Compression values

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Gannet
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Compression values

Postby Gannet » Mon Apr 30, 2012 9:23 am

I am repairing/restoring an FV which has a compression figure of 75psi. Is this low? Should I strip it down and examine/repace the piston rings? I am not keen to do this unless I really have to.

It does start and run in a big water bucket but difficult to assess its performance, although it doesn't seem to run very smoothly.
(My SJM when tested with the same pressure gauge gives 130psi).

Look forward to your advice.
Thanks

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Collector Inspector
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Re: Compression values

Postby Collector Inspector » Thu May 03, 2012 10:37 am

Use on a transom. Should be OK at that for a start. Older motors run well without larger number comps because, basically, they are used to it?

If it does not satisfy then yes new rings.

Compression is based on bore wear, piston wear and ring wear for any engine.

Use and enjoy untill a pain to you. Remember that comp comes up when an engine is used if it has been idle for a time.

B
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Charles uk
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Re: Compression values

Postby Charles uk » Thu May 03, 2012 11:15 am

That is high for an SJP & your FV is about average.

Has anybody else seen that much in a 40 series powerhead?
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Gannet
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Re: Compression values

Postby Gannet » Thu May 03, 2012 4:17 pm

Thank you very much for your help.
From your comments, It would seem that 130psi is a very high figure and that 75psi is an acceptable value, with a likelyhood/expectation that it will increase with use.
Thanks.

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Re: Compression values

Postby Collector Inspector » Sat May 05, 2012 6:45 am

My SJM that I am proud of was last run a while ago now, just at fast idle in the tub to loosen it up. As I noted in my showing it off vid...........very tight rebuild. Go to my I have a Minus topic.

Anyway it is far from run in but I grabbed it out and did a comp test as it has been slumbering under its blanky.......

136PSI. Pic Below with same quality guage as used for .........................

The Featherweight run last weekend was 115psi

The WSC the weekend before that was 65psi

Gannet has a good one by the looks

Cheers

B
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Gannet
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Re: Compression values

Postby Gannet » Sat May 05, 2012 9:54 am

Hi,
Thanks for this info. I have seen various comments such as ' good compression' etc, but have never been able to quantify it.
For a 40+/- it would appear that 120 to 140 is a good figure. I haven't been very consistent in noting whether the bore was oily ie whether it had a bit of added 'laid up' oil in the bore. this would make a difference.
Anyway the figure of 75psi for my Little Forty I now think is low. Action required.

Thanks again.

Jeremy

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Re: Compression values

Postby Collector Inspector » Sat May 05, 2012 11:41 am

Jeremy

Now watch the replies from this.....................?

The earlier 40s had a shorter stroke, just bear that in mind and check your engine via S.O.S. before doing something that may be a waste of effort? The shorter stroke means a lower psi in compression. Also means that they ran very much smoother at idle. Yeh?

A nose out of joint will surface but does not matter.

Lots of hits as topic transpires, cool!

Welcome aboard Mate!

Regards

B
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charlesp
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Re: Compression values

Postby charlesp » Sat May 05, 2012 12:30 pm

Sounds OK to me. I still have a soft spot for these little fellers.

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Oyster 49
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Re: Compression values

Postby Oyster 49 » Sat May 05, 2012 12:35 pm

I still have a soft spot for these little fellers.


Me too, infact I have just bought another one :D

Gannet
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Re: Compression values

Postby Gannet » Sat May 05, 2012 11:04 pm

B,

Thanks for your comments. I think that you are right that I am not being entirely sensible in the work that i am doing on my FV. Although some people would argue that just owning a couple (or more) Seagulls is pushing 'sensible' to the extreme!

Excellent photos of your pressure test set up and very tidy motor. Do you think that the extra volume of the hose would produce a lower maximum pressure? Certainly the compression ratio, I would have thought, would be lower. My gauge fits directly onto the 18mm adaptor via a NRV.

On another matter, I would be grateful for your comments, being a new boy to Seagulls. I am in the process of replacing the corroded steel plate between the crankcase and cylinder block with a brass plate. Do yo think this is wise? My reasoning is that brass is easier to work, won't corrode and being softer should seal better. Any comments?

