565" face to face.

The squish would be the decider given the stick out on 750's.
 
NickZ said:
I've gone thru several threads discussing milled heads, but this 1973 750 engine I'm rebuilding came with barrels that appear to have been milled down about 0.050", to where it is practically flat with the fin at rear.

View attachment 96322

The RH-6 head that came with it appears to be normal.

Can I assume that the issues this presents are to be dealt with in the same ways as discussed for milled down heads?

Has anyone else out there seen this done?
Click to expand...

The issue is squish clearance and compression ratio. I used to mill the cylinders on my race bikes, both 750s and 850s, to get a higher CR. I milled the cylinders instead of the heads, because I had paid a fair amount of money to get Axell to port the heads, and didn't want to modify them. At the time, I assumed that it would be easier to just buy new cylinders if needed. I milled the cylinders enough to get higher CR with the pistons I was using, Hepolite Powermax for the 750 and stock Hepolite for the 850. As I recall, I took .060" off the 750s and .080" off the 850s, but that was a long time ago, so don't relay too much on my memory. My criteria was .040" of squish.

Ken
 
I have 2 sets of 77mm barrels here milled 0.020" that I use with a Fullauto head milled 0.020" Like Ken I didn't want to go for broke on the head.

Back in the day I had a set of 77mm barrels with 0.060" milled off used with a head that also had 0.040" removed, but this was desperation with a fully sphered short stroke head to get some compression.

Assuming pump petrol and depending what you are looking for on compression, most likely you will just want a compression plate at the base. Maybe one of JSMs thicker gaskets too.

If it was me I would probably find a way to use this as an excuse to fit new AN 750 through bolted barrels with standard bore pistons :rolleyes:
 
Its a 750 flange cylinder not an 850 cylinder.

The top face being close to the top fin means little and does not automatically mean it has been faced outside the factory and that has not been verified measurement wise as a number by the OP.
What is the - 0.050" based off, a factory machining drawing perhaps ?

I checked another 750 cylinder (perfect stock bore in as close to new condition as could be imagined)
The overall height is close to the one I measured this morning but the top gasket face is higher to the top fins (noticeably lower/closer to on the other) but the bottom flanges are different thickness's which might suggest the lower face and spigots were done first then the top face off that.
The fins vertical placement varying within that measurement being a casting to be machined.

Perhaps that 750 flange difference (top face close to top fin) is part of why some fail and others hang in.
 
Time Warp said:
Its a 750 flange cylinder not an 850 cylinder.

The top face being close to the top fin means little and does not automatically mean it has been faced outside the factory and that has not been verified measurement wise as a number by the OP.
What is the - 0.050" based off, a factory machining drawing perhaps ?

I checked another 750 cylinder (perfect stock bore in as close to new condition as could be imagined)
The overall height is close to the one I measured this morning but the top gasket face is higher to the top fins (noticeably lower/closer to on the other) but the bottom flanges are different thickness's which might suggest the lower face and spigots were done first then the top face off that.
The fins vertical placement varying within that measurement being a casting to be machined.

Perhaps that 750 flange difference (top face close to top fin) is part of why some fail and others hang in.
Click to expand...
'Its a 750 flange cylinder not an 850 cylinder.'

I didn't think that was in dispute, you can see that, and he says '73 750.

But if I understand you, on the basis of the barrel type, you question if there really has been 0.050" milled off, which is a good point. I had just accepted he knew what had been milled off, perhaps not.

The rest of your post confirms that whilst measuring the deck height is useful, the OP really needs to do a trial assembly, to determine he has adequate squish and an acceptable compression ratio for his application.

The issue with this might be piston choice, which might be influenced by the results of the trial assembly, but need to be present to do it!

We don't know if there are pistons present with the barrels or what the OP intends to use. What he also hasn't told us is what he wants to achieve?

If this is a road engine build, he most likely won't want the compression that someone else may have worked to achieve.

If it is a race engine build, then those barrels are not a good start point.
 
If there's enough ring land you could just machine the piston for ideal squish band ? , (dependant on rod type) steel or aluminum, or fit correct thickness head gasket, base gasket , tight-ish squish is good on most engines , for tumble and swirl, and CC it all up and calculate comp ratio.
 
Thanks for the replies.

I based the 0.050" number off comparing these barrels to another 750 set I have from a 1971 engine. There is about 0.050" of material above the top fin on that one that isn't there on this one.
This bike came to me fully disassembled. My objective has been to rebuild this as a stockish road bike. Not looking for higher compression. I didn't notice the deck issue until I began measuring the bores. I am going to have it bored .020" over and I have a set of .020 EMGO pistons that I could use.

