The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: snowbeard on 18.03. 2009 04:47
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So in looking at my engine, the 57 spitfire scrambler, verified as such, does the HHC mean high compression?
it's stamped on the crankcase under the serial numbers... never really thought about it much but the specs seem to indicate it should be...
should I be using a better gasoline than the 85 octane?
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HHC is code for high compression and performance camshaft (357).
85 octane is too low; go with 93 minimum.
David
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wow, so were all our spitfires made this way?
and should I have noticed pinking or anything from using such low octane stuff? also if I advance the manual advance, I don't find that I actually get better power at me revs, in fact sometimes it drops!
I don't get kickback on starting with it retarded, so I don't think it's slipped, and I think I was at least within a degree or so of correct...
needs gas today, so I'll try the high test and see what I can tell. gotta try that video again with the camera pointed the right direction!
THANKS!
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wow, so were all our spitfires made this way?
Yup, hence the name "spitfire cam" for the -357 cam.
The HHC, however, does not mean that your bike still has high compression pistons in it. Typically, with stock 9:1 or more, I don't think you could avoid pinging on anything less than 93 octane and even then they tend to be real pingy. I'm guessing you either have lower than stock compression and/or retarded timing. Either that or you haven't gotten it hot and pushed it yet.
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well I have been very gentle on 'er. I've taken two rides over 100 miles, and then I'm still not really giving it all she's got by far. I don't have a tach or speedo, but I have had 'er up to 75-80mph by the GPS, even that was on a flat straight stretch and only coming from 60 mph.
I siphoned out most of the gas that was in it this morning and luckily still made it to the station, filled up with 91 as that's the highest we get around these parts. maybe I should pick up a little octane booster to top it off? either way I could feel it just in the few miles to work, and I'll see what I get on the ride I plan for lunch!
I plan to have the head off and into the machinist tonite (wish me luck) and I'll be able to get a better idea of what the pistons are then. I'll be back!!
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ok, so the head is off! the side that was burning oil from the worn guide and rough valve stem actually looks better than the "cleaner" side!
the pistons are marked BSA, but I didn't have time to clean them off to get the number on them. here's a pic.
(https://www.a7a10.net/forum/proxy.php?request=http%3A%2F%2Fimg.photobucket.com%2Falbums%2Fv632%2Fsnowbeard%2Fpistons.jpg&hash=4cb9431dabf8a9ad8b4f97dda308fb36404614fa)
so would a high comp piston have to be domed?
the 91 octane gas, by the way, makes a noticable difference!! well, it did before I tore 'er all down... it only took me an hour and a half to have all of it off to this point!
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Are you going to lift the barrels and do the rings while they head is away?
It seems a good time to check pistons and bores.
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I've been told that by a good friend who is a mechanic as well, so I suppose it's a good idea.
I looked at them, ran the pistons up and down, the right cylinder that has been leaking extra oil thru the valve guide is superb, super smooth and clean.
the pistons themselves are rock solid in the bores, no shifting about on thier rings, etc.
the other side, the only dry spot in my pic, looks to actually have a small bit of pitting on the inside wall. :-(
I've often admitted just how much I don't know on here before, so I'll ask, if the pistons seem tight, the compression was good (100+ on both sides), what would it hurt to run it a while longer this way? I don't mean to be a dumbbutt DPO, and I certainly don't want to do any damage to this beautiful machine, but honestly, if I'm going to have to rebore, get new pistons and rings, etc, what harm could I cause running it another year in this condition? again, I don't say that as an "I've seen it all and know what I'm talking about", I honestly want to know the repercussions?
from what I (think I) know, I guess I might get some of the combustion gasses into the crank cases thru the minor pitting, and without knowing the tolerances of my skirt clearance, etc, do I run the risk of the piston wobbling off to the side and jamming(?) what else am I not even vaguely aware of?
I'm a bit of the thought to pony up on the head work this year, put it back together and run it thru the season, hoping to get the super rocket done right in the meantime, since it is in even further disassembly, and then trade off, doing the full bottom end, barrels, etc on the spitfire?
am I being a dipstick? (I'll keep it clean as we have expressed that this is a family site ;-) )
thanks for the great advice everyone!!
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Snowbeard,
You have also heard me say that others here have more experience and may be better sources, nevertheless, I have an opinion. If you can assume that the cylinder pitting is inconsequential, as in, it's smooth overall, it's not enough to vent compression, it's not in a circumferrential pattern that a ring might trip over, I say, trust the bores and pistons until or unless there is a reason to think otherwise.
In my previous rebuild, before the crank was starved of oil, I reused my existing pistons and honed away years of rust on the bores. The result was pistons way out of spec for fit, yet the engine ran fine and, to me, sounded good until the main bearing dried up. In the first rebuild, I was completely ignorant of the crankshaft sludge trap and did not clean it out after the engine sat for 24 years. (Now I expect someone will tell me that the loose pistons brought on the oil shortage.)