Thanks for your help.

Jeremy

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Charles uk
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Re: Compression values

Postby Charles uk » Sat May 05, 2012 11:36 pm

Electrolytic corrosion.
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Re: Compression values

Postby Collector Inspector » Sun May 06, 2012 9:49 am

Thanks for that Jeremy,

The pressure guage has a shraeder valve in the bit that screws into the spark plug hole so the line and the guage just "Pumps Up" to whatever the maximum compression is..........or how much the engine can do. When equal, that is the guage reading.

Electrolytic corrosion

Oh Yeh!, I have only been involved so far with my own ongoing resto of my FVP and corrossion? Never seen anything like it!

I had the head welded and machined and the crankcase base, under the original steel gasket is a mess. I can get around that so stay tuned to the FVP restore post later.

So, these things had a steel gasket at the base contacting aluminium and Iron block. Then they had a steel gasket between the aluminium head and the Iron block at the top.

The unknown steel of the head/barrell studs contact everything in the mix.

These old things are, well, lets face it behind the 8 ball when first put into production with all that. Mind you, to have one still running or in a condition to restore has to be an achievement in its self aye?

I would imagine that head and case aluminium was not the same mix back then which adds more unknowns.

So, with all of that, just add salt water as an electrolite and we have what we have in our hands to deal with.

Copper gaskets

I have never seen a head with copper gasket original corroded. So that is iron block/copper/aluminium. Seems to me that that was a fix by Seagull?

I am going to use a copper head gasket, modified and sealed between layers around the induction port, for my FVP base gasket.

I am going to use a copper head gasket, as normal, top end.

The gaskets from BS or John actually fit with just the base dimensions being fiddly but no real issue in sealing. Some trimming is required of course. Post that later.

I reckon, if your Old Dear runs, tune it the best you can and use it before pulling it apart...................there will be nasty inside as above.

Cheers

Bruce
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Charles uk
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Re: Compression values

Postby Charles uk » Sun May 06, 2012 10:21 am

What's your answer to the gasket thickness question on the FV's? Double head gaskets?

The cylinder compression in the early short stroke FV is somewhat compensated for by the different exhaust port positioning & size, making the trapped stroke less than 10 thou different, & that could well be in the manufacturing tolerances.
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Oyster 49
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Re: Compression values

Postby Oyster 49 » Sun May 06, 2012 11:25 am

Here are a couple of pics of my FV, as stripped and then with the cases cleaned up. There was just enough sealing face left to be able to re-use the crankcases without repair. I made a replacement base gasket out of stainless, and used a normal copper head gasket. The stainless one took ages to make!

Personally I don't see how a brass one can be any worse than the steel originals, and if the engine is kept away from salt water I think it will be ok. I will do what Bruce is doing next and modify a copper head gasket to make the base gasket.

I have a pair of steel gaskets in good condition to use as templates, I have also got a spare set of crankcases in excellent condition if anybody is in dire need.
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Gannet
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Re: Compression values

Postby Gannet » Sun May 06, 2012 2:59 pm

Thanks for all the interesting comments.
It is strange that the ali heads don't (?) seem to suffer the same corrosion level as the crankcase inspite of being connected to a copper gasket. I was also wondering whether the ali spec was diferent, resulting in enhanced corrosion resistance. Although I would have thought that electrolytic corrosion would still take place irrespective of the spec.
Anyway, I am going to try to replace the bottom steel plate with a copper one of 1.5mm thickness. To reduce corrosion I have painted the relevant crankcase with chromate primer and will of couse add gasket goo.
Another thought is to provide a sacrificial zinc anode. I dont know how or where. My boat diesel engine (A BUKH 20HP) which is raw salt water cooled is fitted in the block with an anode (attached to a brass plug), which needs replacing annually. I believe that this is typical for boat engines.
Would a very thick layer of zinc rich paint on the studs help? Galvanised studs?

In reality how many hours is one of our seagulls going to be full of salt water every year? ( I take it as read that they will be flushed out after use).
If an FV/FVP has survived 60 years of poor standard of care, it is likely to last longer, with good care than most of us!(?).

Jeremy


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