Author Topic: Ive got my wrong head on I think  (Read 2416 times)

Offline spyke

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Ive got my wrong head on I think
« on: 15.01. 2010 20:08 »
Hi chaps,

After reading a BSA related link on this site on cylinder heads last night, Im afraid the Road Rocket head sitting in my garage is in actual fact a Shooting Star one (its stamped 67-1122).
I recall talking some time ago to the owner of quite a famous BSA shop  /rebuilder of A10s and he said that " putting a Shooting Star head on a A10 makes a lovely engine ".Bearing in mind my recent post on wanting a soft engine anyway will this head be ok, and will it need modifying or is it only suitable for a 500 and ive got to spend yet another 200 squid! I wasnt planning on.

Cheers Spyke
A10 spitfire style

Offline MG

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Re: Ive got my wrong head on I think
« Reply #1 on: 15.01. 2010 20:14 »
Hello Spyke!

We had discussed this topic recently:
http://www.a7a10.net/forum/index.php/topic,1807.0.html

I think it might be worth a try to use it as is. I suppose the smaller valves would compensate the effect of the pinch-off edge a bit making it softer.
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

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Online muskrat

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Re: Ive got my wrong head on I think
« Reply #2 on: 16.01. 2010 20:47 »
G'day Spyke, works for me. See MG's link.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online chaterlea25

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Re: Ive got my wrong head on I think
« Reply #3 on: 16.01. 2010 22:12 »
Hi All,
During the last couple of weeks I have been involved with an A10 "Rocket or RGS " replica
the chap who bought it recently asked me to try and solve a misfire!!!
(can you see this is going to be a long story??)
After going through all the usual and unusual things to no avail including compression testing changing mag and carb
I thought the compression pressure was kinda high I read 175psi on the kickstart, so I started the bike on the good cylinder and then checked the misfiring one, compression was erratic????

So I took off the head for a look see!!! the exhaust valve spring on the misfiring side had lost all its tension and was only barely closing the valve *eek* *eek* *eek*
On further examination I saw that it was fitted with an unmodified A7SS head!!!!
The pistons had been clouting the head around the edge of the combustion chamber!!
I decided after looking at the bores I had better lift the cylinder block for a look see
silicone goo instead of a base gasket obvoiusly hadn't helped
The cam/followers are in a sad state and the bores had been scored by the shrapnel flying about

Nothing for it now but a complete reconditioning of the engine *sad2* *sad2* *sad2*

Most of you will know that the 650 alloy head uses different caps collets bottom collars etc than the rest of the range
I have a spare 650 head which is bare and would require a complete recon to use
All this adds up to a considerable sum on top of any labour costs and the top dollar price paid for the bike in the first place

I probably shouldnt say any more until the owner goes back to the seller with the photographic evidence and the estimate for the rebuild

To return to the original topic I would think from this that fitting an unmodified A7SS head to an A10 that has 7.5 or more comp pistons is not too good an idea!
Either modify the head to 650 spec (not cheap) or use flat topped pistons and the consequent poorer breathing of the 500 head with smaller valves

Regards
John O R
1961 Super Rocket
1963 RGS (ongoing)

Online muskrat

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Re: Ive got my wrong head on I think
« Reply #4 on: 17.01. 2010 07:38 »
G'day all,
             John O R, had the barrels been shortened ? At TDC my 10 1/2:1 pistons only come to about 1/8" from the top of the barrel so even with an unmodified A7SS head The piston would not touch. I have CC'd the combustion chambers  after assembly and both are 31cc = 10.48:1. I did chamfer the head to not have a step and use iron head A10 valves with A7SS hardwear, all good. pic of head and hard to see but piston is at TDC in barrels.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline MG

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Re: Ive got my wrong head on I think
« Reply #5 on: 17.01. 2010 09:55 »
Maybe its been A7 barrels, too? Like muskrat said, usually the pistons should stay clear of the head's edge.
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

Austria

Online bsa-bill

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Re: Ive got my wrong head on I think
« Reply #6 on: 17.01. 2010 10:03 »
I'm sure I read that putting a SS head on an A10 was sometimes done as a  racing mod to increase compression, if that is true it must have been safe enough.
Is it possible that it is the pistons that are wrong ( pin to crown ) ?

All the best - bill
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

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Re: Ive got my wrong head on I think
« Reply #7 on: 17.01. 2010 12:39 »
That may be the case Bill. In the A7 for racing I used Rocket 3 pistons with A10 rods shortened by 2.5mm to get the right height. God knows what may have been done to that motor by PO. I would hate to have got one of my motors 2nd hand and tried to work out what was going on !! Hee-Hee.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline MG

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Re: Ive got my wrong head on I think
« Reply #8 on: 17.01. 2010 13:40 »
I still think that much of the power gain lies in the pinch-off edge. When done properly, you have more grunt and less tendency for detonation due to better turbulence in the combustion chamber.
I once fitted the 350cc head on the 500cc Kawa (early 80s bike) of a friend. This resulting in just a 1mm edge, the bike went like hell, teaching several 650 rice cookers a lesson.
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

Austria

Richard

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Re: Ive got my wrong head on I think
« Reply #9 on: 17.01. 2010 14:48 »
I had a standard A7ss head fitted on my 54 Plunger when I bought it no mods had been made and it ran ok, since then I have changed it for a cast one and sadly the bike has been sold.
Richard

Online chaterlea25

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Re: Ive got my wrong head on I think
« Reply #10 on: 17.01. 2010 17:09 »
Hi Muskrat and all,
Muskrat on your head in your pics the head has been chamfered around the edge to make clearance
On the engine I stripped the pistons are original BSA std size.
On the spare A10 head there is a recess to accomodate the protruding piston crown,
As I said no base gasket wasnt helping
I will post some pics to show the "mess"
Cheers
John O R
1961 Super Rocket
1963 RGS (ongoing)

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Re: Ive got my wrong head on I think
« Reply #11 on: 17.01. 2010 18:47 »
Ok here are the first few photos,
For comparison purposes I sat a piston on the A7 head and then the A10 head
The standard piston will sit down into the recess in the A10 head

3rd photo shows where the piston kissed the head

More to follow

John
1961 Super Rocket
1963 RGS (ongoing)

Offline MG

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Re: Ive got my wrong head on I think
« Reply #12 on: 17.01. 2010 18:57 »
Man, that engine must have pinged under load  *smile* *eek*
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

Austria

Online muskrat

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Re: Ive got my wrong head on I think
« Reply #13 on: 17.01. 2010 20:43 »
G'day all,
             the fun and games we have working on old stuff that others have tinkered with. maybe my pistons have a shorter crown height as I didn't have to chamfer the head for it to fit but did so as not to have masked areas where fuel might not be burnt.
Spyke, bring your pistons up to TDC, place the head on top without a gasket. Do the faces meet ?
 If they do  get the grandkids plasterscene and roll up two worms about 1/8" and place across a piston. Fit the head and then remove it. Check the plasterscene for your clearances, note that at TDC on one cyl the valves will be open about a mm or so. You need to have at least 1mm clearance at the closest point.
 If they don't meet you will have to chamfer the head as I have, see pic above.
Either way you can use the A7 head.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online Brian

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Re: Ive got my wrong head on I think
« Reply #14 on: 17.01. 2010 21:25 »
I cant help but think that BSA put A10 heads on A10's and A7 heads on A7's for a reason.........................................