A quick measurement gives me about 4.550" face to face, but I will attempt to give it a more precise measurement and check for parallel top to bottom later today.
 
NickZ said:
Thanks for the replies.

I based the 0.050" number off comparing these barrels to another 750 set I have from a 1971 engine. There is about 0.050" of material above the top fin on that one that isn't there on this one.
This bike came to me fully disassembled. My objective has been to rebuild this as a stockish road bike. Not looking for higher compression. I didn't notice the deck issue until I began measuring the bores. I am going to have it bored .020" over and I have a set of .020 EMGO pistons that I could use.

A quick measurement gives me about 4.550" face to face, but I will attempt to give it a more precise measurement and check for parallel top to bottom later today.
Click to expand...

That was my whole point.
I took the time to go measure two 750 cylinders and they were just as you describe, one high the other noticeably lower gasket face to top fin and based on this reply the low one is within 0.010" to your number.
That low one has a thicker base flange by default (It seems)

My remark after that regarding squish was if the cylinder was actually decked / planed / resurfaced which was an unknown.

An actual factory height measurement would be more relevant in general (including to me) ?
I only used a Vernier, to get a truly accurate measurement would need zeroed off step blocks in the mill and checking with the DRO E/W/N/South.
 
Last edited:
Time Warp said:
That was my whole point.
I took the time to go measure two 750 cylinders and they were just as you describe, one high the other noticeably lower gasket face to top fin and based on this reply the low one is within 0.010" to your number.
That low one has a thicker base flange by default (It seems)

My remark after that regarding squish was if the cylinder was actually decked / planed / resurfaced which was an unknown.

An actual factory height measurement would be more relevant in general (including to me) ?
I only used a Vernier, to get a truly accurate measurement would need zeroed off step blocks in the mill and checking with the DRO E/W/N/South.
Click to expand...
Yes, I further measured 4 points with vernier calipers with cylinders on granite flat plate. Average of the 4 measurements was 4.550" and variation between the measurements was around .007". I'm not going to rely on the variations until I can remove what appears to be gasket sealer residue that remains on some areas of the base flange.

So I see what you are saying. With this measurement, and your measurements, the case can be made that these barrels came from the factory like this. However, if it ever became necessary to plane the top face, you would be taking metal from the fins as well.
 
I was curious enough to search and this was the first thread that came up.

Commando cylinder deck height

Does anybody know what the distance between and mounting flange and top flange should be on a 750 cylinder? When I check the position of the pistons in the bores at TDC with no base gasket the pistons are up 0.050" above the top of the cylinder. I'm using MAP rods and GPM flat top pistons. The...
www.accessnorton.com www.accessnorton.com

Of course anything could have happened to a cylinder over 50 years old including the ones I have.
Maybe Andover Norton have a drawing noting that + - flange to top face height.

@Madnorton

I have to admit being a little surprised seeing the pistons higher than the deck height first removal of the 750's head.
 
I measured again with mill DRO. Pretty much same result: 4.549" - 4.555" depending on location.
I also measured the cylinder barrels from the 1971 engine: 4.560".

I'm relieved to learn that the first one likely was not messed with. I'll still probably check all the valve clearances etc. when I assemble, just to be safe.
Thanks again for the helpful info.
 
NickZ said:
I measured again with mill DRO. Pretty much same result: 4.549" - 4.555" depending on location.
I also measured the cylinder barrels from the 1971 engine: 4.560".

I'm relieved to learn that the first one likely was not messed with. I'll still probably check all the valve clearances etc. when I assemble, just to be safe.
Thanks again for the helpful info.
Click to expand...
Thats good, the close face to fin is most probably just an issue of where they put in the jig back then.
 
NickZ said:
If that's not too much trouble. It would be interesting to know how common the flat deck is.
Click to expand...
well, I have just measured 4 750 barrels: 3 were in the range 4.555-4.559. One was 4.571, and interestingly that one had the lowest deck height left above the fins, almost none at the rear.

The two 850 barrels were 4.555 and 4.563.
 
jan nelder said:
well, I have just measured 4 750 barrels: 3 were in the range 4.555-4.559. One was 4.571, and interestingly that one had the lowest deck height left above the fins, almost none at the rear.

The two 850 barrels were 4.555 and 4.563.
Click to expand...
Ok, thanks. Very interesting. Confirms what was said before.
 
You must log in or register to reply here.
Back
Top