Regarding your compression test, it's hard to say how genuine that is. If you were runnning a lot of oil into one cylinder and not the other, that cylinder would show improved compression due to a better ring seal caused by the excess oil. Squirting a shot of oil into the cylinders during a compression test is an old trick to tell if leakage is occuring at the rings or at the valves/gasket. I am not sure if "100+" compression is a good thing. Your pistons are 8.25:1. In a perfectly sealed chamber (valves, gasket and rings) this would yield right around 125 PSI. I, and, I believe, others here have noted that compression measurements can exceed mere product of compression ratio x atmospheric pressure. Under a previous topic, I surmised that this was due to the adiabatic process (that is, the thermodynamics that makes the heat for diesel fuel to ignite) with the added heat bringing on added pressure. I am not promising to be correct in this, but I haven't yet been told by any of this forum's professorial staff that I am wrong.
Maybe, most importantly, delaying work on pistons and bores means you get to enjoy riding around in the Colorado mountains this season, if not this weekend.
Richard L.
P.S. After you finish the "Red Devil" you will not need to apologize so much for your newbieness (you don't, really, need to now), and you will consider yourself well-versed in many of the nuances, pariticularly the ones you had to do life-or-death battle to resolve.
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thank you for the comiseration and moral support Richard, it is true we see and experience worse neglect of these old bikes than any of us interested enough to even look at this forum would accept...
I'll still bend to the will of the knowledgable, so please do warn me now! :-)
as to my compression test, it was executed with the engine cold and not run for days at least, I just wanted to see what I would get, and I have in fact forgotten my exact results, they may have been more in the 120 range...
HA!! gotta love the internet and an addiction to bragging, I found my post in the measuring compresssion thread where I stated the compression I tested cold was 120psi each cyclinder!!
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just a bit more info, I cleaned off enough of the piston heads to read the stamps,
67-1606
1M/M OS (might be N/M?)
30 12 62
so I'm thinking that this reads part no, 1mm oversized? which would be about 0.040 in? and production date of dec 30, 1962?
so have my barrels been bored over once all the way back in 62?! I might be more inclined to have them redone if they have already been done once, I was quite impressed that it would make it 50 years with little to no molestation...
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I thinnk it safe to say that when the pistons were made is not relevant to when the last rebore occured.
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Do you know what compression ratio your pistons are? They look exactly the same as the pistons in my bike as a matter of interest SRM have posted the following information in the technical section of their website that may be of interest. There is no mention of different compression ratios but they state that the ignition timing for ally heads should be set at 5/16 (35 degree BTDC) and iron heads 9/32 (33 degree BTDC). and that state that this is taking into count modern fuel. I use Tesco super market fuel rated at 99 octane, and my bike just loves it. Interestingly SRM also recommend a cylinder head torque figure for all A10 and A7's built between 1950 and 1963 of 32 foot lbs. There appears to be no distinction between ally heads an iron heads.
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Look at this regarding compression ratio.
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good point about instalation Richard, *red* edit:but what is your link ;)
A101960, no, I no longer know the compression ratio with these pistons, the original setup would have been 8.26:1 but I don't know what those pistons should look like? are these a more standard piston?
that's a very interesting note from SRM, I think I was aiming for 32 BTDC, but have found that I get the most power at "full retard" on my manual advance, so maybe I need to revisit the timing as well! that might be another post.
I am getting the valve seats upgraded at Acme, they will bore out the seats and replace them with a more modern fuel resistant material. I wonder if that has any impact on the timing suggestion...
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oops
http://en.vintage-motorcycle.com/index.php?language=en&site=4&pid=27&id=3339&gesucht=true&marke=&nummer=&beschreibung=8.25
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aha! well those certainly look the part, I guess there's a decent chance I still have the higher compression!
I did put an email in to SRM to inquire after the reasoning behind 35 degrees, we'll see what they say I guess!
at this point I'm second guessing having the head redone at all, but I have it off, I may as well press on!! it's only money, eh?!
thanks for the link!
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Looking at those pistons, I would say they are 8.75:1 or commonly known as 9:1.
Check the numbers in the parts book.
Trev.
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Trev,
It wouldn't surprise me if you knew more than the folks at British Only Austria, but the pistons they show as "8.25:1" are almost certainly the same top geometry as Snowbeard's. My own pistons are full domed, which I am fairly certain are the 9:1 variety. Please, just correct me if I am off the mark.
Richard L.
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sorry, but I haven't got a parts book yet. I haven't had enough confidence that the spitfire specific models will be accurately listed in a non specific parts book from the range of dates I've seen so far.
I did find a site that has listed these part numbers as A10 pistons, 0.060 over, but if the 1 M/M OS means millimeter, that would be 0.040 over...
67-1601 Piston A-10 STD 9-1 pair 2 $177.50
67-1603 Piston A-10 +.020 9:1 pair 4 $180.25
67-1605 Piston A-10 + .040 9:1 pair 4 $160.60
67-1606 Piston A-10 + .060 pair 2 $160.60
67-1607 Piston A-10 + .02 pair 4 $150.00
looked at the bores again today with a little more time, they may need to be cleaned up after all. so how much material does a honing remove vs a rebore?
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Richard L, the original 8.75:1 pistons fitted to A10s were as pictured, as were the 8.25 type. I don't have a 8.75 to hand to measure the difference in height, but I will see if a mate has one handy to check.
I have seen the domed type, the dome rising like a pimple from the centre of the piston with a flat area around it. The set I saw many years ago were in a Hepolite box and marked "Ariel Huntmaster".
Trev.